The Breeding of Blackbird, Song Thrush and Mistle Thrush in Great Britain Part I. Breeding Seasons

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1 Bird Study ISSN: (Print) (Online) Journal homepage: The Breeding of Blackbird, Song Thrush and Mistle Thrush in Great Britain Part I. Breeding Seasons M. T. Myers To cite this article: M. T. Myers (1955) The Breeding of Blackbird, Song Thrush and Mistle Thrush in Great Britain Part I. Breeding Seasons, Bird Study, 2:1, 2-24, DOI: / To link to this article: Published online: 17 Jun Submit your article to this journal Article views: 177 View related articles Citing articles: 19 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 22 November 2017, At: 04:34

2 2EDITORIAL 2(I ) season. We are therefore launched into becoming ornithological actuaries dealing with life tables, death rates and fertility rates. Nest records give a firm basis for these studies. Furthermore these figures from nest record cards are attracting a new interest in birds. Capildeo and Haldane in last year's Journal of Animal Ecology have published tables which give the annual changes to be expected in a bird population if mortality and fertility rates are constant. Nest record cards and ringing returns between them can supply the basic data for such calculations. So the biomathematician will be coming to the ornithologist more and more for information, and one of the chief problems that arises over the nest record analyses from an editorial point of view is at what point in the accumulation of cards for any species does publication of the results become desirable. Readers of this number of Bird Study cannot help seeing how much more has been extracted from the Song Thrush cards (now numbering between 3,000 and 4,000) than was possible in It may therefore be better that preliminary papers should be much curtailed, confining themselves to brief statements on overall clutch size, breeding season and success and leaving the analysis of variation in time and place until plenty of cards have been sent in. What I, as Editor, wish to avoid, is the multiplication of papers all to the same pattern and all consuming many pages with tables. If each species dealt with is approached as a new problem, then new ideas will be struck during the analysis, even if it is a short one. Those who fill in the cards will feel more rewarded for their trouble and the statisticians will feel that the results have already been subjected to a rigorous scrutiny before passing into their hands. THE BREEDING OF BLACKBIRD, SONG THRUSH AND MISTLE THRUSH IN GREAT BRITAIN PART I. BREEDING SEASONS by M. T. MYRES (Edward Grey Institute of Field Ornithology, Oxford) Received 31 August, (A Publication of the British Trust for Ornithology) SUMMARY 1. Information on 9,50o nests of the Blackbird, Song Thrush and Mistle Thrush, covering the years , has been analysed. 2. The methods of analysis are explained. Care was taken to exclude observational bias. The limitations of such a ' nest record ' scheme are discussed. 3. The breeding season of these species in Great Britain lasts from March to July, that of the Mistle Thrush being the shortest, that of the Song Thrush the longest. 4. The pattern of breeding varies markedly from year to year. There are 3-4 peaks of laying during the season.

3 1955 BREEDING OF THRUSHES 3 5. The three species coincide in the peaks and troughs of their egg-laying activity. 6. The blizzard at the end of March 1952 considerably affected the pattern of breeding in that year. 7. Breeding starts earlier in the south than in the north of England, but all over southern England seasonal fluctuations in laying coincided. 8. The pattern of breeding in is compared with the mean temperature and rainfall at Oxford. The number of clutches begun during March is correlated with the temperature about 5 days before. From mid-april onwards mean temperature has little effect, though prolonged heat waves may delay the laying of new clutches. 9. Thus in an early season the number of clutches begun during March may fluctuate with the temperature, but in a late season breeding may decline smoothly after a first peak. Details are given of a comparison between laying dates and temperature and rainfall in 1952 and INTRODUCTION This is the first part of an analysis of nest record cards for the three common British thrushes carried out by the present author and D. W. Snow and deals only with the timing of the breeding season. Parts II and III on clutch size and breeding success by D. W. Snow remain to be published. Since 1948 the British Trust for Ornithology has received 5,700 nest record cards for the Blackbird (Turdus merula) and 3,500 for the Song Thrush (Turdus ericetorum). These are more than for any other species and it has seemed more profitable to write one comparative report than separate ones for each species. The Song Thrush records up to 1947 have already been described by Silva (1949), but there is no previous analysis of Trust data for the Blackbird. Records for both species since 1948 have been analysed, but for most purposes data for only have been used. Only those 1953 records which were returned by I May 1954 have been included. 35o cards for the Mistle Thrush (Turdus viscivorus) for the same years have also been inspected. Figures not given in full in this paper are deposited at the Edward Grey Institute. Nest record analyses, besides that for the Song Thrush already mentioned, have to date been published only for the Robin (Erithacus rubecula) by Lack (1946, 1948), for the Spotted Flycatcher (Muscicapa striata) by Summers-Smith (1952) and for the Greenfinch (Chloris chloris) by Monk (1954). The very much larger number of records for Song Thrush and Blackbird has enabled us to make a more detailed analysis than was possible for these other species. In particular, it has been possible to examine the influence of weather on the pattern of breeding in each year. Adding together the data for all years provides a composite picture of the breeding season which is different from that of any particular year, and may obscure important annual differences. Due weight must also be given to the possible observers' error inherent in such a nest record scheme as this (see Nelder, 1952 and 1953, and Gibb and Campbell, 1953). Until sufficient cards are available for species with long breeding seasons (and the requisite number depends on the species and particular topic of investigation), detailed analyses cannot be made. For instance since 1951 about I,000 cards a year have been returned for the Blackbird and also for the Song Thrush, but these have only just

4 4 BREEDING OF THRUSHES been sufficient to reveal fluctuations in the breeding season and in clutch size. Our experience of the cards has impressed upon us that the nest records scheme is unable to provide accurate information about certain aspects of breeding biology, which are best investigated by individual students. But, as we hope this paper will show, there are many aspects of the subject which individuals cannot investigate alone, and about which only such a co-operative scheme as this can provide adequate information. The geographical distribution of the cards is uneven. There are too few being returned from Wales, Ireland or the Channel Islands for these to be analysed as yet. England and Scotland were separated, and England was divided, as by previous authors, into two main ` regions ', North and South, along the southern boundaries of Lincoln, Nottingham, Derby and Cheshire. There were enough cards to sub-divide South into four `areas' in order to investigate local differences. It may be possible in later analyses to bring this down to a district level where such factors as habitat, exposure and height above sea level might be investigated. Convenience of handling and the uneven distribution of observers caused us to divide South England as follows: Peninsula: Cornwall, Devon, Dorset, Somerset. South-east : Hampshire, Wiltshire, Berkshire, Sussex, Surrey, Kent. East : Norfolk, Suffolk, Essex, Middlesex, Hertford, Huntingdon, Cambridge, Bedford. Midlands : Northampton, Buckingham, Oxford, Gloucester, Worcester, Warwick, Leicester, Rutland, Hereford, Shropshire, Stafford. Peninsula and East were obvious entities. Northampton, which lies mainly on the Jurassic belt, was included in Midlands with all the counties to the west of it. The Thames valley was arbitrarily chosen to separate the Midlands from South-east. METHODS The date when the first egg of each clutch was laid was used in describing the breeding season (cf. Silva, who used the last egg). Many nests fail before the clutch is complete so that this method is less wasteful of records. For most purposes the dates of the first egg were gathered into 5-day groupings. Dates for first eggs were obtained with varying degrees of exactness : (I) Precise dates. (a) Clutches found during laying. In this case the date of the first egg could be inferred exactly or almost exactly, as one egg is usually laid each day, most commonly before midday. These alone were used in the histograms (Figs. 1, 2, 6 and 7). (b) Calculated dates. If the date of hatching was known, the date of the first egg could be calculated by subtracting 13 days plus the clutch-size minus one. (13 days is the mean incubation period given in Witherby et al. (1938) and was confirmed from our own records.) This estimate is accurate to within two or three days. If the date of fledging was known, a less reliable estimate could

5 1955 BREEDING OF THRUSHES 5 be obtained by subtraction of 26 (for the Mistle Thrush 28) days plus the clutch-size minus one. Similarly the date of the first egg could be calculated if the age of the young in the nest was stated on the card. - (2) Bracketed dates. An estimate, within restricted limits, of the day on which the first egg was laid could be obtained if more than one visit were paid to the nest during incubation or during the fledging period. The largest possible ` bracket ' is one of 12 days with an error of ±6 days from the middle date. We had originally intended to take the middle date and include all of these clutches in our graphs, but to ensure that the graphs for all species were exactly comparable we included only those bracketed dates, of 5 days or less in extent, which fell completely within a period (cf. Silva who had to use nests which had been visited only once). As discussed later for 1952, this exclusion of records results in the first part of the season being under-represented, as many of the early nests could only be assessed as large brackets'. But in Tables I IV and the Appendix all bracketed dates were used (even including nests only visited once). When the accurately dated clutches found during laying are plotted day by day, the resulting histograms are very similar to the 5-day graphs (see Figs. 1 and 2). Minor discrepancies (e.g. under Blackbird 1953 and Song Thrush 1952) are due (i) to the arbitrary grouping of clutches cutting across a sudden change of numbers in the histograms, or (2) to the fact that bracketed and precise dates do not always correspond exactly. Most of the smaller peaks and troughs in the graphs cover more than one 5-day period, which greatly reduces the possibility that they are due to sampling accidents. The use of a 10- or even a 7-day period may obscure the seasonal fluctuations. Sometimes it produces a misleading impression of a season with two, or even a single, peak (e.g. in Lack, 195o). When Silva's data for the Song Thrush were re-analysed in 5-day periods (omitting the clutches which provide more than a 5-day bracket, which she had included) the two peaks she had found in the generalised breeding season were shown to be really 3 or 4 peaks (see Fig. 2), as occurred in the Song Thrush in OBSERVERS' BIAS It was obvious that many people, besides schoolboys, tend to search for thrushes' nests most actively in April ; the effect of this is augmented by the fact that later nests are better concealed by the summer vegetation than earlier ones. At boarding schools, however, active nest-recording may occur only during term-time, i.e. in March and May, but not in April. This is evident not from the date of the first egg but from the date on which the nests are found. Participators are usually members of the school natural history society and so do not often fill in cards at home. This would make it desirable, for certain purposes, to exclude the contribution of some observers (e.g. from area analyses of the breeding season, though not of clutch-size or nesting success). These observers raise the peaks in one or other months in the

6 6 BREEDING OF THRUSHES 2(I) breeding season graphs, but the position of the peaks as a whole was not usually affected. For instance, South-east England in 1951 has a very large March peak which is due mainly to one observer (Fig. 3). But as we have been chiefly concerned to compare the species we have found it too costly in cards to exclude them. When the season is broken down into histograms, of course, the heights of the peaks are not the main points of interest (for these are shown better in the 5-day graphs). It is then the day-to-day fluctuations, and the correlation of these with, for instance, the fluctuations in temperature a few days before, that are of primary concern. These are not greatly affected by an individual's concentration of nest-finding into one month. ANNUAL AND SPECIFIC DIFFERENCES IN THE BREEDING SEASON General Pattern of Breeding. The laying dates in South England in the years are plotted in Figs. 1, 2, 4 and 5. The earliest clutches were laid in the last few days of February, and late nests were recorded from J my and even August. From these records the start is more or less synchronous throughout the population. After this the effects of weather and predation result in a quick loss of synchrony varying with the stage of the season and the local conditions. However, the start of breeding may be different in different habitats. This cannot be ascertained from the nest records as most observers find their early nests in gardens. Field observation indicates, however, that the main start of breeding of the Blackbird in deciduous woodland may be as much as a fortnight later than in gardens. The graphs show 3-6 peaks in laying, a number that corresponds more closely with the number of nests actually begun than with the number of broods successfully fledged. It is thus only at the start of the season, and in re-layings after a period of unfavourable weather, that the clutches will be in phase together. Predation and other causes will then quickly put them out of phase. The form of the breeding season of the Blackbird and Song Thrush is quite different from that revealed in the previous Trust analyses of the Spotted Flycatcher (a summer visitor) and the Greenfinch (whose young are fed mainly on seeds). These birds do not start to breed until 1-2 months later than the thrushes and have fewer broods. The influence of weather and food is likely to be different for them. Lack (1950), using data from the note-books of the late A. Whitaker, compiled graphs of these three thrushes, which in the sense that they covered forty years' observation in one district (Derby-Yorks border) may be regarded as `average' seasons. These showed Mistle Thrush, Song Thrush, and Blackbird reaching their peaks of laying in successive weeks (first three weeks of April). The nest records agree as to the order of starting of the species, but it should be emphasised that for most of the breeding season the Mistle Thrush is overlapped by the other species, and it is only a small proportion of the population which starts absolutely earlier. Lack notes, and we can confirm, that the length of the Mistle Thrush's breeding season is less than that

7 1 955 BREEDING OF THRUSHES 7 of the other two species. But, as will become apparent, it is probably not true that the breeding season reaches a peak in any of the three species so late as Whitaker's data suggest. The paucity of cards for the Mistle Thrush in June may be due partly to the nests being placed higher from the ground, and being even less easy to find in the summer vegetation, than are the nests of the other two species. Figs. i and 2 show that 1952 had the earliest start of breeding, then 1951, then 195o (see Fig. 5), and that in 1953 the season began latest. The proportion of the whole seasons' clutches laid in March (see Appendix) indicates the same order of breeding in the four years but with 195o and 1951 interchanged. The Appendix also shows that when breeding starts early the distribution of clutches through the season is not necessarily weighted in favour of March nests at the expense of June nests. Of the two generally late years, 1951 and 1953, the former had a large June percentage of nests (in North England and Scotland) and the latter had instead a high April percentage, which in the Blackbird was the same in all three geographical regions (54%). The Song Thrush had its least variable April figures in the other late year 1951 (41-42%). The implication is that the breeding season is more alike in the three regions in late years than in seasons when breeding begins early in March in the south of the country. The Appendix confirms that the Song Thrush starts laying earlier than the Blackbird (see also Table III) but suggests that there are also relatively more thrushes' nests in June. This is reflected in the proportion of nests in April which (with two exceptions) was lower in the Song Thrush than the Blackbird. The May proportion was similar in the two species. Silva records that in the Song Thrush ` as many as 72% of the recorded clutches were laid in April and the first half of May', but admits that this is probably too high. Our highest figure for the same period is 61% in Scotland and 49% in South England. For April plus the whole of May in the South it only varied between 63 and 69%, which are both below her figure. Bull (1946) describes the breeding season near Auckland in New Zealand in He also found that the Song Thrush started earlier and ended later than the Blackbird. There was a preliminary period of ' winter laying' (2.5% of all nests) from 26 June to 3 July, in the Song Thrush, followed by a non-laying period until the first clutches of the main season on i 1 August. The first Blackbird clutches were not laid until 27 August. Bull found that the main season consisted in both species of about four peaks, and that the two species fluctuated together. The season in 1944 began earlier than in Detailed descriptions of the years. 195o. The start of the season (Fig. 5) was earlier than in but later than in It apparently consisted of about three peaks, and the one at the end of April was prolonged The similarity of the graphs of all three species is at once apparent ; all the main peaks and troughs are exactly or very

8 8 BREEDING OF THRUSHES2(I) nearly coincident. Proportionately fewer Blackbirds than Song or Mistle Thrushes started laying during the first favourable period (22-26 March). (The number of Blackbirds starting in the period March did not drop relatively so low as that of Song Thrushes). Consequently the second peak in 6-10 April was lowest in the Mistle Thrush and relatively highest in the Blackbird. This is reflected in the histograms in which the day when the most Song Thrush clutches were begun was zo March (calculated dates included) while in the Blackbird it was 6 April. The Mistle Thrush figures were few but closely followed those of the Song Thrush, even in showing a third peak in the first period of May. But the first peak was seemingly more prolonged and relatively higher than in either of the other species. In the Song Thrush there was no ` peak of breeding', as suggested by earlier writers for thrushes, and there is considerable similarity between the graph and that of Silva's revised graph (see Fig. 2). It seems likely that, whereas the breeding season in 1953 was most like the average' described by Lack (1950), 1951 was the most ` typical' of the four years, showing 3-4 peaks. The three peaks cannot have represented three successively fledged broods as they occur within a month of each other ; hence the second peak was a combination of delayed first layings and repeat layings, and the third peak must have consisted of second broods and re-layings. r052. The start of breeding was earlier than in any of the other three years. But there was a great blizzard in the last few days of March, snow being reported from the whole of the South of England and from Lancashire and Yorkshire. Of 84 Song Thrush and 74 Blackbird nests under observation at the time, 56 and 46 respectively (i.e. 2 in every 3) were recorded on the cards as failing directly because of the severe weather. The proportion may be exaggerated, however, as only those cards on which weather was mentioned were counted. The blizzard, besides destroying large numbers of nests already begun, obviously held up the start of new clutches, so that when it was over there was a great outburst of breeding, which was closely concentrated within a few days (6-13 April) and the number of clutches begun on each day in the period was abnormally large. Consequently the highest peak day did not occur in the first peak of breeding (as it did in 1951 or 19J3). Ash (1951) describes the effects of a oneday snow storm (4-6 in. of snow) in Berks on 26 April, Five out of fifteen Blackbird and Song Thrush nests were destroyed. But the 1950 nest record cards do not show a trough in laying after this snowstorm nor a later peak of re-laying. As already mentioned a larger proportion of the cards for early than late nests give only bracketed dates ; when those with more than a 5-day extent are incorporated (Table I) the first Song Thrush peak is shown to have been even higher than the second one, though for the Blackbird inclusion of all bracketed dates enlarges only the second March peak. It is possible, therefore, that exclusion of some of the nests recorded will make the earliest March peak relatively smaller than it should be in other years also.

9 1 955 BREEDING OF THRUSHES 9 TABLE I NUMBER OF CLUTCHES BEGUN PER 5-DAY PERIOD IN FIRST PART OF SEASON IN SOUTH ONLY March April Song Thrush Blackbird.. I to (to be compared with Figs. I and 2) Thus, although both species had peaks at about the same time, the Song Thrush in 1952, as in 1951, had its first peak relatively higher, and about 5 days earlier, than the Blackbird. The latter in contrast had the second peak higher relatively than the Song Thrush ; some Blackbirds clearly were not sufficiently stimulated to lay during the first favourable period. The peak in April in the Blackbird histograms is not represented in the Song Thrush histograms, in which there is a peak in 28 April-4 May which is itself not truly represented on the 5-day graph. Both probably consist mainly of second broods of the few birds whose first nests had survived the blizzards. Both Blackbird and Song Thrush showed a slight resurgence of activity in mid-june. The records for the Mistle Thrush agree with those for the Song Thrush. The Mistle Thrush had only one period of laying activity in March ; it was apparently less affected by the blizzards, for the peak in Crjo April is relatively smaller than in the other species. It shows the peak in the first period of May (presumably second broods) The Song Thrush again reached its peak of laying one 5-day period earlier than the Blackbird. The Mistle Thrush reached its peak at the same time as the Song Thrush. Breeding started later than in any of the previous years. The main peak was followed in all species by a gradual falling off, interrupted b y 3-4 minor peaks, which appear synchronous. The season differed from the others in showing no marked troughs until towards the end of breeding. The graph does not drop so sharply in the Blackbird in April as it does in the other species and so does not show the subsidiary peak in April, though this is clear in the histograms. The trough in May was marked in both species. The day with the maximum number of clutches was 27 March in the Song Thrush and 3-4 April in the Blackbird, i.e. in this season which was late the two species were more nearly synchronised. But large numbers of clutches were begun on two earlier days and there the Song Thrush was ahead. Discussion. In any year the three species agree in the timing of their peaks and troughs of laying. This means, presumably, that they respond similarly to environmental conditions which initiate or inhibit breeding. But after the first peak only a proportion of the population is able to respond by laying at any one moment as many birds will be at another phase of the cycle, with eggs or young.

10 I0 BREEDING OF THRUSHES Severe weather evidently holds up laying. The re-layings after the blizzard in 1952 presumably account for the fact that the peak, which in 1951 and 1953 occurred in 26 April-9 May, is hardly represented. There was a marked reduction (by about one-quarter) in the number of nests begun in May (see Table II). The number of Song Thrush and Blackbird clutches begun after 25 April (the start of the usual 'third' peak) in 1952 was actually less than in 1951, though the total of all dated clutches rose from 1951 to 1952 by a third. It is true that attempts at late broods must have come later in May, and may be less well recorded on account of the reduction in nest-finding later in that month. There is a likelihood therefore that when breeding starts early, and unsuitable weather occurs late in March, the pattern of breeding may be profoundly affected in that year, if the population is already far advanced with its first nests when the bad weather occurs. TABLE II THE REDUCTION OF CLUTCHES LAID AFTER 25 APRIL, 1952, WHICH MAY HAVE BEEN DUE TO THE BLIZZARD (SOUTH ONLY) Number of clutches started after 25 April Song Thrush Blackbird Total of dated clutches in season Song Thrush Blackbird % clutches after 25 April Song Thrush Blackbird , Note. As measured by the X' test the proportion of clutches after 25 April in both species in 1952 is significantly different from the proportion in 1951 and (P < o.o1). Three to four peaks form the usual fluctuation in breeding, but in 1953, which was a late season, the fluctuations are much reduced. In March in most seasons the number of clutches begun in any period is closely related to the fluctuations in mean temperature as will be shown later. In April and later this is not so ; the graph for 1953 shows an initial peak of breeding activity followed by a smooth slope to the end of the season as pairs of birds become out-of-phase and breeding declines. The interpretation suggested is that there are several favourable periods stimulating the three species of thrush to lay ; the Mistle Thrush responds most quickly (perhaps to a slightly lower threshold temperature), then the Song Thrush and lastly the Blackbird. But the Mistle Thrush is not so far ahead as to respond to early favourable conditions to which the Song Thrush does not respond. Similarly a higher proportion of Song Thrushes than of Blackbirds responds to the first favourable conditions that affect them.

11 ^955 BREEDING OF THRUSHES I I GEOGRAPHICAL AND SPECIFIC DIFFERENCES IN TILE BREEDING SEASON Regions. If there are any geographical differences in the time and form of the breeding season they may first be detected by comparing Scotland, North and South England. From Scotland there were too few clutches, and these were too widely dispersed, to justify plotting as a graph. The percentage of clutches begun in each month in each of the three regions is shown in the Appendix and the extreme limits for March are summarised in Table III. It is apparent from the trends of the figures in both species that a greater percentage of the total clutches recorded in South than in North England were laid in March. TABLE HI GBOGYAPHIC.AL DIFFRIENOND THE PROPORTION OF MARCH CLUTCHES Song 77uufb Blackbird Scoria d 0-13% 0-6 % North England 12-23% 7-18% South England 22-29% 14-25% In any one year or month the two species agreed remarkably ; thus the two highest Scottish figures (in the Appendix) for June occurred in 1951 and 1952 in both species. They agreed in having no March nests in Scotland in The season in Scotland does not reach its peak until the end of April. Venables and Venables (1952) state that in Shetland breeding of the Blackbird starts in the second half of April. In 1951 the season began later than in 195o or 1949 due to a cold and late spring. But in the South of England breeding started later in 1 95o than in 1951, an example of the differences caused by local climatic effects. In Holland it appears that the breeding season of the Song Thrush hardly starts until April, with a single peak in April (see Table II in Silva, 1949). If so, it raises interesting problems of comparison e.g. with our own populations in East Anglia and Kent which are closest to the Continental shore and climate. If the Dutch data had been grouped in 5-day periods (instead of 10-day) a peak might have emerged in 1-9 April. The season there may prove a little shorter and less fluctuating, as well as later, than ours. Comparable information for the Blackbird is desirable. Detailed description of the areas. Subdividing South England into four areas tested whether regional differences were being obscured in Figs. 1 and 2. In fact the areas agreed very well although the number of clutches was often small. The graphs of the Blackbird clutches from three of the four areas are plotted (with North for comparison) in Fig. 3 to demonstrate this. Clutches were found to be later, or less concentrated in March, in the North than in these four southern areas.

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14 I^ BREEDING OF THRUSHES 2(1) 20 NORTH to MIDLANDS EAST SOUTH-EAST 20 ^ 1O tii 'IV ' V ' ID '. DI ' Cr 'V ' IQ ' m ' iy ' V ' 23 Figure 3. Blackbird. Commencement dates of clutches ( ) in three subdivisions of South England and in North England. Grouped in 5-day periods. Ordinate=number of clutches; abscissa=months The March peak in both species was unusually high in the South-east owing mainly to the great activity of one observer. All the areas show the deep trough in mid-april and agree in the two peaks at the beginning and end of the month, though the exact period in which the highest point occurs varies a little. The Blackbird was perhaps a little later in the East than the Song Thrush. There was a minor peak in mid-may in most areas The Song Thrush and Blackbird had almost identical breeding seasons, and the effect of the blizzard was noticeable in all areas. Only in the Midlands was there a marked drop between the first and second peaks. The first peak was relatively higher in two areas in the Song Thrush. It is clear (esp. South-east and Midlands) that the main Song Thrush recovery peak was 6-Io April while in the Blackbird it was April, especially in the North. The late April early May sub-peaks vary from place to place, being largest in Scotland, but there are traces in most areas of the revival in the second half of May in the Song Thrush and of mid-june clutches in both species The start of breeding in all areas was delayed until the last ten days of March, though there were one or two clutches recorded from every period earlier. In the North the summit of the first peak was even later. The mid-april trough showed well in both species in Midlands, but in most areas there were signs of a sub-peak in the middle of this period. The sub-peak at the end of the month occurred in most areas and the one in mid-may in South-east and Midlands. The percentage of clutches in March for the four areas of South and for 'North England show, in all years a definite geographical trend (Table IV). The South-east was earliest and the Midlands (which are away from the sea and adjacent to the North) latest. The figures for other months also indicate a similarity between the Midlands (and also East) and the North, but they are less conclusive. In some cases the figures for East were closest to South-east. in other cases to the Midlands. It

15 1955 BREEDING OF THRUSHES 15 might be supposed that breeding would start earliest in Peninsula but this is not proved by the present figures. There are indications not only that the South-east, but also the East may be earlier than Peninsula in some years. But the South-east percentage includes the large sample of early nests from one observer mentioned above, so we cannot certainly claim this. Probably the factor determining where the earliest clutches are laid in any year is the pattern of weather movement in northern France, the English Channel and Bristol Channel in the last few days of February or the first few days of March. TA= IV THE PERCENTAGE OP CLUTCHES LAID IN MARCH ACCORDING TO AREA Song Thrush Mean 1951 Blackbird Mean North Midlands East.. 20 (34) (22) Peninsula. (27) (33) (22) 27 (26) 17 (22) 22 South-east (24) 30 z South (mean) Note. All these percentages have been calculated from over loo cards for each area except those in parentheses, all but one of which are based on 50 loo clutches. The figures for North and South (mean) are significantly different as measured by the X 2 test in both species in 1951 (P < 0.0!) and in the Blackbird in 1952 (P<o.o5). THE INFLUENCE OF WEATHER A breeding season which is composed of a series of well-marked peaks of laying could be due (I) to an uninterrupted succession of synchronised broods or (2) to an extended laying season in which the laying and re-laying of clutches are concentrated in periods of favourable weather. It has already been shown that the pattern of breeding differs markedly from year to year and so cannot be due to (I) alone. Furthermore the intervals between peaks are often too short. In this section I hope to show that weather does have a profound modifying influence on the pattern of breeding, especially in determining the moment in the early spring when laying begins. Figs. 4 and 5 compare the breeding season of Blackbird and Song Thrush in the South of England with the mean temperature and rainfall (per 5-day period) in the springs and summers of The weather data were kindly supplied by the School of Geography, University of Oxford. Oxford is in the centre of the South of England and when the 1952 figures were compared with those from Uppingham in Rutland, kindly sent by Mr. K. G. Messenger, we found the same fluctuations the more northern figures being 2-5 F. below those from Oxford.

16 16 BREEDING OF THRUSHES 2(z) NO. OF CLUTCHES ` J^ TA 1 0- o O- S, C- 40- RAINFAL 1 l NY. ) ^ ^ ^ v RAINFALL (I NJ ( / 1953 TEMPERATURE (SF.) 1 17 ra IV F m Figure 4. The breeding seasons of 1951 and 1953 compared with mean temperature and rainfall at Oxford (5-day periods). Unbroken line= Blackbird ; broken line = Song Thrush ; abscissa = months. I NO. OF CLUTCHES As is shown below, the number of clutches begun in any period in March and the first half of April appears to be correlated with the mean temperatures one 51-day period earlier. From mid-april onwards, however, the temperature has little effect except possibly in the opposite direction because when the mean temperature exceeds about 5o F. for any length of time breeding may be inhibited. This is suggested particularly by the sharp rise in mean temperature about the end of May in 1953 and 1950 ; at this time rainfall appears to cause periodic revival of egg-laying. The threshold temperature in March that has to be reached before breeding begins is about 40 F. This has been marked, for convenience, on the graphs by a straight line. Breeding can evidently start in the first few days of March but unsuitable weather may delay it until the first period of April a delaying effect of a month in some years. The threshold temperature is clearly higher than 4o in February, and lower in the last periods of March and during April The second half of January was very cold. In the first week of February the temperature rose sharply and in the fourth 5-day period it was high. But it fell again and for. the last three nights of February it was freezing. Mean temperature rose again and was steady at 45 F. for the first three periods of March.

17 1955 BREEDING OF THRUSHES TEMPERATURE ( F (1 32 I0-- RAINFALL (IN.) AWL. ^ ^^^ _ Is. /SWIM- NO. OF f.:utlifes 60, QO o 1952 TEMPERATURE ( FJ WP"Ii r ^J. RAINFALL (INS 70 í) \ / ^ n \` 1 AIL l ^ NO - OF LLL'TLRES ^ 1 II IC r2y iq Figure 5. Comparable data to figure 4 for 195o and /\J Breeding began then, but only on a small scale until the mean temperature rose to 5o F. in the fourth period. The subsequent drop in temperature is also represented in the clutches starting at the beginning of April, and it is possible that temperature had some influence on breeding until the end of April. In the first week of June mean temperature rose sharply at Oxford the mean minimum for the month was the highest ever recorded and the mean maximum had only been exceeded once. January was dry and February wet. Thereafter precipitation was evenly distributed through the season at Oxford. Snow was recorded only on April January temperatures were above 40, and were followed by a cold spell in early February. The temperature thereafter was close to 4o, but in the first week of March it fell again. When it rose to 44 in the third period it was at once followed by the start of breeding on a large scale. It again fell below 4o in the last period of March and this reduced, but did not completely stop, the laying of clutches. When the temperature rose yet again, more clutches were laid. The high temperatures at the end of April may have been partly responsible for the third peak of breeding. The fluctuations in rainfall were closely parallel to those in temperature in February and March, and it was moderately wet i 7

18 Ió BREEDING OF THRUSHES 2(I) when breeding began. There was much rain at the time of the second (resumed) peak in breeding. Snow fell on 1-2 January, 31 March, and 7 and 13 April A warm first half of January was followed by a cold spell. Temperatures just exceeded 40 0 in the second half of February. January and February temperatures taken together were lower in 1952 than in But January and February 1952 were the sunniest on record at Oxford. For the three months December-February there were hours the previous highest being hours in Temperatures only rose much above 40 (maximum 48 ) in the first week of March. Breeding began at once, but a drop in temperature in mid-march caused a temporary hold-up. In the last week of March gales and blizzards swept the country ; the maximum temperature fell to 33.9 F. on 29 March, which was the lowest for the second half of March ever recorded at Oxford, and the mean for the period was only 0.5 higher than this. Many birds lost their nests, but were unable, owing to the previous low temperatures, to start again in the first period of April. The return to normal temperatures resulted in a remarkable rush of laying. Breeding is not obviously correlated with temperature after this. The season ended early. Rainfall in January and February was lower than in the other three years. But it was rainy when the temperature rose and breeding began in early March. The rest of the month was dry until the blizzards. April too was dry and the resurgence of breeding in the first half of May may be related to the fact that it was wetter then. The fourth period of May and the first half of June were slightly wetter also, and breeding continued then ; the Song Thrush particularly shows this. Snow fell in January (2, 17, 20, 23, 24), March (5 in. lying on 3o, 2 in. lying on 31) and 2 April This year was particularly instructive. The temperature was well over 40 in the last two periods of February, but fell in the first week of March. Evidently the high temperatures (sufficient in 1952 a week later to allow breeding) were not able to initiate breeding in 1953 (cf. also 195o). The temperature remained about 40 until 20 March, when it rose to 48 and breeding began in earnest. But the temperature remained level until the beginning of May and the number of clutches begun during that time was unrelated to it. June was one of the least sunny Junes on record at Oxford and the maximum temperature of 50 on 3 June was almost the coldest June day on record there. The start of breeding again occurred in a wet period (there had been no rain since 20 February). The periods of laying activity in the first and fourth periods of May appear to be related to the amount of rain just before each period. There was rain on a small scale during June, which may explain the long continuation of breeding by a small number of birds. Snow fell only in January (1, 3-7, 30) and February (3, 8, 11-14). Thus the graphs suggest that the number of clutches begun in any 5-day period is partly determined by the weather conditions about 5 days before ; both temperature and rainfall may have an

19 1955 BREEDLNG OF THRUSHES NO. OF CLUTCHES 15 (BLACKBIRD) ^ NO.OF CLUTCHES (SONG THRUSH) RAINFALL (IN.) A Irl --- P--L ^ 4. nni.- 0F3, A. _ I t i fl._, III 1:37 8 V ( II Figure 6. Commencement dates of clutches of Blackbird and Song Thrush ( compared day by day with maximum and minimum temperatures and rainfall at Oxford. Abscissa=months. For explanation of letters see text and Table V. effect in March and April, but in May and June rainfall may be the only influencing factor. Daily fluctuations in the weather. In Figs. 6 and 7 histograms for the Blackbird and Song Thrush clutches started each day in 1952 and 1953 (South only) are placed alongside the daily maximum and minimum temperatures and rainfall at Oxford (i) to show how soon large changes in temperatures stimulate breeding and to see whether minor daily fluctuations produce detectable effects and (2) to find whether the fluctuations seem most closely related _ to the maximum or the minimum temperatures. In the following brief description of the figures possible correlations are pointed out but it must be emphasized that these are suggestions only. Far more cards from more circumscribed areas, whose weather data are known, are needed to show how closely temperature changes affect individual birds. In Table V the points marked on the graphs are listed with the length of time before their supposed ' effects ' occured. These are rough estimations only The histograms show marked fluctuations in breeding early in the season. The maximum and minimum temperatures rise and fall together. After 3o April the minimum temperature is always 40 or over and no correlations between temperature and laying of clutches are certain.

20 20 BREEDING CF THRUSHES 2(1)

21 1 955 BREEDING OF THRUSHES 1I TABLE V INTERVALS BETWEEN WEATHER CHANGE AND 'EFFECT' ON BREEDING Point marked Delay (days) (see Fluctuation figs. Approx. (temp.+ Song Black- 6& 7) date rain) Thrush bird March A 1-7 Inean+min. rise, also rain?? BII I mm. fall 5 5 Bs15 min. fall C mean rise D1-Ds 26-3/4 min. fall 2 4 E April f 1-3 if mean, rise 1 5 if min., rise Ft 20 max. fall F2 25 min. fall 2?2 ' G 28 mean fall 2 I-2 H 3o mean rise 4 4 Delay (days) Fluctuation Point Approx. (temp.+ Song Blackmarked date rain) Thrush bird March a 4 max. rise b min. rise c max. rise A 20 mean rise 4 6 B 23 max. rise 4 6 C 26 max. fall D 29 min. rise+?rain 5 5 April E 1-2 min. rise+? rain F14 min. fall Fs 8 min. fall G rz min. rise H mean rise I { if max, rise 5 none K 27 min. rise? none 4 The onset of breeding occurred some days after the minimum temperature rose above freezing and when the first rain for 12 days had fallen (A). From $I B2 the minimum temperature was low, and there is a corresponding trough in the number of new clutches of both species a few days later. There was a general rise of temperature at C which was followed by a second peak of laying, but during the blizzard night temperatures were again below freezing (for 9 nights). The restarting of breeding was sharp-2-3 days after the minimum temperature rose above freezing. Between E and F I day temperatures fluctuated widely and may be the cause of the large fluctuations from day to day in the number of clutches begun after the main peak of re-laying was past. In the 6 days F 1 F, first the day and then the night temperatures fell. Both species showed a trough during this period. There was a short rise of temperature between F z and G which apparently resulted in more clutches ; this was followed by a drop in the number begun, perhaps caused by G. The rise of temperature at H may have been responsible for the simultaneous rise in both species 5 days later In contrast to 1952 minimum temperatures only exceeded 4o on a few nights before the middle of May. Although the

22 22 BREEDING OF THRUSHES 2(I) temperature changes and fluctuations in laying are much smaller, the influence of temperature can perhaps be traced, for the spring of 1953 also differed from 1952 in that the maximum and minimum temperatures did not rise and fall together for much of the time. Possible effects of the one or the other may therefore be examined, rather than the combined effects of both. Thus a, b and c, though coming too early to cause detectable laying of clutches, show extra-warm days and extra-warm nights occurring independently. The rise in temperature from A to B was followed by the onset of breeding. There was a pause in the middle of this possibly traceable to a pause in the rise of the maximum and a fall in the minimum temperature. There was a sharp drop of temperature at C followed by a decline in the laying. Thereafter day maxima did not alter more than a few degrees for four weeks, but night temperatures rose sharply at D. The drop between D and E, the rise at E, and the drop at F apparently caused simultaneous effects in both Blackbird and Song Thrush and show the possible effects of night temperatures (which occur, of course, when the birds are inactive). In the next three weeks the night minima only rose above 42 on 4 nights and it is possible that these changes are responsible for some of the fluctuations in the number of new clutches begun in the second half of April and first week of May. There was at Oxford a 12-day dry period (L) in the first half of May towards the end of which the number of new clutches declined. Shortly after the ensuing wet period (M) however there was a revival of laying in both species (minimum temperatures also rose at this time). From Table V three things may tentatively be suggested. (1) It takes about 5 days for temperature changes to influence the course of laying early in the season but that later its effects occur in about z days. (2) Minimum temperature is probably more important than maximum temperature in producing these effects, especially as regards low temperatures holding up breeding. (3) The Song Thrush is sometimes quicker in responding to these changes ; in the clearest case of all, the outburst of breeding after the blizzard in 1952, the Blackbird responded a day later than the Song Thrush. ACKNOWLEDGEMENTS Thanks are due to all the nest recorders who made this paper possible ; also to Dr. David Lack for suggesting the analysis be made, and to him and to R. E. Moreau for helpfully criticising the manuscript. REFERENCES ASH, J The effect of a snow-storm on breeding birds. Brit. Birds, 44:57-9. BULL, P. C Notes on the breeding cycle of the Thrush and Blackbird in New Zealand. Emu, 46:

23 1955 BREEDING OF THRUSHES 23 GIBB, J. & CAMPBELL, B Letter. The use of data on Nest Record Cards. Brit. Birds, 46:119. LACK, D Clutch and brood size in the Robin. Brit. Birds, 39 : , LACK, D Further Notes on clutch and brood size in the Robin. Brit. Birds, 41 :98-104, LACK, D The breeding seasons of European birds. Ibis, 92: MONK, J. F The breeding biology of the Greenfinch. Bird Study, 1:2-14. NELDER, J. A & Letters. The use of data on Nest Record Cards. Brit. Birds, 45 : 430 & 46:384. SILVA, E. T Nest Records of the Song-thrush. Brit. Birds, 42:97 III. SUMMERS-SMITH, D Breeding biology of the Spotted Flycatcher. Brit. Birds, 45 : VENABLES, L. S. V. & U. M The Blackbird in Shetland. Ibis, 94 : WITHERBY, H. F. et al The Handbook of British Birds, Vol. 2. London Scotland North England South APPENDIX PERCENTAGE OF CLUTCHES BEGUN IN EACH MONTH: BLACKBIRD March April May June Numher of clutches Scotland North England I I 184 South Scotland North England South I Scotland North England South t 722 Note. When the figures for March are compared with those in the other three months added together a trend from South to Scotland which is :tatitically significant, as measured by the x 2 test, is found. In x951 and 1952 P<o.or and in 1950 P<o.o5 (see Table 4 for South North comparisons).