Breeding of Collared Doves Streptopelia decaocto in rural Oxfordshire, England

Bird Study ISSN: 0006-3657 (Print) 1944-6705 (Online) Journal homepage: http://www.tandfonline.com/loi/tbis20 Breeding of Collared Doves Streptopelia decaocto in rural Oxfordshire, England H. A. Robertson To cite this article: H. A. Robertson (1990) Breeding of Collared Doves Streptopelia decaocto in rural Oxfordshire, England, Bird Study, 37:2, 73-83, DOI: 10.1080/00063659009477043 To link to this article: https://doi.org/10.1080/00063659009477043 Published online: 24 Jun 2009. Submit your article to this journal Article views: 851 View related articles Citing articles: 4 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalinformation?journalcode=tbis20

Bird Study (1990) 37, 73-83 Breeding of Collared Doves Streptopelia decaocto in rural Oxfordshire, England H. A. ROBERTSON Edward Grey Institute of Field Ornithology, Department of Zoology, South Parks Road, Oxford OX1 3PS, UK Breeding of Collared Doves was studied for 3 years in rural Oxfordshire. They laid eggs from February to October, mostly between April and mid-august. In rural Britain, breeding starts and finishes about one month later than at similar latitudes in eastern Europe although the populations have been separated for less than 50 years. The reasons for this are not clear. Collared Doves laid on average 3.8 clutches per year. Of completed clutches 89% were of 2 eggs, the rest of 1. Most pairs began a new clutch while still feeding fledged young, a few while the previous brood were still in the nest. Mean hatching success was 59%, fledging success 69%, and overall breeding success 41%. Success varied with season, peak production coming from eggs laid in May to July. Losses of eggs and chicks were mainly from predation, especially early in the season. Mean annual productivity was 3.1 young per pair. By feeding on super-abundant, predictable and persistent supplies of commercial crops, Collared Doves have the potential for high productivity. Predation by humans has increased substantially in recent years and this has probably brought Collared Dove populations in Britain to near stability after logarithmic population growth following colonization about 30 years ago. The Collared Dove is a recent colonist of Britain. Breeding was first recorded in 1955, 1 after a spectacular spread north-westwards across Europe this century from their ancestral home in Asia and the Middle East. The population of Collared Doves in Britain initially grew logarithmically,2 3 but has since slowed. For example, the Common Birds Census index increased each year to a peak in 1982, followed by 4 successive decreases and then a slight increase in 1987, so that over the 10 years 1978-1987 the index has decreased by 6% overall.' The cause of the sudden expansion in range of Collared Doves is not known but suggestions include: recent changes in climate s a mutation of a favourable gene,' and changes in agricultural practices and hence food supplies in or near the edge of their pre-expansion range i.e. in the Balkans.' The remarkable spread has been related to the ability of the species to reproduce rapidly and for dispersal Present address: Ecology Division, DSIR, PO Box 30379, Lower Hutt, New Zealand. movements to be orientated towards the north-west, 8-10 taking them into intensive cereal-growing areas of northern and western Europe. Shortly after Collared Doves arrived in Britain, many people published notes on their breeding but only Coombs et al. I have studied the breeding of Collared Doves from a large sample of nests. They made fortnightly visits to a population of up to 15 pairs breeding in a disused orchard at Soham, Cambridgeshire, from 1971 to 1977. They also related the observed egg-laying seasons to the seasonal changes in the gonad volumes. 11 In the ancestral range of the Collared Dove, studies of the breeding biology have been conducted by Rana 12 in India, and Marchant 13 in Iraq. In continental Europe, detailed studies have been made by Hofsetter, 14 Tomasz, 15 Lachner, 11 Nowak, 17 Gnielka & Wolter, 18 Kubik & Balat, 19 Gnielka, 20 Saemann, 21 and Pikula & Kubik. 22 In this paper I describe the breeding biology

74 H. A. Robertson of a rural population of the Collared Dove, and compare the Collared Dove in Britain with populations in the ancestral range and elsewhere in Europe, to provide some insight into changes that have accompanied their spread. METHODS This study was carried out in rural districts of north-west Oxfordshire within 30 km of Oxford (51 45'N, 1 15'W). Collared Doves were nesting on rafters or machinery in farm buildings, and feeding mainly on spilt or stored grain and stock-feed. During the normal non-breeding season (November February), suitable nest sites were checked at least once each month, because elsewhere in Europe eggs are reported to have been laid in all months. 23 From March until October, suitable nest sites were checked at least once a week and often daily. I recorded clutch size, egg sizes, laying order, incubation periods, nestling weights, fledging periods, inter-clutch intervals and number of clutches per season, and I ringed all nestlings with BTO rings. Adult birds were not marked, but there was a maximum of 6 pairs at any one farm so, from records of a few birds with distinctive plumage or behaviour and from the chronology and location of nests, I had little difficulty In identifying the successive nesting attempts of each pair through the breeding season. The maximum length (1) and breadth (b) of eggs were measured to the nearest 0.1 mm with dial calipers and a volume index was calculated as V = 1b 2/1000. Eggs were weighed to the nearest 0.1 g with a Pesola spring balance, and reweighed at intervals during incubation to determine the rate of weight loss. For the analysis of breeding seasons, I calculated the date of completion of each clutch from the known laying dates and from hatching dates. I discovered that Collared Dove eggs could be aged accurately for 3 days because freshly laid eggs had a clearer area under the shell at each end which gradually contracted towards the tips and finally disappeared. From the relationship between fresh egg weight and egg dimensions and the rate of weight loss of the eggs, 24 I was able to calculate laying dates for the few remaining clutches. For the analysis of the fate of eggs I defined 10 possible outcomes: Abandoned: the clutch was abandoned at any stage after the first egg was laid; Infertile: on dissection, no sign of embryonic development was visible; Addled: the embryo had died before hatching or during the hatching process; Egg gone: the egg had disappeared without trace, or was found away from the nest, where it must have been carried by a predator, or by a parent bird; Out of nest: the egg was found broken directly beneath the nest; Broken: the egg shell was cracked. This applied only to eggs that failed to hatch; some cracked eggs did hatch successfully; Hatched: any nestling that completely left the egg shell; Young gone: any nestling that disappeared from the nest, including those that fell accidentally, or those found dead in the nest but which had obviously been killed by a predator; Young died: any nestling that died of unknown causes in the nest; Fledged: any nestling that successfully left the nest, plus 8 nestlings (which were within a day or two of fledging) that were collected for crop samples at the end of the study under a Nature Conservancy Council licence. These outcomes are not always independent and it was difficult to decide whether one category could precede another. Thus Coombs et a1. 10 considered that many Collared Dove eggs taken by predators had already been deserted. I defined predation as either 'egg gone', or 'young gone', excluding those chicks known to have fallen accidentally from the nest. However, it is likely that some chicks that fell from the nest were subsequently eaten by a predator that had nothing to do with the fall. Furthermore, although some eggs 'out of nest' were a result of predation, most were probably caused by careless adults, or birds that were surprised and knocked eggs out as they left the nest. Collared Doves usually re-nested in the same nest or very close by. Inter-clutch intervals were calculated for those pairs that I knew had re-nested. The interval was calculated by subtracting the date when a clutch or brood was abandoned, or when the last nestling had fledged, from the date when the first egg was laid in the following clutch. Negative values re-

Breeding of Collared Doves 75 sult from a few pairs beginning one clutch while the previous brood was still in the nest. Annual productivity was calculated by averaging the number of young produced per pair, for pairs with records over the complete season. RESULTS Breeding seasons During the study, the first egg was laid on 14 February and the last on 4 October; the frequency of new clutches was fairly constant from early April until the middle of August, with a slight peak in May (Table 1). The distribution of the egg-laying season was not significantly different between the 3 years of this study. Nor was the average season in Oxfordshire significantly different from that reported for a rural population at Soham, Cambridgeshire (Table 2). Studies of the Collared Dove in its ancestral range and in the rest of Europe show very significant differences in egg-laying seasons between different parts of the species' range (Table 2). The most striking difference is between British and European populations: the egg-laving season of Collared Doves studied in Britain is about one month later than in continental Europe. Clutch size and egg size 89% of completed clutches were of 2 eggs, the rest of only 1, There were no significant differences in the proportion of 1-egg and 2-egg clutches through the year. As others have noted, ""'= there seems to be a slight tendency for more 1-egg clutches to be laid at the ends of the season, particularly at the beginning; however, in all studies to date, few 1-egg clutches have been laid and the differences are not significant. Twice, a clutch of 2 eggs was laid in a nest containing 1 egg left from an earlier clutch, thus giving the appearance of a 3-egg clutch; however, the old egg could easily be identified by its low weight to volume ratio. Table 3 shows the sizes of eggs measured in this study. In 2-egg clutches, the second egg laid was significantly larger than the first. The mean fresh egg weight was 10.0 g (sd = 0.8 g, n = 88). Incubation and nestling period As I visited nests only once a day, the 'incubation time' is defined here as the date on which the eggs were found to have hatched minus the date on which they were found to have been laid. The mean value was 15.0 days (sd = 1A, range 14-19, n = 20) for first eggs and 14.5 days (sd = 0.9, range 14-18, n = 20) for Table 1. Breeding seasons and clutch size of Collared Doves in Oxfordshire, 1981-1983. Only completed clutches are included in this table. Differences between years are not significant (Kruskal Wallis H = 3.90, P > 0.05) Number ofeggs laid Clutch size Month 1981 1982 1983 Total Mean sd February 0 0 1 1 1.00 March 1 8 4 13 1.86 0.38 April 6 9 29 44 1.91 0.29 May 8 13 36 57 1.97 0.19 June 12 16 22 50 1.92 0.27 July 12 9 26 47 1.88 0.33 August 10 4 17 31 1. 82 0.39 September 2 4 17 23 1.77 0.44 October 0 2 ll 2 2.00 Total 51 65 152 268 1,89 (!. 3 1

ó 76 H. A. Robertson d,._ ^ y a. E W ^.i.-. c ^ 5 y 3 ^ y c V V ^ v^ ^: R ^ s~ 77: ^[- m ^o v G 3 C 4) L ó ^ ^ s R H. a. ^ Crj y CD n C > o 2 `) v =>. ^_ v O ^ U L ó3 ^ V^ R 7; 4 ó ^ ^ O R^. -5 - V 9 ^- : ^ m è c 4 II Y ^ C] v > Vo-, ^ 4 ^ C a. ^,, C x ^ v d N R 4.? W ^ cf, N M^i ^ N ON N O^ N.-N^ HI"OC ^ 01 ^ N N ^.0 C' CC ^ N. ^ mc^^^rn m c c c.-0oh0 00 V V O O O 00 O CC =0 O G r 0 I o I H -. ti I V V V 1" ('n i 'cr. csi so ti^ r N^ M ^ ^--i N ^ ^ N,CC n^h -m-^ 00 d' NN. H H LI) ti 0, Ht. LI) ^ N ^ N p1n ^ OC O N OC C, N N ^C N Lr) V"r. N N N N N N--i NH. ^ 0 N,C O IS) N N^ u) Lf> Hi HIH. O C O H N a' N N r+ V O 0 r» H c 0 0 V V V second eggs. The difference (0.55 ± 0.51 days) is highly significant (paired t = 4.82, P < 0.001) and indicates that, on average, the first egg laid hatches half a day ahead of the second egg. The difference is also significantly less than one day (paired t = -3.94, P <0.001) which indicates that although the eggs are laid on successive days and incubation begins when the first egg is laid, the incubation time is shorter for the second egg. The mean nestling period was 18.3 days (sd = 1.9, n = 90). Although they hatched on average half a day later, second chicks tended to leave the nest at the same time as their elder siblings, so they had shorter nestling periods (mean difference 0.73 days, sd = 0.77, n = 22, paired t = 4.45, P < 0.001). There are no significant effects of brood size, years or months on nestling periods, although there is a suggestion that nestlings at the ends of the breeding season had longer nestling periods than those in the middle (Kruskal-Wallis H = 8.3,0.10>P>0.05). Number of clutches, inter-clutch interval, and productivity Collared Doves, like most pigeons, are multibrooded. The number of clutches laid by each pair in a year depended on the number of unsuccessful nesting attempts: if a nest failed, the pair often re-nested quickly (Table 4). The normal minimum time between losing a clutch or a brood and starting a new one was 6 days. This agrees with the observations of Coombs et a1. 10 that behavioural interactions and follicle growth precede egg-laying by 5-7 days in doves. The few examples where birds relaid less than 6 days after losing a brood were at nests where nestlings from the previous brood had died a few days before they were due to fledge, and so a new cycle had already started. Although relaying was usually prompt following a failure, Collared Doves started their next clutch significantly sooner if the preceding clutch was successful (Table 4). Three pairs began to lay while nestlings from a preceding brood were still in the nest, twice in new nests but once in a nest with 2 chicks that were ready to leave. Twenty-eight clutches (65%) were started less than 6 days after the chicks had fledged although Collared Doves continue to feed their fledglings for up to a week. 21 There

Breeding of Collared Doves 77 Table 3. Size (+sd) of Collard Dove eggs in Oxfordshire. The difference in volume between first and second eggs in a clutch is significant (paired t = 11.0, P < 0.001), but between 1-egg and 2-egg clutches is not (t = -0.6, P > 0.05) Length (mm) Breadth (min) Volume index Sample size 1-egg clutches 31.30 24.00 18.07 31 (1.04) (0.84) (1.57) 2-egg clutches 31.28 24.13 18.24 209 (1.32) (0.66) (1.45) first 30.68 24.10 17.84 53 (1.21) (0.63) (1.35) second 32.14 24.28 18.97 53 (1.12) (0.68) (1.33) All eggs 31.28 24.11 18.21 240 (1.29) (0.69) (1.46) minimum 28.1 21.1 13.2 maximum 34.6 25.7 21.5 Table 4. Comparison of intervals between successive Collared Dove nests (in days). Negative values occurred when a clutch was begun before the previous brood had fledged. The difference between the medians for successful and unsuccessful nests is significant (Mann-Whitney U-test:. P < 0.001) Category Median Mean sd Range n Successful 4.4 6.0 6.4-4to28 43 Unsuccessful 7.3 9.7 6.3 1 to28 43 Total 6.1 7.9 6.3-4 to 28 86 Table 5. Number of successful and unsuccessful clutches laid in a season by each pair of Collared Doves Number of successful nests 0 1 2 3 4 5 Total Number of 0 1 1 1 2 5 unsuccessful nests 1 4 3 2 1 10 2 2 5 4 3 I 15 3 2 - I 1 4 4 I I 2 5 - I I 6 1 - I Total 5 11 II) 7 1 2 38

78 H. A. Robertson Table 6. Number of young fledged per pair each season. Differences between years are not significant (Kruskal-Wallis H = 5.1, P > 0.05) No. young fledged 0 1 2 3 4 5 6 7 8 Mean sd 1981 3 2 1 2 1 1 1.90 1.79 1982 1 1 1 2 2 2.43 1.51 1983 1 3 3 3 4 1 2 1 3 3.90 2.49 Total 5 6 5 7 7 2 2 1 3 3.11 2.31 Table 7. The number of Collared Dove eggs laid each month, and their fates, for nests with known histories in the years 1981-83 combined. Hatching success and overall breeding success varied significantly between months (Mann-Whitney U-tests, P < 0.01). Fledging success did not, though it was significantly correlated with hatching success (r = 0.8, P <0.05). Predation ('egg gone' plus 'young gone') was significantly higher before June than from June onwards (x2 = 11.0, P < 0.01) Month Feb/Mar April May June July August Sept/Oct Total Total 16 48 59 52 49 33 25 282 abandoned 3 6 3 3 4 4 3 26 infertile 0 0 0 0 0 3 0 3 addled 3 4 4 1 1 1 0 14 gone 1 9 9 3 7 0 2 31 out of nest 2 7 1 6 2 4 2 24 broken 2 2 6 4 1 1 1 17 Hatched 5 20 36 35 34 20 17 167 Hatching success (%) 31 42 61 67 69 61 68 59 Young gone 0 6 9 7 5 2 5 34 Young died 3 1 4 0 7 3 0 18 Fledging success (%) 40 65 64 80 65 75 71 69 Fledged 2 13 23 28 22 15 12 115 Breeding success (%) 13 27 39 54 45 45 48 41 is no difference in the inter-clutch interval between months (Kruskal Wallis H = 0.9, P > 0.05). The number of successful and unsuccessful clutches laid within a season by each pair, and the total number of young fledged by each pair in a season are given in Table 5. Although there were generally continuous breeding histories for most pairs through a season, it should be stressed that these figures are minima, as some pairs that laid a few clutches in a season may have bred elsewhere, unknown to me. One pair laid 9 clutches (a total of 15 eggs) in one season, but most laid 3 to 5 clutches (mean 3.8, sd = 1.6). Up to 4 different nest-sites were used by each pair. Up to 5 broods were successfully raised in a season by some pairs, and 3 pairs each managed to raise 8 chicks in a season (Table 6). There appears to be considerable variation in average annual productivity between years, but this is not significant. Breeding success Table 7 summarizes the breeding success, and causes of failure, of nests for each month during the study. All eggs laid whose ultimate fate is known are included in this table, including those from incomplete clutches. Breeding success was poor early in the season but was consistent at about 50% from June to October.

Breeding of Collared Doves 79 Predation was most important early in the season, but otherwise there is no obvious systematic seasonal variation in any of the causes of failure. There are significant differences between years in hatching success and overall breeding success, but not in fledging success (Table 8). Farm to farm variation in breeding success was high within years, mainly because of different levels of predation. At Hordley Farm, 4 pairs raised a total of 5 young from 28 eggs in 1981, whereas at Twelve Acre Farm, 6 pairs raised 14 young from 28 eggs in the same year (x2 = 5.1, P < 0.05). At Hordley Farm, where there was a resident flock of Jackdaws, 12 eggs or chicks were preyed upon, whereas at Twelve Acre Farm no eggs and only 2 chicks disappeared (x 2 = 7.7, P < 0.01). Many eggs in this study were apparently destroyed when careless parents either broke them or tipped them from the nest (15% and 20% of all egg losses, respectively). Two such losses occurred when I flushed birds from nests, and others were possibly caused by farm-workers. A few nests were abandoned because of farming activity, particularly during egg-laying or early incubation. Of the 18 nestlings that died in the nest, 7 from 4 nests were abandoned when they became infested with mites (probably Ornithobursa; C. Bowden, pers. comm.). These were the only nests in which I recorded mites on nestlings. Although the chicks were well fed, they were not brooded; they presumably died from hypothermia rather than from any direct effect of the mites. At least one egg hatched in 60% of completed nests, and 49% of all nests successfully produced at least one fledgling (Table 8b). Other studies show considerable variation with no systematic trends, except that breeding success in Europe appears to be higher than in the ancestral range (Table 9). Where less than 40% of eggs or 30% of nests were successful, predators have been blamed for most losses. Rana 12 considered that predatory birds (mainly Indian House Crows Corpus splendens) destroyed about 60% of eggs laid by Collared Table 8. Breeding success of Collared Doves (a) Annual variation in terms of eggs laid Eggs Chicks Breeding Year Eggs laid hatched (%) fledged (%) success (%) 1981 52 56 55 31 1982 69 44 70 30 1983 161 67 72 48 Total 282 59 69 41 Tests of differences between years: hatching success: X 2 = 11.5, P < 0.01 fledging success: X2 = 3.1, P > 0.05 breeding success: X 2 = 9.1, P < 0.05 (b) Annual variation in terms of completed nests Broods Hatching producing Nests at least at least Nesting Year completed one egg (%) one chick (%) success 1981 27 67 61 41 1982 34 53 61 32 1983 81 77 74 57 Total 142 69 69 48 Tests of differences between years hatching success: X2 = 6.3, P < 0.05 fledging success: X2 = 1.8, P > 0.05 breeding success: X 2 = 6.4, P < 0.05

80 H. A. Robertson Table 9. Comparison of Collared Dove breeding success between different studies (a) Expressed in terms of Eggs Site laid bers of eggs Eggs hatched (%) Chicks fledged (%) Breeding success (%) Reference India 109 40 80 32 12 Iraq 77 64 55 35 13 Hungary 32 97 100 97 15 Czechoslovakia 436 86 79 69 19 East Germany 1359 28 21 West Germany 175 --- 54 14 Cambridgeshire 317 42 62 26 10 Cheshire 73 51 86 44 10 Oxfordshire 282 59 69 41 This study (b) Expressed in terms of number of nests Number Eggs Chicks Breeding Site of nests hatched (%) fledged (%) Success Reference Iraq 55 56 48 29 13 Hungary 16 100 100 100 15 Czechoslovakia 229 89 87 77 19 East Germany 588 65 74 49 20 East Germany 715 36 21 West Germany 93 60 14 West Germany 188 73 16 Oxfordshire 142 69 71 49 This study Doves in his Indian study area. Daily losses of eggs to predators appeared to be consistently higher than losses of nestlings in all studies. Comparisons between studies must be cautious, however, because the frequency of visits to nests is important. For example, Coombs et 01. 10 visited their Soham study site only once a fortnight, and so their results are biased towards success because breeding attempts that failed early in the nesting cycle were probably missed; had my visits been fortnightly instead of, on average, every 2 days, I would have missed 11 nesting attempts out of 20 in which eggs disappeared during incubation or 7% of all 156 nests I recorded. DISCUSSION Throughout its range the Collared Dove has a long breeding season (e.g. Ah & Ripley 25 for India, Glutz & Bauer23 for Europe). However, there is a striking difference in breeding season between Britain and the ancestral range, and even other parts of Europe. In the Rajasthan Desert, India, there was a minor peak in spring (April), and a major one during the monsoon period (August September). 12 These peaks corresponded well with marked seasonal fluctuations in testis weight, follicular size and ovary weight (Rana 72 cf, Murton & Westwood, 3 p. 270), and food availability. 12 26 Roonwaa1 27 stated that all Indian pairs have at least 2 broods and most have 4 or 5, but from Rana's 12 data it seems unlikely that many pairs could breed successfully more than 3 times in a year, once in spring, and twice in the monsoon. In Iraq, Marchant 13 found peak egg-laying between 20 April and 20 May (43% of all nests), with doves generally breeding for about 4 months from May to August inclusive; usually only 3 broods were attempted. Collared Doves in continental Europe both start and finish breeding earlier than British ones. The breeding season of Collared Doves in Britain is under photoperiodic control: 25 the gonads are regressed during the short winter days, and most individuals reproduce only between March and September or early October, when the daily photoperiod exceeds about 12

Breeding of Collared Doves 81 hours. 3 The most important ultimate factor affecting the timing of the breeding season of most birds is the food supply, either for the laying female, or for feeding the young. 29 Of factors that are known to influence food availability and hence timing of breeding seasons between populations, the effect of latitude is particularly marked, as spring arrives about 4 or 5 days later per degree northward shift in latitude, and the summer is shorter. This latitudinal effect is usually associated with a contraction of the breeding season. In Britain, the breeding season of Collared Doves is shifted back by one month but not shortened, with a late start to the breeding season resulting in a late finish: Murton 28 found experimentally that Collared Doves under constant conditions have a breeding cycle of 9-10 months before a refractoriness develops, whereas under natural conditions the gonads have regressed before this. Eastern Europe is colder in winter and early spring than Britain and so the breeding season of Collared Doves should have been later in eastern Europe. By analysing data from Saemann, 21 I found that periods of sub-zero temperatures and snow cover appear to delay the onset of breeding of Collared Doves in Karl-Marx City, East Germay, even though food supplies were apparently constant: the proportion of clutches begun before the second week of March was significantly more in 1972 than in 1971, when snow was lying on the ground, and the difference in the proportion of new nests built by the same time was even more dramatic (Kolmogorov Smirnov test D = 10.86, P < 0.05 and D = 20.40, P < 0.001, respectively). Once the snow disappeared there was a sudden burst of breeding activity. A number of studies have shown that if extra food is provided, laying dates of passerines can be advanced. 30-32 Furthermore, the same species breeding in different habitats may breed at slightly different times. 33-35 Of these, Cramp's34 study is particularly pertinent because he found that Woodpigeons breeding in London began 3 months earlier than did those in a rural locality in Cambridgeshire. This was attributed to the greater food supply in urban habitats early in the season and in rural habitats late in the season. Both studies of Collared Doves in Britain have been conducted in rural habitats, whereas the east European studies used in this comparison were in urban sites. However, there is no evidence to suggest that food supplies for Collared Doves are better in urban than rural areas early in the season, nor the reverse late in the season. Indeed, Coombs et a1. 10 found no statistically significant seasonal trends in food availability, mean body weight, or variance of body weight of Collared Doves at an urban site at Ellesmere Port, Cheshire, and food supplies in rural areas in Oxfordshire did not appear to improve between February March and April May; if anything, they got worse as stored grain was used up and spring sowing of cereals was completed. They also found that the mean gonad size of Collared Doves in an urban area closely paralleled the breeding season at a distant rural site, and their observations suggested that the egg-laying seasons at the 2 sites did not differ appreciably. Collared Doves showed a seasonal increase in hatching success between February and May, but thereafter it remained fairly constant. There was little variation in the ability to rear nestlings through the season. Collared Doves, like doves in general, lose most eggs to avian predators, especially crows and birds of prey. 10,13,20,21,26 Known avian predators of Collared Dove eggs or chicks at my study sites included Jackdaws Corvus monedula, Sparrowhawks Accipiter nisus and Little Owls Athene noctua. According to Coombs et a1. 10 predators take eggs of larger species of doves when the parents are forced to leave their nest to feed. Thus predation rates may reflect changes in food supply. In India, Rana 12 found that the incubation period was 13.3 days in the monsoon, when food was abundant, but 18.5 days in spring, when food was limited. Using Rana's 12 data, I calculate that daily predation rates were similar in the spring and in the monsoon (5.8 and 5.5%, respectively) but that the differences in incubation period led to 69% of all eggs laid being preyed upon in the spring but only 55% in the monsoon. The Collared Dove is a commensal of man throughout its range. Its spread into the intensively cultivated areas of Europe in the last 50 years has been very spectacular. In Britain it has apparently filled an 'empty niche', or one which is sufficiently wide that interspecific competition is not an important factor in determining population levels of the species

82 H. A. Robertson involved. By feeding on super-abundant, predictable and persistent supplies of commerical crops, Collared Doves are able to have a long 8. breeding season. They lay a series of small 9. clutches, each of which has a low probability of success, but which combine with high postfledging survival and long life-expectancy to 10. give the high potential productivity reflected in the spectacular expansion of range and logarithmic population growth achieved during the early stages of colonization. Populations of Collared Doves in Britain seem to have 11. peaked. In Britain, there has been more predation by humans in recent years, because Collared Doves are now regarded as an agricultural pest through eating and fouling stored grain products. Legal protection has been partially removed so that Collared Doves are now 12. destroyed where they cause economic damage. Such damage is actually very minor corn- 13. pared with natural wastage but Collared Doves are potential vectors of diseases such as 14. foot-and-mouth virus because of their close association with livestock. 3ó ACKNOWLEDGMENTS I thank Dr C.M. Perrins for supervising this work done on a National Research Advisory Council of New Zealand Post-graduate Fellowship. I am grateful to B.M. Fitzgerald, J.E.C. Flux, J.J.D. Greenwood, and R. Hudson for critical comments on the manuscript. REFERENCES 1. Richardson, R.A., Seago, M.J., & Church, A.C. (1957) Collared Doves in Norfolk: a bird new to the British list. British Birds, 50, 239-246. 2. Murton, R.K. (1966) A statistical evaluation of the effect of Woodpigeon shooting as evidenced by the recoveries of ringed birds. The Statistician, 16, 183-202. 3. Murton, R.K. & Westwood, N.J. (1977) Avian Breeding Cycles. Clarendon Press, Oxford. 4. Marchant, J. & Whittington, P. (1988) 1986-87 CBC index report. BTO News, 157, 7-10. 5. Kalela, O. (1949) Changes in geographic ranges in the avifauna of northern and central Europe in relation to recent changes in climate. Bird Banding, 20,77-103. 6. Mayr, E. (1951) Speciation in birds. In Proceedings of the Xth International Ornithological Congress, Uppsala. pp. 91-131. 7. Dyrcz, A. (1957) On the biology and distribution 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. of the Collared Turtle Dove in Poland. Zool. Poloniae, 7, 433-454. Fisher, J. (1953) The Collared Turtle Dove in Europe. British Birds, 46, 153-181. Hudson, R. (1972) Collared Doves in Britain and Ireland during 1965-70. British Birds, 65, 139-155. Coombs, C.F.B., Isaacson, A.J., Murton, R.K., Thearle, R.J.P., & Westwood, N.J. (1981) Collared Doves (Streptopelia decaocto) in urban habitats. J. Appl. Ecol. 18, 41-62. Murton, R.K. & Westwood, N.J. (1975) Integration of gonadotrophin and steroid secretion, spermatogenesis and behaviour in the reproductive cycle of the male pigeon species. In Neural and Endocrine Aspects of Behaviour of Birds (Eds P. Wright et al.), pp. 51-89. Elsevier, Amsterdam. Rana, B.D. (1975) Breeding biology of the Indian Ring Dove in the Rajasthan Desert. Auk, 92, 322-332. Marchant, S. (1963) The breeding of some Iraqi birds. Ibis, 105, 516-557. Hofstetter, F. B. (1954) Untersuchungen an einer Population der Tiirkentaube. J. Om. 95, 348-410. Tomasz, J. (1955) Contributions to the ecology of the Indian Ring Dove. Aquila, 59-62, 101-143. Lachner, R. (1963) Baitrage zur Biologie and Populationsdynamik der Turkentaube (Streptopelia decaocto decaocto). J. Orn. 104, 305-356. Nowak, E. (1965) Die Tiirkentaube. Die Neue Brehm-Bucherei. A. Ziemsen Verlag, Wittenberg. Gnielka, R. Sr Wolter, W. (1970) Die Besiedlung der Stadt Halle/Saale durch die Tiirkentaube- Streptopelia decaocto (Friv.). Apus, 2, 100-114. Kubik, J. & Balat, F. (1973) Zur Populationsdynamik der Tiirkentaube Streptopelia decaocto (Friv.) in Brno, CSSR. Zool. Listy, 22, 59-72. Gnielka, R. (1975) Zur Brutbiologie der Turkentaube Streptopelia decaocto. Orn. Mitt. 27, 71-83. Saemann, D. (1975) Studien an einer Grossstadtpopulation der Tiirkentaube Streptopelia decaocto im Siiden der DDR. Hercynia, 12, 361-388. Pikula, J. & Kubik, V. (1978) Die Brutikologie der Turkentaube Streptopelia decaocto im milieu der Stadt Brno. Acta Sci. Nat. Brno, 12, (10), 1-40. Glutz, U.N. & Bauer, K.M. (1980) Handbuch der Vogel Mitteleuropas. Vol. 9. Akademische Ver -lagsgesellschaft, Wiesbaden. Furness, R.W. Sr Furness, B.L. (1981) A technique for estimating the hatching dates of eggs of unknown age. Ibis, 123, 98-102. Ali, S. Ripley, S.D. (1981) Handbook of the Birds of India and Pakistan. Vol. 3, (2nded.). Oxford University Press, Delhi.

Breeding of Collared Doves 83 26. Rana, B.D. (1976) Observations on the food of the Indian Ring Dove Streptopelia decaocto and the Little Brown Dove Streptopelia senegalensis. Z. angeus Zool. 63, 25-30. 27. Roonwaal, M.L. (1940) On the subspecies of the Ring-dove, Streptopelia decaocto (Frivaldsky). Rec. Indian Mus. 42, 437-452. 28. Murton, R.K. (1975) Ecological adaptation in avian reproductive physiology. Symp. Zool. Soc. (Lond.), 35, 149-175. 29. Perrins, C.M. & Birkhead, T.R. (1983) Avian Ecology. Blackie, Glasgow. 30. Kallander, H. (1974) Advancement of laying of Great Tits by the provision of food. Ibis, 116, 365-367. 31. Yom-Tov, Y. (1974) The effect of food and predation on breeding density and success, clutch size and laying of the Crow (Corvus corone L.). J. Anim. Ecol. 43, 479-498. 32. Ewald, P.W. & Rohwer, S. (1982) Effects of supplemental feeding on timing of breeding, clutchsize and polygyny in Red-winged Blackbirds Agelaius phoeniceus. J. Anim. Ecol. 51, 429-450. 33. Perrins, C.M. (1965) Population fluctuations and clutch-size in the Great Tit (Parus major). J. Anim. Ecol. 34, 601-647. 34. Cramp, S. (1972) The breeding of urban Woodpigeons. Ibis, 114, 163-177. 35. Newton, I. (1976) Breeding of Sparrowhawks (Accipiter nisus) in different environments. J. Anim. Ecol. 46, 425-441. 36. Robertson, H.A. (1984) Ecology of the Collared Dove Streptopelia decaocto in relation to other British Columbidae. Unpubl. D. Phil. Thesis, University of Oxford, UK. (MS received 27 July 1987; revised MS accepted 7 August 1989)