Development and behaviour of Little Tern chicks Stephen Davies In the summers of 1977 and 1978, at Gibraltar Point, Lincolnshire, I made observations on 15 nests of Little Terns Sterna albifrons on a shingle spit. I noted the behaviour described below on numerous occasions, both with the aid of powerful binoculars and from a hide placed within a few metres of nests and young. The biometric data and feeding counts were taken in 1978. Little Terns usually lay a first clutch of three eggs, but subsequent clutches may be of two or a single egg. None of the pairs I observed succeeded in raising three chicks. Development The following information forms a background to the subsequent sections. The growth of Little Tern chicks falls roughly into two stages. In the first stage, often to 14 days, their weight quadruples, much of the body down is replaced by feathers, the beak more than doubles in length and assumes the adult shape, and the body increases considerably in size. The primary pins are first noticeable when the chick is three or four days old, and are fully grown (about 20 mm) at the end of this stage. The egg tooth disappears about the fifth day. The second stage begins when the primaries break pin, and ends at about 28 days when the chick is fully fledged. In this period, the rate of body growth slows markedly, and most of the food taken goes into the growth of the flight feathers. Within four to six days of breaking pin, the primaries grow about 15 mm; five to seven days later, they are about one-third grown and the chick is capable of flying distances of up to 100m, but cannot sustain flight. Within another week, the chick becomes fully fledged and the primaries are fully grown. A newly hatched Little Tern weighs about 7.0g (Marples& Marples 1934). A well-fed chick that I weighed 24 hours after hatching was 9.8g; at four days, another weighed 12.1 g. One of a family of three chicks raised in poor weather starved after five days, when it weighed 10.2g. The smaller of its brood weighed 12.5g the next day, and at 12 days 17.Og, when its sibling (only a few hours older) weighed 34.8g; in its stage of development, the smaller chick was several days behind the larger. Another 12-day-old chick weighed 29.4g, and three days later 37.9g. The primary pins of one six-day-old chick were about 8 mm long. Chicks with pins of 15-20 mm were variously of eight and ten days, but the smaller of the two mentioned above did not reach this stage until 12 days. Chicks with newly emerged primaries of l-5mm were 12 days old, while another of 13 days had primaries about 8mm out of pin. Four chicks with primaries about 15mm out of pin were between 14and 17 days old; four to six days later they were capable of weak flight and their primaries appeared to be about one-third grown (the longest primary of a Little Tern is about 170 mm: Marples & Marples 1934). [Brit. Birds 74: 291-298, July 1981J 291
292 Little Tern chicks Protection and defensive behaviour For the first two or three days, Little Tern chicks are brooded almost continuously. Typically, the female parent 'tents' her wings and the chicks dig with their feet until they can shelter beneath her sides and wings (plate 170). Brooding becomes less frequent as the chicks get older, but, particularly in inclement weather, chicks of more than 20 days are sometimes brooded. When not being brooded, the chicks avoid attracting attention to themselves, except when being fed; usually they are quiet and still, sheltering downwind of vegetation or stones in bad weather. Chicks were often observed to crouch in footprints and other depressions in the ground, and, on at least a dozen occasions, chicks of 12 days or more were seen to dig scrapes in which to crouch. The female remains nearby for most of the day, and it was unusual for the chicks to be unattended for more than half an hour. 170. Female Little Tern Sterna albifrom 'tenting' wings to shelter chicks, Lincolnshire, July 1978 (Stephen Danes) The adult Little Terns warn their chicks of approaching danger by their cries. They attack predators on the ground (e.g. stoats Mustela erminea or grass snakes Natrix natrix), or 'nuisances' (e.g. rabbits Oryctolagus cuniculus or Grey Partridges Perdix perdix), by diving on them repeatedly; flying predators (gulls Lams, raptors and crows Corvus) are fearlessly harried in the air. All Little Terns in the vicinity join in these attacks. Any birds, even roosting or resident waders, which approach the chicks too closely may be attacked by the parent terns. If the chicks from one nest approach too near another nest, they are attacked by the resident adults and defended by their own parents. These attacks appear fierce, but I have not seen a Little Tern chick killed by another Little Tern. If the colony is disturbed by human beings, the adults fly above the intruder(s) calling. If the nest is approached when the chicks are too small to leave, the adult terns may dive upon the intruder, but I have not known them to strike the person. By their second or third day, the chicks scatter from the nest and hide at an approach, and from that time the adults do not usually swoop upon the intruder.
Little Tern chicks 293 171. Seven-day-old Little Tern Sterna albijrons, showing camouflage effect of cryptic plumage against sand and pebbles, Lincolnshire, July 1977 (Stephen Danes) Newly hatched chicks crouch in the nest when they are disturbed. When about three days old, they scatter from the nest when they hear their parents' alarm calls. They make full use of their cryptic plumage (plate 171) by crouching among stones of their own size, in the tidal wrack, under sticks or twigs, or in available vegetation. This pattern of defensive behaviour continues until the chicks fledge, but, as they get older, they scatter farther. At about one week, they may run as far as 20m before crouching, and will readily ford puddles and small creeks when seeking a safe hiding place. When the chicks can fly short distances, they may do so to reach vegetated areas before seeking cover; given sufficient warning, however, they are as likely to run as to fly to shelter. Fully fledged chicks take to the air when danger threatens and fly around with the calling adults. Because Little Terns nest on open shingle, newly hatched chicks are exposed to the weather and predation: at Gibraltar Point, most of the chicks were led by their parents as soon as possible from the nesting area to an adjacent saltmarsh with hummocks of cord-grass Spartina surrounded by stretches of mud. The female abandons the original nest when the chicks are two or three days old. She digs brooding scrapes nearby and calls her chicks to her; often, she makes several such scrapes in quick succession and uses only one. At Gibraltar Point, the chicks moved to the saltmarsh when about five days old; for some families this meant a move of up to 100m. Once the chosen area of the marsh had been reached, the chicks tended to remain there until they fledged, unless disturbed by tides, predators or intruders. At colonies on isolated shingle bars, it is not generally possible for adults to move their chicks to protective vegetation; R. B. Wilkinson, however, has told me (verbally) that, at such a site at Saltfleetby, Lincolnshire, chicks were, in one year, moved across at least 1 km of mud to the nearest saltmarsh.
294 Little Tern chicks 172. Adult Little Tern Sterna albifrons feeding sand eel Ammodytes to chick, Lincolnshire, July 1977 (Stephen Davies) Feeding of chicks Little Terns regularly fish over water only a few centimetres deep: for example, over the advancing tideline. Almost all the food brought to the chicks is caught by the male parent. The female feeds herself during her brief absences from her chicks. Sometimes, when the fishing is good, she joins with the male in bringing food to the chicks. At Gibraltar Point, the adult terns rarely travelled more than 1 km from the nesting colony when seeking food for their young. At low tide, they fished in standing saltwater ponds and in the Wash; at high tide, they fished in the flooded runnels on the beach and in a tidal river and its creeks adjacent to the nesting area. The chick is first fed within a few hours of hatching, by which time it is dry and fluffy. The chicks appear able to recognise their parents' calls from the moment they hatch, perhaps learning them while still in the egg; they ignore the calls of other adult Little Terns. The male calls as he flies in with food. Sometimes, on reaching the chicks, he rises vertically to up to 20m and then dives down to the chicks. This same 'towering' flight can often be observed when the male feeds the incubating female. On hearing the male's call, the chicks run from underneath the female (plate 173) or from where 173. Chicks starting to run from beneath brooding female Little Tern Sterna albifrons to food-bearing male, Lincolnshire, July 1978 (Stephen Dames)
Little Tern chicks 295 they are crouching, calling loudly and flapping their wings; the first to reach the incoming male receives the food. Young chicks appear to peck automatically at the black tip of the adult's bill, which is no doubt an important visual stimulus to the chick, in the way that the red beak spot of the Herring Gull Larus argentatus is to that species' chicks (Tinbergen 1953). On three or four occasions, I noticed that the chicks did not call when food was brought, probably because they were satiated; in these cases, the food was given to the attending female, who held it until it was accepted by a chick. Apart from their louder cries, the behaviour of older chicks being fed is the same as that of the younger ones (plates 174 & 175). Even after they are fully fledged, the chicks depend on their parents for food for several weeks while they master the art of fishing for themselves. Fledglings fly to ground near the area where their parents are fishing for them. Because of the staggered hatching of the eggs (usually over 24 hours), the first chick receives a number of feeds before the younger ones receive their first. The first hatched is, therefore, the largest and most mobile, which 174 & 175. Above, fledgling Little Tern Sterna albijrons crouching in scrape and. below, a few moments later, on hearing calls of its food-bearing parent, Lincolnshire. July 1978 (Stephen Davits)
296 Little Tern chicks gives it an advantage over its siblings in claiming food. It is only when the largest is satiated that the younger chicks are able to reach the food-bearing adult first. At times of scarcity, the oldest chick is fed at the expense of the younger ones, one or both of which may starve. When starvation occurs, it is usually within a week of hatching, most often within the first two days. Older chicks appear to be able to live off their fat during times of shortage, but their growth may then be temporarily retarded (see 'Development'). The adults have difficulty supplying sufficient food to their chicks during periods of prolonged rain or strong winds when the surface of the water is choppy. At Gibraltar Point, most (probably more than 90%) of the food brought to the chicks consisted of crustaceans (Crustacea); most of the items identified were prawns (Natantia). The balance of the diet comprised fish, including sand eels Ammodytes. These observations agree with Collinge's (1926) analysis of Little Tern stomach contents. The largest food items that adult Little Terns were observed to eat were fish about 8cm in length. The food brought to the young is necessarily much smaller. Small items such as prawns were easily swallowed by chicks of all ages. Chicks were observed to take small fish (about 4cm) in their second day; at about two weeks, they could accept fish of about 8cm. When fish were presented to the chicks, they held them by the gills and swallowed them head first; with comparatively large fish, the chick sometimes stood with its neck stretched and the fish's tail protruding from its beak for several minutes before swallowing the fish: this behaviour was noted in chicks as young as three days. If the offered food was dropped, it was retrieved by the adult and re-presented, sometimes to each chick in turn. Often, the adult flew away momentarily before representing. On perhaps four or five occasions when bad weather made it difficult to catch suitable food, the parents brought items which were too large for the chicks to take; these were repeatedly offered, but eventually one of the adults ate them. The rate at which Little Tern chicks are fed appears to depend on two factors. First, as one would expect, there is a diurnal feeding pattern: whatever the height of the tide, a relatively high rate of feeding is sustained early in the morning when the chicks are very hungry and, again, in the evening. Secondly, there is a marked tendency for the feeding rate to increase during the last two hours of the rising tide and, to a lesser extent, of the falling tide. That Little Terns feed most actively in the last few hours of the rising tide is not surprising, because the prawns which comprise such a large part of their diet often bury themselves in the sand and emerge as they are covered by the incoming tide. In an attempt to provide quantitative confirmation of the above observations, the feeds given to a pair of chicks were counted over three 12-hour periods divided into six two-hour intervals (table 1). On the days of these counts the high tides fell at 06.39, 10.39 and 15.14 BST respectively; the weather remained consistently the same throughout each 12-hour period. Within the two-hour intervals when high rates of feeding were recorded, feeds were sometimes brought to the young at the rate of one per minute for periods often to 15 minutes.
Little Tern chicks 297 Table 1. Rate of feeding chicks by adult Little Terns Sterna atbi/rom during three 12-hour periods divided into two-hour intervals, Gibraltar Point, Lincolnshire, 1977 and 1978 High-tide positions are marked by* 1-2 NO. OF FEEDS IN EACH TWO-HOUR INTERVAL OF 12 -HOUR PERIOD 3-4 5-6 7-8 9-10 11-12 Count 1 (06.30-18.30 BST) High tide(06.39) Count 2 (07.00-19.00 BST) High tide (10.39) Count 3 (06.30-18.30 BST) High tide (15.14) 16 * 10 35 5 24 * 11 9 14 12 9 2 8 11 9 22 * 22 * 18 33 In count 1, the coincidence of tidal and diurnal feeding patterns is reflected in the two obvious peaks in the feeding rate in the morning and in the evening. In count 2, the later tide displaces the morning peak rate, as would be expected. In count 3, the pattern is less clear: the feeding rate increases markedly on the last two hours of the rising tide, and continues to increase in the evening after the high tide; but the peak in the first two-hour interval, by which time the early morning 'rush' should have slowed considerably, is unexpectedly high. It would be useful to repeat these counts at a site where the monthly spring tides fall in the middle of the day, rather than, as at Gibraltar Point, in the early morning and late evening: at such a site, the diurnal and tidal patterns of feeding should be more easily distinguished. There is no question of Little Terns feeding at night. I was at the colony 24 hours per day for three months in each year and I was often up all night (watching for foxes Vulpes vulpes). At night, there is complete silence, with none of the calling from chicks which inevitably accompanies feeding. The colony comes to life about dawn and feeding commences soon after. So far as I could tell, on,the few occasions when I went through the colony at night, only one adult was with the chicks, so perhaps the males roost away from the colony. The conclusions drawn from the daily observations and from the counts are as follows. Fishing is easiest for the terns in the last few hours of the rising tide. Under reasonable weather conditions, the adults are capable of feeding their young at a high rate, whatever the height of the tide; but, except for the early morning and the evening when the chicks insistently demand to be fed, they feed the chicks at a high rate only when the fishing is easy, and thus conserve their energy. The male probably feeds himself when the fishing is less easy: that is, at low tide. The female feeds herself mainly when the fishing is good; as a result, her chicks are not left unattended for long periods. At these times, she sometimes helps the male in feeding the chicks. Summary In the summers of 1977 and 1978, 15 nests of Little Terns Sterna albifrons were studied at Gibraltar Point, Lincolnshire. Three aspects of the development of the chicks are discussed:
298 Little Tern chicks the rate of growth, including some information on weights and the growth of primary feathers; the protection of the chicks by the adults, and the behaviour of threatened chicks; the behaviour of adults and young when the latter are fed, the chicks' diet, and the influence of the tides and the time of day on the rate at which chicks are fed. References COIXINGE, YV. E. 1926. An investigation of the food of terns at Blakeney Point, Norfolk. Trans. Norfolk Norwich Nat. Soc. 12: 35-53. MARPLES, G., & MARPI.ES, A. 1934. Sea Terns or Sea Swallows. London. TINBERGEN, N. 1953. The Herring Cull's World. London. Dr Stephen Davies, c/o Gibraltar Point Nature Reserve, Skegness, Lincolnshire