Adoption of Chicks among Thick-Billed Murres

Abstract

Thick-billed Murres (Uria lomvia) breed in dense aggregations on cliff 'edges. Like Common Murres (U. aalge), they build no nests, the single egg being laid on bare rock. Breeding murres of both species rarely leave their egg or chick unattended, but they may do so in the event of attacks by predators, in response to disturbances (e.g. nearby rockfalls; Gaston and Nettleship 1981), under conditions of extreme food stress (Birkhead and Nettleship 1984), or occasionally in response to severe attacks by mosquitos, or heat stress (A.J.G. pers. obs.). During periods of desertion, eggs may roll away from the site and chicks may run up to several meters from the site in order to seek shelter in crevices, or to avoid predators. However, parents recognize their own egg by its color and markings (Tschanz 1959, Gaston et al. 1993), and their chick by its call (Tschanz 1959). The chicks also are capable of recognizing the calls of their parents and actively seek to return to them once they return to the site. When their own egg is lost, murres sometimes incubate the egg of another bird, either appropriating one left unattended, or even stealing one from under an incubating neighbor. Chicks hatched from such appropriated eggs are reared in normal fashion. When foreign eggs have rolled from adjacent sites, birds may attempt to incubate more than one egg, but in such cases no more than one hatches (Gaston et al. 1993). Tschanz (1959, 1979) and Birkhead and Nettleship (1984) observed Common Murres protecting foreign chicks that were temporarily deserted by their parents during a period of food shortage. Wanless and Harris (1985) reported two cases where neighbors helped to rear chicks that also were fed by their own parents. None of these observations involved complete fostering. However, Perry (1940) described an apparent adoption of a neighbor's chick by a Common Murre pair. In this note, we describe four cases in which Thick-billed Murre chicks were adopted and reared by foster parents. Three cases involved natural events, while the fourth involved a chick displaced during banding activities. All observations were made at Coats Island, Northwest Territories, Canada in 1993 and 1994. Observations were made daily in both years throughout the chick-rearing period. Methods used were the same as those described by Gaston et al. (1993). Our observations caused no disturbance to breeding birds, so they cannot account for the three apparently natural adoptions. Case 1.-A five-day-old chick was present at site D154 on 25 July 1993, but was missing on 26 July, when a chick of similar age was being brooded by a bird at site D54, about 1.5 m directly below D154. The pair at D54 already had a chick that was one to four days old at that time. The ledge that they occupied supported two other pairs, one of which was incubating an egg that was eventually deserted after it failed to hatch within the normal incubation period. The other was occupied by a pair that was never known to have laid an egg. Both members of the pair at D54 were banded as breeders at the same site in 1986 and had definitely bred at the site annually since 1988. They brooded both chicks continuously throughout the rearing period, one chick usually being under a wing and the other under the breast. Fighting between the chicks was not observed. The pair at D154, also both uniquely banded, remained at their site and made no further breeding efforts. They were never seen to land anywhere except at their own site. Feeding was observed at D54 twice, but we did not see which chick was fed. The plumage development of both chicks was normal. The larger chick (presumably the older, adopted chick) departed from the site at 2023 EST on 14 August, making a successful glide to the sea. Both foster parents were present when it left, but neither interacted with it as it prepared to depart, or accompanied it (for descriptions of adult and chick behavior at departure, see Tschanz 1959, Gaston and Nettleship 1981). Unless it was adopted at sea (some observations suggest that this may occur; G. Gilchrist unpubl. data), the chick would have perished because chicks are fed by the male parent for at least a month after leaving the colony (Scott 1990). The smaller chick left at 2153 the next night, accompanied by both parents, although, as is normal in Thick-billed Murres (Gaston and Nettleship 1981), the female returned to the breeding site within 10 min and continued to occupy it for several days thereafter. Case 2.-At site P23, the egg disappeared on 25 July