Influence of hatching order on growth rate and resting metabolism of kestrel nestlings

Hatching asynchrony in altricial birds may result in a competitive disadvantage for the youngest nestlings compared to older siblings. We studied the effects of a size hierarchy on the growth rate of Eurasian kestrels Falco tinnunculus chicks in nests with and without access to supplemented food in western Finland. Body mass stopped increasing on the 19th day after hatching while body size, estimated by a combination of bone and feather lengths continued to increase at least until fledging at 26 days. Body condition, reflecting muscle and fat, did not change markedly during the growth period from the 12th day to fledging. Body temperature and resting metabolism were usually lower in nestlings 12 days old than in nestlings at fledging. Growth of body mass, size and condition, and resting metabolism were delayed in last-hatched nestlings aged 19 days. Just before fledging, last-hatched nestlings attained a similar body mass and size, and had a similar resting metabolism to those of older siblings. At fledging, only in nests without access to supplemented food was the body condition of last-hatched chicks lower than that of its siblings, but in nests with access to supplemented food no such difference was detected. Our results highlight that the level of lipids in the last-hatched nestling can be affected by the food restriction imposed by hatching order.