CHARACTERISTICS OF THE BODY MASS GROWTH IN SEMIALTRICIAL AND ALTRICIAL BIRD SPECIES DURING NESTLING PERIOD
The dynamics of body mass growth were studied in nestlings of 22 semialtricial and altricial bird species based on materials collected in 7 regions of Russia in 1976–2013. The species of the birds belonged to 4 orders and 13 families. The results of the study indicate the nonuniform growth of nestlings in different bird species. Only birds of seven species among the investigated ones were found to reach or exceed the mass of adult birds. For nestling period, nestlings of open-nesting species, such as hooded crow (Corvus cornix), rook (Corvus frugilegus), magpie (Pica pica), fieldfare (Turdus pilaris), song thrushes (Turdus philomelos), and goldfinch (Carduelis carduelis) do not reach the weight of adult birds, and their growth continues after they leave the nest. For the nestling period, only nestlings of swallow (Hirundo rustica) among closed-nesting species reach the mass or exceed the definitive one, whereas nestlings of jackdaw (Corvus monedula), starling (Sturnus vulgaris), wryneck (Jynx torquilla), tree sparrow (Passer montanus), and great tit (Parus major) continue to grow after leaving the nest. The body mass of birds on the day of their hatching before their departure from the nests and the mass of adult birds depends on the species type, the duration of the nestling period, the size of bird species, and on their definitive size. The average specific growth rate of body mass and its maximum values for the nestling period are also related to these factors. The maximum specific growth rate in small-sized and medium-sized bird species was observed on the 0–1st day of life, in large bird species – on the 2nd–4th day. The specific growth rate did not depend on the type of nesting, but it was inversely related to the duration of the nestling period and definitive sizes of birds.  Keywords: postembryogenesis, semialtricial and altricial bird species, open-nesting species, closed-nesting species, specific growth rate, nestling period