Growth peculiarities of beak length as a morphometric parameter were studied in nestlings of the red-footed falcon (Falco vespertinus Linnaeus, 1766) during the nesting period, before leaving the nest, in an anthropogenic habitat (within disappearing forest belts) in the Manych River valley, Rostov-on-Don Region, Russia. Nestlings of the red-footed falcon were revealed to emerge asynchronously, the order of their hatching generally corresponding to the egg-laying sequence and this process taking place in a short time, within two–three days. Based on median values, depending on year, the parents of the study species nurture 2–3 nestlings. The postembryonic beak growth rate of “younger” and “older” nestlings was found to be uneven, because at the time of hatching the degrees of their maturity differed. The beak of “younger” nestlings grew 1.1 times more intensely compared to the beak of “older” nestlings. Beak length increased 2.0 times during their stay in the nest. During the first week of nesting life, nestlings showed the greatest absolute increase in beak length, but in “younger” nestlings, this occurred on the 4th day, versus the 2nd day in “older” nestlings. Beak length did not differ significantly among nestlings of different ages by the end of the nesting period. During the time spent by nestlings in the nest, the beaks of both “younger” and “older” nestlings generally attained the length characteristic of adult beaks.
Ermolaev et al. (Mon,) studied this question.