Dynamics of the Onset of White-Tailed Eagle (Haliaeetus albicilla) Breeding in the Central Part of the Volga–Kama Region and Possible Explanations

Abstract

This work is aimed at determining the onset of breeding of the White-tailed Eagle and the reasons that affect its breeding phenology in the central part of the Volga–Kama region (Republic of Tatarstan, together with the adjacent areas of the republics of Chuvashia and Udmurtia, as well as Samara and Ulyanovsk oblasts). In the Republic of Tatarstan, the bulk of data were obtained in 2011–2016; in Ulyanovsk and Samara oblasts, in 2012; in the Republic of Chuvashia, in 2013, 2015, 2016; and in Udmurtia, in 2016. An analysis of the onset of breeding was carried out on the basis of photo records of the White-tailed Eagle nests with nestlings primarily at ages of 30–60 days and some younger than 30 days, in 2011–2016. The age of nestlings was determined by the photo tables comparing the degree of development of their plumage. When establishing the date of the first egg laying, the age of the elder (or the only) nestling and the incubation time, which amounts to 37 days, were taken into account. In total, 119 cases of breeding were analyzed. For additional control of calculations concerning the earliest periods of breeding, at the beginning of the breeding season, on February 23, 2015, distant observations of a group of 16 nests were carried out using optical devices along the upper reaches of the Kuibyshev reservoir. More precise data on the dates of egg laying were also obtained using a surveillance camera at an eagle nest in the Volga–Kama Biosphere Nature Reserve. According to the results of calculations, the period of the first egg laying extends from February 22 to April 11 (n = 119). Most of the clutches (70.6%, n = 84) are laid in the period from February 22 to March 15, and the maximum occurs in the first five days of March (26.05%, n = 31). A comparison with previously published data shows that, on average, the eagles start breeding half a month earlier than they did in the 20th century. In our opinion, the shift towards earlier breeding dates is primarily caused by the presence of artificial food supply at local wintering sites and increased species numbers in the Volga–Kama region. The White-tailed Eagle’s status in the region partially changed from the migrating breeder to resident breeder. The distances from the breeding territories to the wintering grounds with abundant of forage shortened significantly. The influence of climate warming on the time of egg-laying cannot be excluded either.