Comparison of nestling nutrition and growth of Black-crowned Night Heron, Nycticorax nycticorax, and Little Egret, Egretta garzetta

Abstract

Comparison of nestling nutrition and growth of Black-crowned Night Heron, Nycticorax nycticorax, and Little Egret, Egretta garzetta by Ali Uzun and Belgin Uzun Black-crowned Night Herons (Nycticorax nycticorax Linnaeus, 1758) and Little Egrets (Egretta garzetta Linnaeus, 1766) are both tree-nesting species which breed in a mixed colony at Poyrazlar Lake (40°50’N, 30°28’E) in north-western Turkey. In order to determine the ecological separation of the two species and to assess their different nutrition strategies, we examined and compared the diet and growth of nestlings of both species. The study was conducted between April and June of 2005. Observations were carried out on a weekly basis throughout the breeding season, but were also carried out on a daily basis over 30 days between early May and early June, when the young hatched. Night observations were carried out with the help of two bulbs of 1500 Watt for illumination and a power generator. The type and amount of food of the young was assessed through direct observations and through analysis of vomits in the laboratory. Vomiting is a typical behaviour of herons and egrets when disturbed. Direct observations extended throughout the day, while the vomit samples were collected twice a day, once in the morning and once in the evening. Each day we selected different fledglings for vomit sampling, to prevent any negative influence on reproduction. Altogether 800 samples of vomit (400 from each species) were collected in 20 days. The fledglings and nests were marked to avoid confusion. The amount (weight) of vomit collected per day was considered as the relative daily amount of food taken by the young. Approximately 250-300 pairs of Night Heron and 400-450 pairs of Little Egret breed in the mixed colony at Poyrazlar Lake. Both species arrive in early April and leave in late November, when their young are fully grown. Breeding starts in the third week after their arrival. The average clutch size was 3.34 in Night Herons (21 x 2, 37 x 3, 29 x 4 and 13 x 5 eggs) and 6.76 in Little Egrets (17 x 2, 41 x 3, 25 x 4 and 17 x 5 eggs). The incubation period of Night Herons was found to be 21-22 days, and 23-24 days in Little Egrets. The fledglings grow rapidly within the first ten days after hatching. Their weight can increase up to 2-3-fold within the first 5 days (Table 1). At the time of hatching, young Little