Egg-Color Dimorphism and Breeding Success in the Crow Tit (Paradoxornis webbiana)

Abstract

This study was conducted during the 1988-1989 breeding seasons in Dongguk University Forests (area A) at Namyangju-gun, and during the 1993-1994 breeding seasons in Puyong-ri (area B) at Yangpyong-gun, in Kyonggi-do, South Korea. The eggs of the Crow Tit (Paradoxornis webbiana) had two color types (blue and white) in both areas, but no nests had mixed blue and white eggs. In area A (n = 53), blue clutches (75.5%) were more frequent than white ones (24.5%). In area B (n = 62), the ratio of blue clutches (59.7%) to white ones (40.3%) was not skewed significantly from 1:1. In each area, neither the egg-laying period nor clutch size varied with egg color. Twenty females having two or more successive nests had the same egg color, irrespective of their male mates. However, of seven males that mated with different females, eggs in the nests of two males changed from white to blue. In each area, blue and white clutches tended to be widely distributed throughout the study area. Breeding success was related to nest height above ground, but not to egg color or species of nest tree. Nest predation seems to have been caused mostly by Eurasian Jays (Garrulus glandarius) in high nests, by Common Cuckoos (Cuculus canorus) in midlevel nests, and by snakes (Natrix) in low nests.