Ardea
Official journal of the Netherlands Ornithologists' Union

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Bijlsma R.G. (1982) Breeding season, clutch size and breeding success in the Bullfinch Pyrrhula pyrrhula. ARDEA 70 (1): 25-30
From 1970 to 1979 several aspects of the breeding biology of the Bullfinch were studied on the Southwest Veluwe in the central part of The Netherlands. Most information was gathered in the Sysselt, a forest of 350 ha near Ede (52.03 N, 5.40 E). Nests were searched and checked systematically at least once every five days. Egg laying occurred from April through August, two peaks reflecting first and second layings (Fig. 1). Start of egg laying was correlated with temperature. Two layings per pair per year (plus one or more repeats) was usual Clutch size varied from 3-7 eggs (n = 421, = 4.67,. S.D. = 0.69) and showed a seasonal variation (Fig. 2). First layings were largest, followed by second and repeat layings successively (Table 1). Small yearly variations in clutch size were found (Table 2). Of 1321 eggs laid, 825 hatched and 661 young fledged, overall success being 50,0%. Breeding success in clutches of 4, 5 and 6 eggs hardly differed, but the absolute number of fledglings per laying increased with increasing clutch size (Table 3). Breeding success increased from April onwards and was highest in clutches started in August (Table 4). From 1974 to 1979 279 clutches were found in the Sysselt: 114 layings failed completely and 86 nests suffered partial losses. Most of the total losses occurred during the egg stage and were primarily due to predation and desertion.


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