Factors affecting the breeding success of the grey heron ( Ardea cinerea) in northern Poland

The aim of this study was to estimate the importance of factors affecting the breeding success of the grey heron (Ardea cinerea) in three colonies in the northern Poland (Mosty, Kiersity and Kty Rybackie), differentiated in population size, habitat type and feeding areas used by birds. The highest mean number of fledglings (39–51 days old) per nest in 2000–2002 combined was recorded at Kty Rybackie (3.2±0.91 SD). Lower values were recorded at Kiersity (2.5±1.11) and Mosty (2.8±1.20). Breeding success increased with the size of the heronry (r₇=0.77, P<0.05) and was the highest in the biggest, at Kty Rybackie (716–879 nests). In the studied colonies, the number of feeding visits was the most important factor affecting breeding success. At Kty Rybackie and Kiersity, chick mortality significantly decreased with increasing numbers of feeding visits per nest. The highest breeding success was recorded in the colonies with feeding areas situated nearby (Kty Rybackie and Mosty). Siblicide was also a very important reason of chick mortality in those colonies (54% and 32% chicks died in nests where sibling aggression was observed). Other factors, independent of the level of feeding, like predation, human disturbance, weather condition and experience of parent birds, seem to be of negligible importance.Communicated by F. Bairlein