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The influence of climate conditions on breeding phenology of the Grey Heron Ardea cinerea L. in northern Poland

Jakubas D.

Polish Journal of Ecology

2011

Vol. 59, nr 1

179-192

The breeding phenology of the Grey Heron Ardea cinerea L. was investigated in three colonies situated on the Baltic Sea coast in Poland in 1999-2002. The heronries differ in localization (inland vs coastal) and local climate conditions. In inland location, air temperatures in spring were lower by 1.1-1.5[degrees]C and ice cover was present on foraging grounds longer by 19-29 days comparing to coastal ones. Herons occupied all colonies in similar dates (multi-year median date for all colonies: 5 March). However, birds from one coastal heronry tended to start breeding 9-10 days earlier (multi-year median date: 28 February) than birds from other colonies, what might be possible due to early presence of herons in the vicinity. Accessibility of foraging grounds (lack of ice cover) in spring was an important factor affecting the onset of breeding as dates of colony occupation, egg laying and hatching were positively correlated with the dates of last ice cover occurrence on the foraging grounds (r> 0.75, P <0.05). Intercolony differences in hatching dynamics might result from various frequencies of replacement clutches, and nests of late breeding subadults. In one season (2002), the number of medium-aged chicks per nest and fledging success were negatively correlated with the hatching date. Since chick mortality rate during last three weeks before fledging was not correlated with hatching date (suggesting similar food conditions in the course of the season), worse breeding parameters late in the season could be reflected in the lower clutch size or/and hatching success/chick survival rate during the first three weeks in nests of late breeders (i.e. subadults and replacement breeders).