Climate variation and bird breeding seasons in a primeval temperate forest

Abstract

We investigated evidence for climate warming in the primeval forest of Bia8owieza National Park (E Poland) over the last 33 yr (1975-2007). We examined whether local populations of 4 sedentary birds (Sitta europaea, Poecile palustris, Cyanistes caeruleus, Parus major) advanced their breeding phenology during that time, and how breeding phenology was affected by temperature variation during the period preceding egg laying. Mean yearly temperatures varied strongly across years, with a significant warming (~1°C) trend. Spring temperatures increased significantly solely in the second half of April. Only P. major, which breeds later than the other species, showed a signifi- cant advance (~9 d) in breeding. Breeding dates of S. europaea and P. palustris, which usually lay eggs before mid-April, did not significantly change over time. In all species, the onset of breeding was very variable across years (up to 30 d), as birds started breeding earlier in springs with higher temperatures in the pre-laying period. Such a wide range of phenological plasticity indicates that the birds already possess mechanisms enabling them to accelerate breeding in response to climate warming, provided that warming occurs in the pre-laying period. All species shifted their laying dates concurrently; thus, the order of egg laying (S. europaea, P. palustris, C. caeruleus, P. major) was retained irrespective of the spring earliness. This also indicates that mechanisms allowing birds to respond to their changing environments exist at the community level.