Forest fragmentation is associated with primary brood sex ratio in the treecreeper (Certhia familiaris).

We studied the primary brood sex ratio of an old-growth forest passerine, the Eurasian treecreeper (Certhia familiaris), along a gradient of forest fragmentation. We found evidence that male nestlings were more costly to produce, since they suffered twofold higher nestling mortality and were larger in body size than females. Furthermore, the proportion of males in the brood was positively associated with the provisioning rate and the amount of food delivered to the nestlings. During the first broods, a high edge density and a high proportion of pine forests around the nests were related to a decreased production of males. The densities of spiders, the main food of the treecreeper, were 38% higher on spruce trunks than on pine trunks. This suggests that pine-dominated territories with female-biased broods may have contained less food during the first broods. The observation was further supported by the fact that the feeding frequencies were lower in territories with high proportions of pines. In the second broods, territories with a high forest patch density produced female-biased broods, whereas high-quality territories with a large amount of deciduous trees and mixed forests produced male-biased broods. Our results suggest that habitat quality as measured by habitat characteristics is associated with sex allocation in free-living birds.

Forest management is associated with physiological stress in an old-growth forest passerine.

We investigated how physiological stress in an area-sensitive old-growth forest passerine, the Eurasian treecreeper (Certhia familiaris), is associated with forest fragmentation and forest structure. We found evidence that the concentrations of plasma corticosterone in chicks were higher under poor food supply in dense, young forests than in sparse, old forests. In addition, nestlings in large forest patches had lower corticosterone levels and a better body condition than in small forest patches. In general, corticosterone levels were negatively related to body condition and survival. We also found a decrease in corticosterone levels within the breeding season, which may have been a result of an increase in food supply from the first to the second broods. Our results suggest that forest fragmentation may decrease the fitness of free-living individual treecreepers.

Prey depletion by the foraging of the Eurasian treecreeper, Certhia familiaris, on tree-trunk arthropods.

We examined to what extent breeding insectivorous treecreepers, Certhia familiaris, affect the abundance and the mean size of their prey population on the surface of tree trunks. In order to determine foraging pressure on tree trunks, we observed the parent birds' foraging behaviour in marked squares (25×25 m) at a short (10 m) and long distance (90 m) from the nest, when the nestlings were near fledging. Immediately after fledging, we measured the remaining food abundance on a sample of tree trunks near to and far from the nest. All arthropods longer than 1 mm were collected and identified to family level and their length was measured. Treecreepers foraged for significantly less time on trunks far from the nest than on trunks close to the nest (5 s vs 186 s). Consequently, treecreepers were found to deplete food abundance on trunks close to the nest. During the nesting period, parent birds removed almost twice as many spiders and other arthropods (excluding Formica ants) from the tree trunks close to the nest than from those further away. The size distribution of arthropods was the same between frequently and seldom-used tree trunks, suggesting that arthropod consumption by treecreepers was not size selective. Our direct measurements on the abundance of arthropods provide rare evidence for the ability of predators to deplete their food resources to a large extent.