The breeding ecology and behaviour of Wrens Troglodytes troglodytes under primaeval and secondary conditions

The breeding habits of Wrens inhabiting the primaeval forest of Biatowieza National Park, Poland, are described and compared with those of Wrens living in man‐transformed habitats of Western Europe. Significant inter‐habitat differences in numbers exist in Biatowieza. On average there are 4‐3 territories per 10 ha in ash‐alder, 1 ‐7 territories per 10 ha in oak–hornbeam and 0–4 territories per 10 ha in coniferous habitats. In ash–alder stands the males have smaller territories, build fewer nests and have better pairing success than in oak–hornbeam stands. However, these two habitats do not differ significantly in average clutch size, timing of breeding or nesting success. Over 80% of nests used for breeding in Biatowieza are situated in uprooted trees. Inter‐habitat differences in bird numbers reflect differences in distribution and density of such trees. Low numbers of Biatowieza Wrens cannot be accounted for solely by high mortality away from the breeding grounds. Available data strongly suggest that numbers of both sexes are proximately limited by behavioural interactions on the breeding ground. In west European Wrens, densities are higher, territories smaller, polygyny more frequent, clutches smaller and production of young higher than in Biatowieza. Differences in production of young result from different rates of nest predation. Indirect evidence suggests that the breeding habits of Wrens once living in the primaeval forests of western Europe were similar to those of Biatowieza birds, and that the differences observed nowadays result to a large extent from man‐made transformations of habitat. It is suggested that man's lowering of the predation pressure acting both on proximate (enabling birds to produce more young) and ultimate levels (decreasing relative advantages of dispersed breeding) was the decisive factor allowing an increase in numbers of Wrens in secondary habitats.