The presence of non‐territorial males in Willow Warbler Phylloscopus trochilus populations–a removal study

A total of six (unmarked) and 24 (individually colour‐ringed) male Willow Warblers Phylloscopus trochilus were removed from their territories in two study areas: subalpine birch and lowland mixed forest, respectively. The removals were made during three separate periods throughout the breeding season: (1) pair‐formation and egg‐laying; (2) early incubation; and (3) middle and second half of the incubation period. During Period 1 all of the males removed (n= 10) were quickly replaced by new males, while during Period 2 eight of the 11 males removed were replaced. The new males defended basically the same territorial borders as the males they replaced. During Period 3 only one of the nine males removed was replaced. Since the replacements occurred within a few hours of the removals, it is suggested that the new settlers were non‐territorial males that were already present in the respective areas before the experimental removals were made. The mean wing‐lengths of the two groups of males indicated that the initial occupants of the territories were 2 years old or older in both study areas, whereas the new settlers in the subalpine birch forest (mean Willow Warbler density: 55 territories/km2) were 1‐year‐old birds, and two years old, or older, in the mixed forest area (mean Willow Warbler density: 133 territories/km2).