Females may choose a mate on his own quality or on the quality of his resources, i.e., his territory. We removed willow warbler males and allowed new males to settle, before the arrival of females, in order to test whether the proximate cue for female choice was any male trait or territory characteristics. The experiment indicates that females base their choice on some male trait. No correlation was found between male settlement order and size (tarsus length, wing length), but males arriving early were in better body condition than males arriving late. The most likely male trait for female choice was singing rate, which was a good indicator of male settlement date. The duration of time between mating and egg laying was shorter in early territories, both in the observational and the experimental data. Since male song rate was correlated with territory quality (e.g., food abundance) the ultimate benefit to females choosing males with high song rates could be a high quality territory.

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The breeding sex ratio of Willow Warblers Phylloscopus trochilus in a subalpine birch forest was strongly female biased. Very few territorial males remained unmated and most became polygynous. Bigyny was the rule, but some males probably had three females. Male assistance at secondary nests varied and was probably dependent on temporal distance between the females' nests. There was no significant reduction in reproductive output for these females. Variation in the sex ratio of breeding birds can be explained, at least partly, by variation in the timing of male settlement.