Mate choice in house wrens: nest cavities trump male characteristics

When male birds defend all-purpose breeding territories, females may select mates based on indicators of male or territory quality, or both. However, in non-experimental studies, it can be difficult to determine which traits females prefer because measures of male and territory quality frequently covary. We conducted a series of studies to investigate the traits female house wrens (Troglodytes aedon) use to select a social mate when each male is provided with nest sites of equal quality (i.e., nestboxes). We first compared the phenotypic and territory vegetation traits of males with the time it took them to secure a social mate after returning from the wintering grounds. Male arrival and pairing date were positively correlated and early-arriving males were in better condition and occupied territories in more preferred vegetation than late-arriving males. To test the hypothesis that early-arriving males possessed phenotypic or territory traits that led to their pairing more quickly than late-arriving males, we removed all females after pairs had been established and determined male success in attracting new mates. Male settlement and pairing date with replacement females were not correlated as had been male settlement and pairing date with original females, and male time-to-pairing was not correlated with male song rate, condition index, or territory vegetation. When we manipulated the abundance of nest sites on territories, after males settled but before females arrived, male time-to-pairing, independent of male condition index or territory vegetation, was negatively related to the number of nest sites on a male's territory. These results suggest that under natural conditions females rely on nest-site quantity and, likely, quality when selecting a social mate.