Incidence and Determinants of Double Brooding in Wrentits

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Abstract
The Wrentit (Chamaea fasciata) has been reported to raise only one brood per year. Nesting data on a marked known-age population of Wrentits in central coastal California [USA] revealed that 20% of the breeding pairs during the 4 years 1982-1985 attempted a second brood after successfully fledging young from an earlier brood. The probability of a pair attempting a double brood significantly decreased the later the first clutch was completed. No second broods were attempted if the first clutch was completed later than 5 May (14% of breeding pairs) or if the first attempt failed (41%). The remaining pairs (24%) attempted only one brood although they appeared to have the opportunity to attempt two broods. Adults attempting a second brood reduced the amount of postfledging care given to the young of the previous brood. Double-brooding individuals tended to be older, have better nest survivorship, and fledged significantly more young per season than single-brooding individuals. Survival of the adults to the next breeding season was independent of the number of broods attempted. Young males had significantly lower nest success than older males and young females began nesting significantly later than older females, which may explain the lower incidence of double brooding in young birds.