What Determines the Rates of Double Brooding in the Western Bluebird?

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Abstract. In many bird species, double brooding (producing a second brood after successfully rearing a first) is a common reproductive strategy that can greatly increase reproductive output. However, the proportion of breeding pairs that produce a second brood varies considerably from year to year. Here, we examine how environmental factors such as weather patterns and individual factors such as clutch size, the female's body mass, and date of breeding affect the rate of double brooding in a population of the Western Bluebird (Sialia mexicana) in New Mexico. More pairs tended to produce second broods in years in which freezing ended earlier and there was warm, dry weather during the breeding season, but this pattern was apparent only when we excluded years with wildfires from the analysis. In addition, pairs that bred earlier were more likely to produce a second brood, and in these pairs females tended to be heavier. Thus both the length of the breeding season and the weather during that time influence ra...