Nesting success in Darwin's small tree finch, Camarhynchus parvulus: evidence of female preference for older males and more concealed nests

Abstract

Age-assortative mating has been documented among birds, but there is little evidence of direct benefits to females that pair with older males. In this study, I examined nest concealment in Darwin's small tree finch and tested the following predictions: (1) nest concealment increases with male age, (2) females prefer males that build well-concealed nests, and (3) females receive direct benefits through reduced predation at concealed nests. Darwin's small tree finch has a pattern of annual moult that makes it easy to observe age-related variation in reproductive success. In this species, males build a display nest that is sometimes used by the female for nesting. Controlling for male age, females preferred display nests for nesting that were well concealed by surrounding vegetation, whereas exposed display nests were rarely used for nesting (although occasionally a female paired with the male that built the unused display nest). Using cross-sectional and longitudinal data, this study found evidence of a female pairing preference for older males, as well as increased nest concealment among older males. Females that paired with older males had higher fledging success (using the longitudinal data), mostly due to reduced predation.