Individuals should fight hardest when they stand to lose the most. Whereas males frequently compete for fertile females, females more often compete for high quality males, male care, or resources required to breed. We asked whether established, territorial female house wrens (Troglodytes aedon) challenged by simulated female intruders fight as if they place more value on retaining (1) their nesting cavity or (2) exclusive access to other benefits offered by males. We randomly assigned house wren pairs to receive one or three nest boxes and then assayed female aggression. The relative costs to losing differed between box treatments. For one-box females, the risk of losing the cavity and territory was higher. For three-box females, the risk of losing the cavity may be lower because intruders may be able to settle as secondary females in the supplemental boxes. In this situation, females would lose exclusive access to males and their territories but would still retain the male’s assistance rearing offspring since male house wrens favour their oldest brood. We found that one-box females were significantly more aggressive. This response may be adaptive, as females that switched territories between broods were significantly more likely to lose their entire nest prior to hatching than females that retained the same territory. We interpret our results to mean that female house wrens value the nest cavity more than other benefits from exclusive access to males and their territories. This work contributes to a body of evidence that females often compete for resources required to breed.
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