EXPERIMENTAL DEMONSTRATION OF A TRADE-OFF BETWEEN MATE ATTRACTION AND PATERNAL CARE

Males should invest in mate attraction, mate guarding and paternal care in relation to the marginal fitness value of each of those behaviours. Since time and energy are limited, trade-offs between these activities are expected. This study demonstrates that monogamous male European starlings (Sturnus vulgaris) decrease their paternal effort in response to increased opportunities to attract additional mates and instead invest in mate attraction. Monogamous males' probabilities of attracting additional females were increased by providing them with additional nestboxes. This resulted in both the rate at which males were visited by prospecting females and the probability that they would obtain secondary mates increasing. Males with an additional nestbox sang more than males with only one nestbox, both before laying and during incubation. Males with two nestboxes spent more time at their nest sites when their fertile females were away before, but not during, egg laying. The experiment affected how much males incubated during the early, but not during the late, part of the incubation period. This makes sense, because males can attract additional females mainly during the early part of the incubation period. Male feeding of nestlings was unaffected by the experiment. The fact that the potential to attract mates affects males' investment in parental care suggests that variation in this potential may contribute to the variation in paternal care between bird species.