Intensity of nest defence is not related to degree of paternity in the willow tit Parus montanus

Abstract

We asked whether willow tit Parus montanus males adjust their parental care according to their paternity in current brood. The origin of the nestlings was determined by using molecular technique, and the studied broods were assigned into extra‐pair paternity (EPP) broods, if at least one nestling was fathered by another male, and truly monogamous broods. Over 3 years, 14 of 40 broods (35%) included EP‐offspring, and 29 of 273 nestlings (11%) were EP‐young. Intensity of parental care was measured with risk‐taking against a potential predator, mounted stoat Mustela erminea. The results showed that risk‐taking by EPP males did not differ from that by monogamous males. Neither was the sexual difference in risk‐taking different at EPP and monogamous broods. Our results are consistent with the hypothesis that males do not adjust their level of care to paternity, perhaps because they have no reliable cues for assessing their paternity. This may be related to the success of mate‐guarding in their breeding environment, closed forests. Guarding is seemingly successful as the EPP levels are rather low, but it is not totally sure making the potential costs, rejection of own young, too high. We also discuss other population characteristics which may further prevent the evolution of paternity assessment in northern willow tits.