Certainty of paternity and paternal effort in the collared flycatcher

Models of optimal parental investment predict that variation in certainty of paternity can affect the optimal level of paternal investment when a male's expected paternity in different nesting attempts is not fixed throughout his lifetime. Several attempts to test this prediction experimentally in monogamous birds have failed to induce a reduction in care by males. This may be because the method used, detaining males, is a poor model for what happens when a male's certainty of paternity is naturally reduced. We caught and detained female collared flycatchers Ficedula albicollis for 1 h immediately after laying on one or two occasions in an attempt to induce variation in certainty of paternity for the males they were mated to. By capturing females immediately after laying we hoped to exploit the existence of an 'insemination window' since males should be very sensitive to female absence during this period. The general effect of the experimental manipulation was consistent with reduced certainty of paternity: males responded by reducing their level of paternal care to nestlings, and males mated to females that had been caught on one morning fed nestlings significantly less often and made a smaller share of feeding visits than males mated to control females. The effects of the experiment were generally weak, however, and we argue that certainty of paternity may be fixed well before egg laying, in which case experimental manipulations are unlikely to have large effects. It is difficult to predict the effects of natural variation in certainty of paternity on levels of male paternal care because differential allocation by females mated to attractive males may act in the opposite direction.