CERTAINTY OF PATERNITY AND PATERNAL INVESTMENT IN EASTERN BLUEBIRDS AND TREE SWALLOWS

Extra-pair paternity is common in many socially monogamous passerine b irds with biparental care. Thus, males often invest in offspring to wh ich they are not related. Models of optimal parental investment predic t that, under certain assumptions, males should lower their investment in response to reduced certainty of paternity. We attempted to reduce certainty of paternity experimentally in two species, the eastern blu ebird, Sialia sialis, and the tree swallow, Tachycineta bicolor, by te mporarily removing fertile females on two mornings during egg laying. In both species, experimental males usually attempted to copulate with the female immediately after her reappearance, suggesting that they e xperienced the absence of their mate as a threat to their paternity. E xperimental males copulated at a significantly higher rate than contro l males. However, contrary to the prediction of the model, experimenta l males did not invest less than control males in their offspring. The re was no difference between experimental and control nests in the pro portion of male feeds, male and female feeding rates, nestling growth and nestling condition and size at age 14 days. We argue that females might have restored the males' confidence in paternity after the exper iment by soliciting or accepting copulations. Alternatively, males may not reduce their effort, because the fitness costs to their own offsp ring may outweigh the benefits for the males, at least in populations where females cannot fully compensate for reduced male investment. (C) 1998 The Association fbr the Study of Animal Behaviour.