CAUSES AND CONSEQUENCES OF VARIATION IN PARENTAL DEFENSE BEHAVIOR BY TREE SWALLOWS

In a three-year experimental study of parental defense behavior in Tree Swallows (Tachycineta bicolor), I presented live ferrets and rat snakes to parents in the vicinity of 113 nests on the 13th and 14th day after chick-hatching. Sex of the defending parent and the identity of the predator being defended against were the most significant determinants of variation in the 13 aspects of parental defense behavior measured. Males defended more aggressively than did females. This may be a correlate of stronger territorial behavior in this sex, rather than a strategic response to differing relatedness to the brood. Ferrets were defended against more strongly than were snakes. This may be a response to greater efficacy of defense behavior against ferrets. Attendance measures of the male and female parent at the nest are highly correlated, whereas intensity measures are much less so. Even those intensity measures that are significantly positively correlated have distributions with many pairs in which one parent does considerable defense and the other does none. I suggest that parents are monitoring each other in the presence of the predator and refraining from defense to get their mates to defend actively. On the basis of observations of defense against many species, I suggest that defense has three functions in Tree Swallows: Intimidation of small nest-site competitors, "moving on," and distraction of larger nest predators. There is evidence that variation in both the costs and benefits of defense are important in affecting its intensity. Despite the large number of potential determinants examined, a large proportion of the variance in parental defense behavior remained unexplained. This large residual variation may be either an adaptation to avoid predator localization of the nest or enhance distraction, or a result of relatively low selective pressures or low frequencies of encounter between predators and swallows.