Sexual Dimorphism and Breeding Success in Tree Swallows and Collared Flycatchers

Abstract

The common pattern of small differences in body size between the sexes of many bird species has been related to a number of ecological factors, including functional differences in niche utilization (Selander 1966, Wallace 1974, Hogstad 1978), sexual differences in nest defense requirements (e.g., Wiklund and Stigh 1983, Safina 1984), and to differential migration strategies (Ketterson and Nolan 1983). While a number of these studies assume that a small degree of sexual dimorphism in body size leads to differential flight performance (see Andersson and Norberg 1981), the possible consequences of such differences have rarely been investigated empirically. Gosler (1987a, 1987b) suggested that sexual differences in the bill length of Great Tits (Parus major) may enhance the ability of pair members to aquire food, through decreased intrapair competition. This same argument has been made for the evolution of reversed sexual size dimorphism in raptors (Newton 1979), typically in reference to differences in wing size/shape and flight performance. This argument rests on the premise that pair members that differ in body size/shape are able to exploit a wider range of prey items. Gosler's (1987a, 1987b) studies of Great Tits provided such evidence and also drew a direct link between the degree of within-pair sexual dimorphism and breeding success. However, more recent work by Matthysen et al. (1991) found no such link among Blue (P. caeruleus) and Great Tits in Belgium. Previous studies have demonstrated significant effects of small differences in wing length and shape on the foraging behavior of several species of birds (e.g., Andersson and Norberg 1981, Lifjeld and Slagsvold 1988). In this paper, we assess the relationship between reproductive success and intra-pair differences in wing length. This study differs from previous efforts by focusing on an aerial insectivore, the Tree Swallow (Tachycineta bicolor), and on a species that feeds by gleaning and flycatching, the Collared Flycatcher (Ficedula albicollis).