The Evolution of Female Body Size in Red‐winged Blackbirds: The Effects of Timing of Breeding, Social Competition, and Reproductive Energetics

Abstract

We examined opposing selective forces on female body size in the sexually dimorphic red-winged blackbird: social competition favoring larger females, and energetic advantages favoring smaller females. Downhower proposed that selection might drive female birds to be smaller than the optimum for survival, if smaller females were able to exceed their energetic requirements for self-maintenance earlier in the season and therefore breed earlier. Since in most birds the earliest breeders fledge the most young, this could favor the evolution of smaller female size, and therefore contribute to the magnitude of sexual size dimorphism in these birds. We tested this hypothesis in 1987 and 1988 by comparing the size and breeding date of female red-winged blackbirds. Consistent with our preditions, early-nesting females had much higher nesting success, but contrary to prediction, larger females bred earlier. We then examined the effects of female size on competition. If large females have an advantage in social competition, and if competition influences breeding date and reproductive success, then larger females might breed earlier. Primary females, the first females to arrive and nest on a territory, were more aggressive than lower ranked females; more aggressive females settled on better territories and laid earlier than less aggressive females; and larger females were more aggressive. Social competition between females may therefore favor large females. Finally, we tested the prediction that selection favoring large females might be limited by energetic constraints on large females. We found that large females had less fat than small females during breeding, and that the levels of fat that females of a given size carried affected breeding date and egg size. Therefore, social competition may favor large females, but reproductive energetics favoring smaller females may constrain selection for large female body size.

Natural Selection and Sexual Size Dimorphism in Red-Winged Blackbirds

“…This hypothesis predicted directional selection favoring smaller females. We did not find this pattern, nor did we find that larger females survived better than smaller ones (Langston et al 1990). …”

“…Both dominance and bioenergetic constraints may favor large body size during winter and early spring. For instance, larger males are (1) dominant over smaller males (and likely females) and will therefore win most disputes for food (Searcy 1979c;Eckert and Weatherhead 1987a; see also Langston et al 1990 for similar results with female red-winged blackbirds); and (2) capable of enduring unfavorable winter weather conditions better than smaller birds, owing to the reduced surface-area-to-volume ratio oflarger birds (e.g., James 1970). Early arrival may be particularly advantageous ifthese males have a better chance of acquiring territories simply by being first on the territory rather than by direct contests (Myers 1981;Shutler and Weatherhead 1992).…”

Individual quality, survival variation and patterns of phenotypic selection on body condition and timing of nesting in birds

“…1a-c). Evidence of stabilizing selection on body mass was not found in female lesser scaup (Aythya affinis; Rotella et al 2003), but was reported in herring gulls (Larus argentatus; Monaghan and Metcalfe 1986) and red-winged blackbirds (Agelaius phoeniceus; Langston et al 1990;Weatherhead and Clark 1994). However none of these studies used com- Table 3 bined capture-recapture-band-recovery models in their analyses.…”

Breeding time, health and immune response in the chinstrap penguin Pygoscelis antarctica

“…Body size (Murphy 1986;Langston et al 1990), age (Hochachka 1990;Burger et al 1996), experience (Nager and van Noordwijk 1995) and foraging skills (Winkler and Allen 1996) may aect condition indices. However, these eects may be mediated through more proximate factors like health status and immune function.…”