Sexual size dimorphism and positive assortative mating in red-backed shrike Lanius collurio: an adaptive value?

Abstract

Piotr Tryjanowski • Jaroslav imeǩ SBrani ovská 31, eské Bud jovice, 370 05 Czech Republič sČě Abstract During field studies in 1997-1999 in South Bohemia (Czech Republic), we found significant differences in size between the sexes in a local breeding population of red-backed shrike Lanius collurio. Males were significantly larger than females for wing length and tarsus length, but had smaller body mass than females. However, there was considerable overlap in the ranges of these parameters between the sexes. Interestingly, pairs were formed at random with respect to wing length and tarsus length, but assortative mating was significant for body mass/body condition. Among tested variables, only male wing length correlated significantly with nestling body mass at day 7. However, clutch size and the number of fledglings strongly depended on differences in tarsus length between mates, but not on body size of mates. Individual improvements in foraging skills and/or courtship feeding rates are proposed as possible explanations for these findings.

Sexual size dimorphism and assortative mating for morphological traits in Passer domesticus

“…In contrast, Tryjanowski and imek (2005) did not find an assortative mating for body size in the redbacked shrike (Lanius collurio). In this species, the greater the difference in tarsus length between mates was, the greater the breeding success was (Tryjanowski and imek 2005), and this could explain the absence of assortative mating for tarsus length in this species. Male and female house sparrows forage in flocks (Summers-Smith 1988), making a diversification of niches unlikely.…”

“…Assortative mating for body size is common in different bird species (e.g., Olsen et al 1998), and can be predicted as a result of mate choice, because body size is frequently correlated with fitness (Widemo and Saether 1999). In contrast, Tryjanowski and imek (2005) did not find an assortative mating for body size in the redbacked shrike (Lanius collurio). In this species, the greater the difference in tarsus length between mates was, the greater the breeding success was (Tryjanowski and imek 2005), and this could explain the absence of assortative mating for tarsus length in this species.…”

“…On the other hand, pairs with a greater number of different phenotypes may use a wider niche and achieve a higher breeding success (e.g., Tryjanowski and imek 2005), which would favour an absence of assortative mating. Such niche use by pairs could favour sexual size dimorphism (Mueller 1990).…”

“…Therefore, high-quality males would mate with high-quality females, leading to assortative mating for the trait used by both sexes to choose mates. Indeed, many bird species show a positive assortative mating for different morphological traits (Olsen et al 1998;Jawor et al 2003;Tryjanowski and imek 2005).…”

Reproductive ecology of two sympatric Tibetan snowfinch species at the edge of their altitudinal range: Response to more stressful environments

“…Like their lowland conspecifics (Zeng, 2008), there were no correlations between the paired males and females in each snowfinch species for any phenotypic characters, suggesting pairs to form randomly with respect to body size. In fact, the most reports for size-assortative mating came from bird species with sexual size dimorphism (e.g., Olsen et al, 1998;Tryjanowski and Simek, 2005).…”