Sex ratio adjustments in common terns: influence of mate condition and maternal experience

Abstract

Adaptive sex allocation has frequently been studied in sexually size dimorphic species, but far less is known about patterns of sex allocation in species without pronounced sexual size dimorphism. Parental optimal investment can be predicted under circumstances in which sons and daughters differ in costs and/or fitness returns. In common terns Sterna hirundo, previous studies suggest that sons are the more costly sex to produce and rear. We investigated whether hatching and fledging sex ratio and sex‐specific chick mortality correlated with the ecological environment (laying date, clutch size, hatching order and year quality) and parental traits (condition, arrival date, experience and breeding success), over seven consecutive years. Population‐wide sex ratios and sex‐specific mortality did not differ from parity, but clutch size, mass of the father, maternal breeding experience and to some extent year quality correlated with hatching sex ratio. The proportion of sons tended to increase in productive years and when the father was heavier, suggesting the possibility that females invest more in sons when the environmental and the partner conditions are good. The proportion of daughters increased with clutch size and maternal breeding experience, suggesting a decline in breeding performance or a resources balance solved by producing more of the cheaper sex. No clear patterns of sex‐specific mortality were found, neither global nor related to parental traits. Our results suggest lines for future studies on adaptive sex allocation in sexually nearly monomorphic species, where adjustment of sex ratio related to parental factors and differential allocation between the offspring may also occur.

Sons do not take advantage of a head start: parity in herring gull offspring sex ratios despite greater initial investment in males

“…Differential mortality among sexes between hatching and fledging appears to increase as food resources decline, and theory predicts that the more expensive sex (requiring more resources) will suffer disproportionate mortality. We failed to find sex biases in mortality during the chick stage, a result similar to studies of common terns (Benito et al ) and roseate terns (Nisbet and Szczys ). Evidence of greater rates of male pre‐fledging mortality, however, was found in western gulls Larus occidentalis (Sayce and Hunt Jr ), black brant Branta bernicla nigricans (Lemons et al ), rooks Corvus frugilegus (Roskaft and Slagsvold ), lesser black‐backed gulls (Griffiths ), and in all‐male clutches of black‐headed gulls (Müller et al ).…”

“…These advantages, however, do not lead to greater fledging rates for sonsor perhaps are necessary for males to fledge in equivalent numbers as females. (Benito et al 2013) and roseate terns . Evidence of greater rates of male pre-fledging mortality, however, was found in western gulls Larus occidentalis (Sayce and Hunt Jr 1987), black brant Branta bernicla nigricans (Lemons et al 2012), rooks Corvus frugilegus (Roskaft and Slagsvold 1985), lesser black-backed gulls (Griffiths 1992), and in all-male clutches of black-headed gulls (Müller et al 2005b).…”

“…The heterogametic sex (females, in birds) could manipulate the primary sex ratio of her eggs in response to environmental and social cues (Clutton‐Brock et al , Alonso‐Alvarez ). In species with asynchronous hatching, altering the order in which eggs of different sexes are laid would effectively lead to sex biases at fledging because the early‐to‐hatch chicks are at a considerable advantage over their younger siblings (Bollinger , Nisbet and Szczys , Savoca et al , Benito et al ). Sex ratios at fledging could also be biased by females depositing more resources in the eggs of a particular sex so that the embryo hatches with a size advantage and a head start in sibling–sibling competition for resources.…”

Facultative adjustment of the offspring sex ratio and male attractiveness: a systematic review and meta-analysis

“…Berkeley & Linklater (2010) Laying/hatch order Cichon, Dubiec & Stoczko (2003), Lezalova et al (2005) and Lislevand et al (2005) Clutch size Benito et al (2013) and Bowers et al (2014) Social dominance rank (maternal) Nevison (1997) and Maestripieri (2002); meta-analyses: Brown & Silk (2002), Schino (2004) and Silk et al (2005) Parental relatedness/compatibility Pryke & Griffith (2009) and Sardell & DuVal (2014) Parental age/breeding experience/pair bond duration Green (2002) and Benito et al (2013) Female mating status/extent of polygyny (sole/primary/secondary female, harem size) Weatherhead (1983), Nishiumi (1998) and Trnka et al (2012) Examples are given of empirical studies that report a significant effect of the factor; note that for all factors we also located studies that find no relationship. References to meta-analyses are provided where possible.…”

Early mortality saves energy: estimating the energetic cost of excess offspring in a seabird

“…Eggs typically hatch with 1-2 day intervals, in the order of laying [27]. Males reach about 5% larger fledging mass than females, but this disparity only develops shortly before fledging [28] and offspring sex does not vary systematically with HO [29]. Recruited offspring on average have a lifespan of about 10 years, but so far the oldest bird in the colony reached an age of 26 years.…”