Offspring sex ratio skew in the sexually monomorphic house martin Delichon urbicum

Abstract

Sex ratio at conception may be under selection pressure, given that male and female offspring differ in the cost of production or generate different fitness returns under specific conditions. We studied adjustments in the primary, secondary and tertiary sex ratio in house martin Delichon urbicum, which is a sexually monomorphic, socially monogamous, colonial bird. Males of this species engage in extra‐pair copulations with heavy males acquiring the highest fertilization success. We analyzed variation in the sex ratio in relation to clutch size and parental characteristics including body condition, wing length, as well as length and pigmentation of the white rump patch during three breeding seasons. The only variable which significantly explained the variation in the sex ratio was wing length of the social father and mother. The proportion of sons among offspring was positively correlated to wing length of the social father and negatively correlated to mother wing length. Social father wing length positively correlated with mean brood body mass at fledging, which may suggest that females that mated with long‐winged males produced sons, which acquired the highest total fertilization success. Consequently, our results indicate that house martin females may adaptively adjust offspring sex composition at egg laying in relation to the characteristics of their social mate.

Nestling sex ratio is unaffected by individual and population traits in the griffon vulture

“…Our results show that, as expected for a monomorphic species ( Fisher 1930 ), secondary sex ratio in nestling Griffon Vultures did not differ significantly from parity. Similar results have been recorded in some monomorphic or slightly sexually dimorphic non-raptor species ( Albrecht 2000 ; Magrath et al 2002 ; South and Wright 2002 ; Benito et al 2013 ; Albores-Barajas et al 2015 ), but not in others ( Whittingham and Dunn 2000 ; Zielińska et al 2010 ). Previous studies also show no significant biases in nestling sex ratio in other populations of Griffon Vulture ( Bosé et al 2007 ) or in other species such as the cinereous vulture Aegypius monachus ( Villegas et al 2004 ), oriental white-backed vulture Gyps bengalensis , long-billed vulture Gyps indicus ( Arshad et al 2009 ), bearded vulture Gypaetus barbatus ( López-López et al 2011 ), or Egyptian vulture Neophron percnopterus ( Sanz-Aguilar et al 2017 ).…”

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

“…On the other hand, parental investment into the sex with highest variance in reproductive success could be theor etically expected under situations such as good maternal condition (Bradbury and Blakey 1998, Whittingham et al 2002), high experience or high maternal quality (Dowling and Mulder 2006). Similarly, if the fathers’ high quality and/or sexually attractive attributes are inherited and have a greater influence on the fitness of sons than on that of daughters, mothers would be induced to skew their reproductive efforts towards male offspring when mated to such males (Charnov 1982, Ellegren et al 1996, Zielinska et al 2010).…”

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

“…Although numerous studies have quantified sex ratios at laying or hatching in order to test for biased sex ratios, conflicting results demonstrate a continuing gap in our knowledge. Many studies of birds failed to detect biased sex ratios (Heinsohn et al , Nager et al , Genovart et al , Husby et al , Zielińska et al ) while other studies found a female bias (Szczys et al , Neto et al ) and yet others reported a male bias (Bednarz and Hayden ). Studies have even recorded variation in offspring sex ratios within the same species sampled at different locations (Rosivall et al ) or at the same location in different years (Rubolini et al ).…”