Sex‐ and age‐biased mortality in wildGreyPartridgePerdix perdixpopulations

Abstract

Differing reproductive effort, individual qualities and local environmental conditions can lead to uneven mortality risk among individuals within populations and may result in survival differences according to age and sex. Identification of factors contributing to unequal operational sex ratios has been important for understanding population dynamics and conservation management. In this study, sex‐ and age‐specific mortality was estimated in three wild Grey Partridge populations from analysis of year‐round radiotracking data from 168 individuals. Survival days were counted in three periods defined individually for each bird: the pairing period (covey break‐up to laying of the first egg); the nesting period (between clutch initiation date and failure of the last nesting attempt, or the date when chicks were 14 days old); and the covey period (the end of the nesting period or joining a group until covey break‐up). Predation was the main cause of mortality. A significant effect of age on survival was found during the pairing period, when older individuals paired off faster and survived better. The highest mortality risk overall was found during the nesting period. Furthermore, significantly higher mortality of females was recorded during the nesting period, suggesting that greater investments in reproduction, behaviour at the nest or the quality of nesting habitats can decrease survival of females and cause a male‐skewed sex ratio. No significant effect of age or sex was found during the covey period, or for the year as a whole, but there was a significant difference in annual mortality rates between the three study populations. Our results confirm age‐ and sex‐specific variation of adult mortality in a ground‐nesting bird with biparental care during the annual cycle, documenting differing sensitivities of various population cohorts to predation.

Geographic variation and environmental correlates of apparent survival rates in adult tree swallows Tachycineta bicolor

“…Survival rates of many bird species can vary with sex (Stutchbury et al 2009, Rymešová et al 2012), owing to differences in sex-specific costs of territorial defense, gamete production, parental care, body size and thermoregulation, and migration distances and tempo (Promislow et al 1992, Schaub andvon Hirschheyd 2009). Higher reproductive costs could lower survival rates, a hypothesis that predicts lower survival for the sex that invests most in reproduction, although breeding-season costs for one sex could be offset by higher mortality of the opposite sex at other times of the year.…”

Mate choice for major histocompatibility complex complementarity in a strictly monogamous bird, the grey partridge (Perdix perdix)

“…It has been suggested [ 12 ] that mate choice based on genetic complementarity is more likely to evolve in long-lived species engaged in lifelong monogamy. Although grey partridges are short-lived birds with a usual lifespan of less than 1 year [ 48 , 56 ], mate choice in just one season should nevertheless be of great importance as it affects lifetime reproductive success of most individuals in the population. Strong selection pressure should also act on social mate choice optimisation in this short-lived species, therefore, and especially in a pairing system with no extra-pair young [ 43 ].…”

“…The grey partridge is a suitable model species for studying MHC-dependent mate choice because its mating system is non-resource based [ 44 ] and a surplus of males enables females to choose a partner from multiple candidates [ 45 , 46 ]. Ideally, partners form lifelong pairs, though high predation rates often result in repeat mate choices [ 47 , 48 ]. Despite a relatively low level of sexual dimorphism in this species, the melanin-based plumage ornament on the breast (‘horseshoe’) is usually absent or smaller in females [ 49 ].…”

Differential mortality of birds killed at wind farms in Northern Portugal

“…The determination of the incidence of differential mortality by sex and/or age induced by wind turbines may provide additional important data for conservation and management purposes (Ewen et al 2001, Buenestado et al 2009 by improving our understanding of the potential consequences for the fitness of populations affected by wind turbine collisions (Schmickl & Karsai 2010, Rymešová et al 2012. Nevertheless, sex and age analysis of birds killed at wind farms has only been performed in a few studies and the carcasses have only been occasionally submitted to necropsy examinations to determine/ confirm the cause of death (Stienen et al 2008, Bevanger et al 2010.…”

“…This sex-biased mortality may have important consequences to population dynamics if it reduces local reproductive success. The occurrence of non-random sex mortality has been reported for several bird species (Martín et al 2007, Rymešová et al 2012, and may be a critical factor in small isolated populations (Stifetten & Dale 2006). For Skylark populations, the high mortality of adult males at the beginning of the breeding season could decrease the number of established breeding pairs, which could affect the reproductive success of the local populations.…”