Abstract
Wild birds differ in size according to their age and sex, adult birds being larger than juveniles. In the galliforms, males are larger than females, in contrast to some groups, such as the raptors, in which the females are larger. Size generally influences the rank hierarchy within a group of birds, although the age, sex, temperament and behaviour of an individual may override its size related rank order. The scaled size of birds according to age and sex affects their physiology and behaviour. Precise details of body-size differences by age and sex are poorly known in most partridge species. We measured 13,814 wild partridges in a homogenous population over 14 years of study to evaluate size differences within a uniform habitat and population management regime. We show that wild Red-legged Partridges have scaled mass, and body- and wing-lengths consistent with age/sex classes. Power functions between mass and body-length (as a proxy for walking efficiency), and between mass and wing-length (for flight efficiency) differ between juvenile females and males, and adult females and males. We discuss these findings and their physiological, behavioural and ecological implications.
Highlights
A whole range of factors act to affect the size of any individual: its age and sex[1,2,3,4], modified by its underlying genetics; its available food resources[5]; and its temperament, which may affect growth through its behavioural characteristics[6,7,8]
Our findings indicate that different age/sex classes of Red-legged Partridges can be distinguished by mass, body-length, and wing-length
Age is frequently associated with reproductive effort, moult, and migration[17,33,34], while sex is associated with dimorphism, sexual selection, and parental division of reproductive effort[16,35,36]
Summary
A whole range of factors act to affect the size of any individual: its age and sex[1,2,3,4], modified by its underlying genetics; its available food resources[5]; and its temperament (i.e., personality), which may affect growth through its behavioural characteristics[6,7,8]. The Red-legged Partridge (Alectoris rufa) (hereafter partridge) is a ground-dwelling species occurring in the wild in Mediterranean habitats and is sexually size-dimorphic (males being larger than females). It is a key prey and a small-game species[23] that employs a social strategy (flocking) to maximise its foraging efficiency and to defend itself against predators.
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