Eurasian Sparrowhawk - Description

Description

The Eurasian Sparrowhawk is a small bird of prey with short, broad wings and a long tail, both adaptations to manoeuvring through trees. Females can be up to 25% larger than males and weigh up to twice as much. When females are larger than males, it is known as reverse sexual dimorphism; this is unusual in higher vertebrates but typical in birds of prey, and most marked in birds of prey which hunt birds.

The adult male is 29–34 cm (11–13 in) long, with a wingspan of 59–64 cm (23–25 in) and a mass of 110–196 g (3.9–6.9 oz). He has slate-grey upperparts (sometimes tending to bluish), with finely red-barred underparts, which can look plain orange from a distance; his irides are orange-yellow or orange-red. The female is much larger at 35–41 cm (14–16 in) long, with a wingspan of 67–80 cm (26–31 in), and a mass of 185–342 g (6.5–12.1 oz). She has dark brown or greyish-brown upperparts, and brown-barred underparts, and bright yellow to orange irides. The juvenile is warm brown above, with rusty fringes to the upperparts; and coarsely barred or spotted brown below, with pale yellow eyes; its throat has dark streaks and lacks a mesial (midline) stripe.

The Eurasian Sparrowhawk's pale underparts and darker upperparts are an example of countershading, which helps to break up the bird's outline. Countershading is exhibited by birds of prey which hunt birds and other fast-moving animals. The horizontal barring seen on adult Eurasian Sparrowhawks is typical of woodland-dwelling predatory birds, while the adult male's bluish colour is also seen in other bird-eating raptors, including the Peregrine Falcon, the Merlin and other Accipiters.

A study, using stuffed bird models, found that small birds are less likely to approach Common Cuckoos (a brood parasite) which have barred underparts like the Eurasian Sparrowhawk. Eurasian Reed Warblers were found to be more aggressive to cuckoos which looked less hawk-like, meaning that the resemblance to the hawk helps the cuckoo to access the nests of potential hosts.

The Eurasian Sparrowhawk's small bill is used for plucking feathers and pulling prey apart, rather than killing or cutting. Its long legs and toes are an adaptation for catching and eating birds. The outer toe is "fairly long and slender"; the inner toe and back toe are relatively short and thick. The middle toe is very long and can be used to grasp objects, while a protuberance on the underside of the toe means that the digit can be closed without leaving a gap, which helps with gripping.

The flight is a characteristic "flap-flap-glide", with the glide creating an undulating pattern. This species is similar in size to the Levant Sparrowhawk, but larger than the Shikra (the calls are however different); the male is only slightly larger than the Merlin. Because of the overlap in sizes, the female can be confused with the similarly-sized male Northern Goshawk, but lacks the bulk of that species. Eurasian Sparrowhawks are smaller, more slender and have shorter wings, a square-ended tail and fly with faster wingbeats. A confusion species in China is the Besra, although A. n. melaschistos is considerably larger.

In Great Britain, Eurasian Sparrowhawks living further north are bigger than their more southerly counterparts, with wing length (the most reliable indicator of body size) increasing by an average of 0.86 mm (0.034 in) in males, and 0.75 mm (0.030 in) in females, for each degree further north.