Kestrels - A Hovering Hunter

The kestrel's old folk name—the windhover—catches the essence of this bird whose most characteristic posture is its skilful, seemingly effortless hovering. The kestrel's Latin name is Falco tinnunculus. 

If you see a bird with wings winnowing, tail fanned out and face looking down; it's probably a kestrel after prey. The kestrel is our most widespread bird of prey; it is also unique in coming to terms with man and becoming an independent and resourceful city dweller. The kestrel has also learned to exploit the thousands of miles of infrequently mown grassy motorway verges which harbour the small rodents that make up its principal food. Thus, a stretch of country motorway is one of the best places to see these slim, long-tailed members of the falcon family. 

Lanner Falcon Watercolour Artwork

Male and females do not look alike. The striking blue-grey head and tail and light chestnut back spotted with black identify the male kestrel, while the female's plumage is barred and streaked in brown and reddish-brown colours. Both sexes have a conspicuous black band on the end of the tail, which is visible when the birds are hovering, and both have creamy underparts streaked with black or dark brown.

Towns and motorways are not, of course, the only haunts of the kestrel. It frequents downland, heaths, moors, mountains, parkland, farmland and cliffs-almost every land habitat barring the middle of thick woods where there is not enough room for its particular hunting style. (In these woods, the sparrowhawk rules).

One secret of the kestrel's success is its ability to adapt to a very varied diet. In-country areas, small rodents such as mice and voles form nearly two-thirds of the kestrel's food, making the bird a distinct benefit to the farmer. However, insects, particularly grasshoppers and beetles and small birds such as finches, are essential diet items.

In towns where small outdoor mammals are in short supply, the kestrel preys principally on small birds such as house sparrows and starlings. Distance is no obstacle if the kestrel knows that a good supply of food awaits it at the end of a journey; for instance, one pair of outer London kestrels flew five miles regularly to hunt at sewage works frequented by starlings.

Skilled hunter

The kestrel is an exceptionally keen-sighted bird—as it has to be since it often hunts from heights of up to 60m (200ft). As it hovers, the kestrel keeps its head still—eyes fixed on the prey far below-while maintaining its position in the air with constant minor adjustments of wings and tail. Thus, even when facing into a high wind, the head remains relatively steady through the body may be wobbling frantically from side to side and the wings and tail lashing furiously.

Once the victim is spotted, the kestrel swoops towards it at great speed. Sometimes catching up short halfway to adjust position, seizing the unfortunate animal with its powerful outstretched talons. Prey dies instantly from the impact or when a talon pierces a vital organ during a struggle. If the kill occurs on a roadside verge or other exposed place, the kestrel flies off at once to a private porch, clutching its prey firmly in its talons. Alternatively, the kestrel will crouch over its kill on the ground, 'mantling' it with outspread wings and tail like an open umbrella. While the prey is being screened this way from other birds, the kestrel can regain its breath after the exertion of the hunt; it is an aspect of behaviour typical of all falcons.

Small birds are sometimes caught on the wing in a swift 'swoop, clutch and away' movement. Carrying the prey off to a suitable perch or post, the kestrel can eat it in peace. Fence posts, telephone wires and tall trees are convenient perches from which the kestrel can watch for prey; its alert, straight-backed posture on these perches are almost as characteristic a pose as the hovering. They swooping down on prey is very effective from this position.

Courtship display 

Kestrels are rather solitary birds, so it is only during courtship that you are likely to see males and females together.

The pair soar up into the sky with extended wings, the male usually higher than his mate. Much wilder manoeuvres follow soaring- the male repeatedly 'stoops' upon the female like a fighter plane in aerial combat, sometimes even brushing her with his wings. Loud, rather shrill 'kee-kee-kee' calls accompany the dramatic displays. 

The soaring display flight is perhaps the only time when you might confuse the kestrel with our other small bird of prey, the sparrowhawk. Both birds are similar enough in size to confuse; both have long tails, and both soar in the breeding season. 

However, one difference between the kestrel and the sparrowhawk as hunters is that the sparrowhawk has a distinctive low-level interception method of killing (as opposed to the kestrel's characteristic hover) and has broad, blunt wings (unlike the kestrel's long pointed ones). Sometimes, however, the kestrel's extended wings can look broad and blunt-ended during soaring.

Choosing a nesting site 

Kestrels, like other falcons, do not build a nest. Instead, the female kestrel will lay eggs on cliff ledges, in hollow trees or, in wooded areas, in the old nest of another bird—particularly that of the carrion crow. And where there are no apparent sites, the adaptable kestrel nests in long heather on the ground on the Orkney Islands, where there are no cliffs or trees.

However, the kestrel can nest in churches in towns, ruined and deserted buildings as substitutes for cliffs. Kestrels have even been discovered breeding in window boxes on high-rise buildings

Raising young 

The female kestrel lays her eggs between mid-April and mid-May at intervals of two to four days. The eggs, from one to five in number, are a rich, mottled red-brown colour. The female incubates the eggs, sometimes assisted by the male for a month before the chicks hatch, to stay a further five weeks in the nest before fledging. Feathers start to appear when the chick is between 12 and 20 days and the chick is able to at anything from 27 to days.

Mortality among the young birds is high - only around 20 per cent survive two years to breeding age. On average, those who do will live for a further two and a half years, while very few can persist well into their teens. 

Starvation is the most significant cause of death, especially of juveniles during their first autumn and winter. Collisions and accidents, shooting, poisoning and disease are other important causes of mortality.

Population Size

According to the British Trust for Ornithology, the kestrel Falco tinnunculus is on the amber list of endangered species. There are thought to be around 31 thousand pairs during the summer. 

Kestrel numbers and vole numbers are closely linked. For example, in 1988-91, the UK population of kestrels was approximately 52,000 breeding pairs, according to the Breeding Bird Atlas. However, there may be around 32,000 breeding pairs of kestrels in 2021, according to some experts. 

Links

https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/animals/birds/kestrel/

https://app.bto.org/birdfacts/results/bob3040.htm

https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z/kestrel/population-trends/