ANIMAL ESTATE client 8.10: KESTREL
FOR ANIMAL ESTATE 8.0: LONDON
SCIENTIFIC NAME: Falco tinnunculus
PROFILE: Although one of the most common European birds of prey, the Common Kestrel population (also known as the Eurasian Kestrel, European Kestrel, Old World Kestrel, Hover Hawk, Windsucker, Windcuffer, Stannel Hawk, Moosie Hawk, Mouse Falcon) has been declining in the last 20-30 years in Britain. According to the Raptor Foundation, between 1995 and 2000 the population was reduced by about 30%. The RSPB currently estimates the number of wild breeding pairs in the UK at 36,800. The Common Kestrel is larger than its American counterpart and The Lesser Kestrel, neither of which are found in the UK, with a length of 32- 50cm, a wingspan of 60-80cm and weighing 140-310g.
RANGE: The Common Kestrel is found throughout Europe and significant parts of Asia. The extent of the population range includes China, parts of the Indian subcontinent, the Middle East, North Africa and vast areas of Southern Africa. They are only partly migratory birds, with some movement taking place from cooler, upland breeding grounds to lowland areas for the winter. The kestrel’s home range does not extend a long distance from the nest – between 1 and 10 km. Territories are often adjusted through the year to take advantage of seasonal variations in the availability of prey and often overlap with other individuals.
HABITAT: Kestrels are adaptable to various habitats but prefer open areas such as fields, heath, shrubland, marshland and rocky areas, notably on the coast (their fondness for rocks or stony areas gives rise its alternative name of ‘stannel’). They are particularly attracted to the side of roads and motorways due to the small mammals which dwell there. Kestrels are also found in cities and urban areas but are again more likely to reside in areas with access to open areas such as heaths, parks, farms and riverbanks. Kestrels do not build their own nests but rather occupy existing structures such as snags, cavities in trees, cliffs, dirt banks, eaves of buildings, former corvid nests or nest boxes.
BREEDING AND REPRODUCTION: Common Kestrels are fairly monogamous, sometimes mating with the same partner for life. The breeding season starts in April or May. Each clutch may be up to 7 eggs and incubation lasts approximately four weeks, with only the female undertaking this task whilst the male goes to hunt. Later on, the sexes share brooding and hunting between them. Kestrel families (often 2-3 young) stay together a month and the young become sexually mature the next breeding season.
PREDATION AND HUNTING: Kestrels have unique hunting techniques and are the only bird of prey thought to ‘hover’ for longer than a few seconds at a time. They hover into the wind (windhovering), which provides enough lift to remain in flight, and remain at a constant level (10- 20m up) in relation to the ground. They are therefore able to remain very still whilst on the look out for prey. Furthermore, they are able to dip their heads further than other falcons, therefore remaining upright in the hunt for food. Whilst flapping their wings very quickly they can keep their heads extremely still. This demands a lot of energy but is compensated by increased rewards from hunting. Kestrels can see ultraviolet light and follow rodent urine trails, which reflect UV, to the animal or the burrow. Whilst their diet is based around rodents in the countryside, they are willing to consume insects and lizards to supplement this. This is particularly important in the city, where open space and rodents on the surface are at a premium. In the city, smaller birds, such as pigeons, occupy a significant part of their diet (Yalden, 1980).
CONSTRUCTING A KESTREL NEST BOX: (From the Owl and Hawk Trust) Nestboxes for kestrels only need to be sited between 3 to 5m (10–16 feet) from the ground if on private land, higher if vulnerable to public access. Nestboxes should not be close to major roads, as emerging young would soon become traffic casualties. Boxes should face onto open land and not be hidden by foliage or thick ivy; the more obvious they are the better. If sites are likely to become obscured by the growth of ivy or tall scrub, this should be removed when the box is installed. When installing boxes during the winter it is important to remember that they may be less obvious to the birds when the tree is fully in leaf. The box entrance should face away from prevailing winds and direct rain. It should be tilted so that rain runs off the front. The box is designed to include a perch but should be placed where there is plenty of perching in front of the box to enable the young kestrels to branch properly. Artificial nest sites should be in place before the end of November to give the best chance of success the following year. It can take three years before a site is used for nesting, although there are often encouraging signs of roosting, such as a build-up of pellets or feathers, before then.
Use non-metallic fixings such as plastic cable ties and nylon bolts when installing boxes in living trees. These materials minimise damage to timber and, if felling takes place, they present no hazard to chainsaw operators and those carrying out mechanised harvesting. For this reason, the wooden mounting boards are primarily designed for these specialised forms of attachment but also allow for more traditional methods, using nails or corrosion-resistant Timberlok screws with washers that can be adjusted or removed if required, if these are the only option. The mounting board is screwed or bolted to the box at installation. To install nestboxes with nylon bolts it is best to select an upright tree trunk with a flat face, free of knobbles and projections. The tree is drilled with a 14mm wood-boring auger bit to a depth of about 38mm (11/2 in). A 15mm diameter 100mm-long nylon hexagon-head bolt is screwed into the hole with a wrench and socket. The box is then hung on the bolt through the pre- cut keyhole at the top of the mounting board. The head of the bolt is then cut off with a hacksaw. A further hole is drilled into the tree through the bottom hole in the mounting board, and a second bolt inserted, tightened and the head removed. Two nylon washer-faced nuts are then tightened on to the stubs of the protruding bolts. They can be slackened each year as the tree grows.
If boxes are fixed too rigidly they distort quickly and eventually break, particularly on fast growing trees such as poplar, willow and sycamore, which should be avoided. A 50mm (2 in) layer of nesting material must be placed in the box for the kestrel eggs. Shredded bark mixed with finely shredded paper can be used. Once used, the kestrels’ own debris, pellets etc form the basis of future nests. If birds have made no attempt to use a box after three to four years it may be useful to consider moving it to another location. Most of the smaller birds of prey will not attack you at the nest.
Inspection is essential. Boxes should be individually numbered and locations and contents accurately recorded every year. Remember to ask for permission from landowners to install or inspect artificial nest sites, and inform them of your successes. Boxes need to be checked once a year and this is best done when the young are small. Most kestrels are tolerant to disturbance at this time, but can be less so when egg-laying and incubating. As a rough guide, they should be inspected in early to mid-June. However, timing can depend on many factors such as spring weather and annual fluctuations in the food supply. Sites should be checked each winter and any unwanted material cleared. If the box has been used by a bird of prey, pellets and lining materials should be left and only removed when they have built up above 100mm or so.
THREATS: During the 19th century, the kestrel was nearly wiped out by gamekeepers but populations had recovered by the middle decades of the twentieth century. A further threat was posed by organochlorides and other pesticides (particularly DDT) and took until the 1970s for the UK population to recover. Recently their numbers have declined (the reason for which is uncertain) and they are now outnumbered by Buzzards, another raptor. The RSPB suggests that one reason for the decline in numbers is due to the effects of intensive farming on the vole population, the vole being a predominant component outside of the city. In the light of the decline in the kestrel population, The Owl and Hawk Trust began an initiative in 2009 to construct over 240 kestrel boxes along Britain’s major roads. This was to see if the absence of nesting sites explained the decline in the population. In May 2011, The London Wildlife Trust began its second kestrel count to gather together reports of public sightings of kestrels in London to ascertain their number in the capital. Kestrel life expectancy is not more than 2 years and many die before the end of the first year.
INTERESTING FACTS: Kestrels are sexually dimorphic, with the female noticeably larger than the male, and, unlike most raptors, differentiate in colour – the male having a grey-blueish coloured head in adulthood. In the Middle Ages, kestrels were considered a lower order of hunting bird, compared with peregrines for instance, and were the only falcon knaves were allowed to train and own. Shakespeare invokes therefore invokes the kestrel as associated with ‘baseness’ and cowardice.
RESOURCES:
The Raptor Foundation
RSPB
The London Wildlife Trust
Wikipedia
BBC Nature
The Hawk and Owl Trust (and guide)
Garden Birds
D.W. Yalden (1980) – ‘Notes on the diet of urban kestrels’. Bird Study. 27(4)
COMPILED BY: Marcus Nyman