White Wagtail

Appearance

The white wagtail is a slender bird, 16.5–19 cm in length, with the characteristic long, constantly wagging tail of its genus. Its average weight is 25 g and the maximum lifespan in the wild is c. 12 years. The nominate subspecies "Motacilla alba alba" is basically grey above and white below, with a white face, black cap and black throat.

Distribution

This species breeds throughout Eurasia up to latitudes 75°N, only being absent in the Arctic from areas where the July isotherm is less than 4 °C. It also breeds in the mountains of Morocco and western Alaska. It occupies a wide range of habitats, but is absent from deserts.

White wagtail is resident in the milder parts of its range such as western Europe and the Mediterranean, but migratory in much of the rest of its range. Northern European breeders winter around the Mediterranean and in tropical and subtropical Africa, and Asiatic birds move to the Middle East, India, and SouthEast Asia. Birds from the North American population also winter in tropical Asia.

Status

This species has a large range, with an estimated extent of more than 10 million km2. The population size is unknown, but it is believed to be large, as the species is described as "common" in at least parts of its range. Population trends have not been quantified, but the species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List. For these reasons, the species is evaluated to be of least concern. The population in Europe appears to be stable. The species has adapted well to human changes to the environment and has exploited human changes such as man-made structures that are used for nesting sites and increased open areas that are used for foraging. In a number of cities, notably Dublin, large flocks gather in winter to roost.

Behavior

The most conspicuous habit of this species is a near-constant tail wagging, a trait that has given the species, and indeed the genus, its common name. In spite of the ubiquity of this behaviour, the reasons for it are poorly understood. It has been suggested that it may flush prey, or signal submissiveness to other wagtails. A recent study has suggested instead that it is a signal of vigilance to potential predators.

Reproduction

White wagtails are monogamous and defend breeding territories. The breeding season for most is from April to August, with the season starting later further north.

Both sexes are responsible for building the nest, with the male responsible for initiating the nest building and the female for finishing the process. For second broods in the subspecies "personata" the female alone builds the nest, which is a rough cup assembled from twigs, grass, leaves and other plant matter, as the male is still provisioning the young.

It is lined with soft materials, including animal hair. The nest is set into a crevice or hole; traditionally in a bank next to a river or ditch, but the species has also adapted to nesting in walls, bridges and buildings.

One nest was found in the skull of a walrus. They species will nest in association with other animals, particularly where available the dams of beavers and also inside the nests of golden eagles. Around three to eight eggs are laid, with the usual number being four to six.

Its eggs are cream-coloured, often with a faint bluish-green or turquoise tint, and heavily spotted with reddish brown; they measure, on average, 21×15 mm. Both parents incubate the eggs, although the female generally does so for longer and incubates at night. The eggs begin to hatch after 12 days. Both parents feed the chicks until they fledge at around 14 days, and the chicks are fed for another week after fledging.

Though it is known to be a host species for the common cuckoo, the white wagtail typically deserts its nest if it has been parasitised. Scientists theorise that this occurs because the wagtail is too small to push the intruding egg out of the nest, and too short-billed to destroy the egg by puncturing it.

Food

The exact composition of the diet of white wagtails varies by location, but terrestrial and aquatic insects and other small invertebrates form the major part of the diet. These range from beetles, dragonflies, small snails, spiders, worms, crustaceans, to maggots found in carcasses and, most importantly, flies in the order Diptera. Small fish fry have also been recorded in the diet. The white wagtail is somewhat unusual in the parts of its range where it is non-migratory as it is an insectivorous bird that continues to feed on insects during the winter.