Common kingfisher

There are seven subspecies differing in the hue of the upperparts and the intensity of the rufous colour of the underparts; size varies across the subspecies by up to 10%. The races resident south of the Wallace Line have the bluest upperparts and partly blue ear-patches.[7][9]

• A. a. ispida (Linnaeus, 1758).[10] Breeds from Ireland, Spain and southern Norway to Romania and western Russia and winters south to Iraq and southern Portugal.
• A. a. atthis Linnaeus, 1758. Breeds from northwestern Africa and southern Italy east to Afghanistan, Kashmir region, northern Xinjiang, and Siberia; it is a winter visitor south to Israel,[11] northeastern Sudan, Yemen, Oman and Pakistan. Compared to A. a. ispida, it has a greener crown, paler underparts and is slightly larger.
• A. a. bengalensis Gmelin, 1788. Breeds in southern and eastern Asia from India to Indonesia, China, Korea, Japan and eastern Mongolia; winters south to Indonesia and the Philippines. It is smaller and brighter than the European races.
• A. a. taprobana Kleinschmidt, 1894. Resident breeder in Sri Lanka and southern India. Its upperparts are bright blue, not green-blue; it is the same size as A. a. bengalensis.
• A. a. floresiana Sharpe, 1892. Resident breeder from Bali to Timor. Like A. a. taprobana, but the blues are darker and the ear-patch is rufous with a few blue feathers.
• A. a. hispidoides Lesson, 1837. Resident breeder from Sulawesi to New Guinea and the islands of the western Pacific Ocean. Plumage colours are deeper than in A. a. floresiana, the blue on the hind neck and rump is purple-tinged and the ear-patch is blue.
• A. a. solomonensis Rothschild and Hartert, 1905. Resident breeder in the Solomon Islands east to San Cristobal. The largest southeast Asian subspecies, it has a blue ear-patch and is more purple-tinged than A. a. hispidoides, with which it interbreeds.

Volunteers create a vertical bank in which common kingfishers have subsequently nested annually

Eggs of Alcedo atthis, MHNT

Like all kingfishers, the common kingfisher is highly territorial; since it must eat around 60% of its body weight each day, it is essential to have control of a suitable stretch of river. It is solitary for most of the year, roosting alone in heavy cover. If another kingfisher enters its territory, both birds display from perches, and fights may occur, in which a bird will grab the other's beak and try to hold it underwater. Pairs form in the autumn but each bird retains a separate territory, generally at least 1 km (0.62 mi) long, but up to 3.5 km (2.2 mi) and territories are not merged until the spring.[7]

The courtship is initiated by the male chasing the female while calling continually, and later by ritual feeding, with copulation usually following.[8]

The nest is in a burrow excavated by both birds of the pair in a low vertical riverbank, or sometimes a quarry or other cutting. The straight, gently inclining burrow is normally 60–90 cm (24–35 in) long and ends in an enlarged chamber.[8] The nest cavity is unlined but soon accumulates a litter of fish remains and cast pellets.[14]

The common kingfisher typically lays two to ten glossy white eggs, which average 1.9 cm (0.75 in) in breadth, 2.2 cm (0.87 in) in length, and weigh about 4.3 g (0.15 oz), of which 5% is shell.[5] One or two eggs in most clutches fail to hatch because the parent cannot cover them. Both sexes incubate by day, but only the female incubates at night. An incubating bird sits trance-like, facing the tunnel; it invariably casts a pellet, breaking it up with the bill. The eggs hatch in 19–20 days, and the altricial young are in the nest for a further 24–25 days, often more.[7] Once large enough, young birds will come to the burrow entrance to be fed.[14] Two broods, sometimes three, may be reared in a season.[8]

The early days for fledged juveniles are more hazardous; during its first dives into the water, about four days after leaving the nest, a fledgling may become waterlogged and drown.[7] Many young will not have learned to fish by the time they are driven out of their parents' territory, and only about half survive more than a week or two. Most kingfishers die of cold or lack of food, and a severe winter can kill a high percentage of the birds. Summer floods can destroy nests or make fishing difficult, resulting in starvation of the brood. Only a quarter of the young survive to breed the following year, but this is enough to maintain the population. Likewise, only a quarter of adult birds survive from one breeding season to the next. Very few birds live longer than one breeding season.[15] The oldest bird on record was 21 years.[16]

Other causes of death are cats, rats, collisions with vehicles and windows, and human disturbance of nesting birds, including riverbank works with heavy machinery. Since kingfishers are high up in the food chain, they are vulnerable to build-up of chemicals, and river pollution by industrial and agricultural products excludes the birds from many stretches of otherwise suitable rivers that would be habitats.[15]

This species was killed in Victorian times for stuffing and display in glass cases and use in hat making. English naturalist William Yarrell also reported the country practice of killing a kingfisher and hanging it from a thread in the belief that it would swing to predict the direction in which the wind would blow.[17] Persecution by anglers and to provide feathers for fishing flies were common in earlier decades,[14] but are now largely a thing of the past.[15]

Pellet of a common kingfisher

The common kingfisher hunts from a perch 1–2 m (3.3–6.6 ft) above the water, on a branch, post or riverbank, bill pointing down as it searches for prey. It bobs its head when food is detected to gauge the distance and plunges steeply down to seize its prey usually no deeper than 25 cm (9.8 in) below the surface. The wings are opened underwater and the open eyes are protected by the transparent third eyelid. The bird rises beak-first from the surface and flies back to its perch. At the perch the fish is adjusted until it is held near its tail and beaten against the perch several times. Once dead, the fish is positioned lengthways and swallowed head-first. A few times each day, a small greyish pellet of fish bones and other indigestible remains is regurgitated.[7]

The food is mainly fish up to 12.5 cm (4.9 in) long, but the average size is 2.3 cm (0.91 in).[7] In Central Europe, 97% of the diet was found to be composed of fish ranging in size from 2 to 10 cm with an average of 6.5 cm (body mass range from <0.1 g to >10 g, average 3 g).[18][19] Minnows, sticklebacks, small roach and trout are typical prey. About 60% of food items are fish, but this kingfisher also catches aquatic insects such as dragonfly larvae and water beetles, and, in winter, crustaceans including freshwater shrimps.[7] In Central Europe, however, fish represented 99.9% of the diet (data from rivers, streams, and reservoirs from years 1999 to 2013).[18] Common kingfishers have also been observed to catch lamprey.[20] One study found that food provisioning rate increased with brood size, from 1498 g (505 fishes for four nestlings) to 2968 g (894 fishes for eight nestlings). During the fledging period each chick consumed on average 334 g of fish, which resulted in an estimated daily food intake of 37% of the chick's body mass (average over the entire nestling period). The average daily energy intake was 73.5 kJ per chick (i.e., 1837 kJ per 25 days of the fledging period).[21]

A challenge for any diving bird is the change in refraction between air and water. The eyes of many birds have two foveae (the fovea is the area of the retina with the greatest density of light receptors),[22] and a kingfisher can switch from the main central fovea to the auxiliary fovea when it enters water; a retinal streak of high receptor density which connects the two foveae allows the image to swing temporally as the bird drops onto the prey.[23] The egg-shaped lens of the eye points towards the auxiliary fovea, enabling the bird to maintain visual acuity underwater.[22] Because of the positions of the foveae, the kingfisher has monocular vision in air, and binocular vision in water. The underwater vision is not as a sharp as in air, but the ability to judge the distance of moving prey is more important than the sharpness of the image.[23]

Each cone cell of a bird's retina contains an oil droplet that may contain carotenoid pigments. These droplets enhance color vision and reduce glare. Aquatic kingfishers have high numbers of red pigments in their oil droplets; the reason red droplets predominate is not understood, but the droplets may help with the glare or the dispersion of light from particulate matter in the water.[23]