Dippers

Dippers

Dippers have the remarkable ability to walk underwater seemingly oblivious of the current: they are the only truly aquatic members of the Passeriformes. One’s first view of a dipper is often that of a dark bird flying fast and low, following the twists and turns of a stream. The Reverend Cotes, the author of Thomas Berwick’s History of British Birds, coined the name dipper in 1804: “It may be seen perched on the top of a stone amid the torrent, in a continual dipping motion or short courtesy of repeated.” The whole body moves up and down vertically during the dipping motion and the blinking membrane moves to give the appearance of a white eye.

Dippers are dark rotund birds like giant wrens (to which they may be related) withal thrush-like bills, stumpy tails, and brown legs with strong claws. The five species are very similar, differing only in range and plumage color. Species only overlap in Central Asia; here the White-breasted dip-per occupies the higher streams and rivers while the Brown dipper occurs at lower altitudes. Dippers swim, dive, and walk into shallow water to obtain their food; they can remain underwater for .30 seconds but most dives are much shorter. They have extra thick body feathers to aid waterproofing and insulation and can survive winter temper-natures down to — 45°C ( — 49°F) if the rivers are not completely frozen; they can even feed under the ice.

They are highly territorial in summer and winter, using a neck-stretching display and chasing to maintain territorial boundaries. Territory size is mainly determined by the extent of stream beds available for feeding.

Dippers breed in early spring when food is most abundant, and are usually monogamous. Both sexes build the nest (in 14-21 days) although females do most of the build-in. Nests are placed among tree roots, on small cliffs, under bridges, and in walls, and sometimes behind waterfalls where the adult must fly through the water to the nest. The bulky nests are often inconspicuous. either because they are built in crevices or because the mossy shell closely resembles the surroundings.

The nestling period is relatively long but nesting success is usually high (70 percent). If disturbed, nestlings may explode from the nest after 14 days (when full body weight is attained); remarkably they can swim and dive expertly before they can fly. When diapers have second broods they are perhaps unusual in often using the same nest (relined); they frequently use the same nest in successive years, often for three or four years. Dippers become very secretive during molt, which is rapid, and the American dipper can become flightless. Most populations of dip-peers are sedentary although some populations move more than 1,000km (625 mi) between summer and winter areas.

Post-fledging mortality is high (over 80 percent in the first six months) but thereafter annual mortality is between 25 and 35 percent. In suitable breeding areas, the habitat is continuously occupied and a non-breeding surplus of birds either does not exist or is small and frequents areas unsuited-able for breeding. Hence replacement of an adult who dies in the breeding season is unusual.

The Latin word troglodytes, from whom the wren family name is derived, means “cave dweller”— a reference to the habit of all wrens of building elaborate roofed nests which they use not only to house eggs and nestlings but also as communal roosts and aids to male courtship. In several species, prodigious nest-building is matched with fine energetic singing by the males. Field studies of the polygynous wrens of European North America suggest that the form and extent of both these activities may be extreme, perhaps a result of strong sexual selection through female mate choice.

Many of the Central American species are thought to be monogamous, while those in the Cactus wren group (genus Ccunpylorhynchus) live in family parties and have evolved cooperative breeding system: independent young help their parents to raise further broods of young. Thus in the wrens, we have a passer form family of exceptional social diversity, even though virtually nothing is known about the habits of most of its tropical species.

Members of all 14 genera occur in the area between Mexico and the equator in South America. The broad, blunt wings, evident poor flying ability, and small size of most wrens have not, however, prevented them from invading some offshore islands. For instance, Cuba has an endemic species in the Zapata wren, and distinct subspecies of the Common wren are found, for example, in Taiwan and St Kilda (off northwest Scotland).

This species, known as the Winter wren in North America, is the sole representative of the family in the Old World. It is therefore thought to have migrated west from Alaska to Siberia, and its range now stretches from the easterners to Iceland. Another species with transcontinental distribution is the House wren, which occurs from the eastern Usutu Patagonia.

These two forms must be very flexible in their habitat preferences and diet, although other species breeding at temperate latitudes, such as the Long-billed marsh wren, undertake seasonal migrations to and from wintering grounds with more equable climates. Other wrens, and especially the numerous forms endemic to Central America, occupy much narrower ecological niches and may be very restricted in distribution.

Based on behavior and form wrens fall into two distinct groups. The majority are small, cryptically colored, secretive, and rather solitary inhabitants of the dense forest understory. They flutter and climb among tangled vegetation in search of tiny insects and other animals that make up their diet.

In a minority are the much bigger Cactus wren and its allies, living in the more opensemidesert habitats of Central America. Although they have a diet similar to that of their smaller relatives, they move much more boldly, perhaps because they are often in small family flocks which should afford them some protection from predators.

All wrens studied in detail seem to be territorial, at least during the breeding season. The role of song in the defense of space is uncertain, but as a family, the wrens are renowned as songsters. Several of the monogamous forest-dwelling species live in pairs all year round and some, including the Musician wren, produce melodic and beautifully coordinated alternating duets.

There seems to be little doubt that the song of the males in polygynous species, such as the Common and Long-billed marsh wrens, serves two roles: in defending territory and attracting several mates trigram is not uncommon. Neighboring males spend a large proportion of their time each morning answering each other across well-defined territory boundaries. When a female enters a territory she is courted vigorously by the occupant who sings and leads her to a nest he has already built. Com-moon and House wrens may have three or four nests ready simultaneously for use both in these displays and by females making breeding attempts. They commence promptly after courtship and the collection of some lining materials for the nest cup their token gesture towards nest construction.

Males of these species may build 6–I 2 nests in the course of a three-month breed-in season, but their efforts are paltry beside those of male Long-billed marsh wrens, who construct as many as 25-3 5 nests over a similar period. They are built-in clusters. Some are even semidetached and appear to have a primarily ceremonial role. Males sing vigorously from these collections of nests and lead any female that appears to several of them in succession. Subsequently, the male builds yet another nest, in which. e female attempts to breed, usually away from his conspicuous courtship center.

Observations suggest that females in these species have a free choice of where to breed, subject to the availability of usable nests. Consequently one can predict that any trait in males that enhances their ability to attract females will be subject to intense sexual selection. Thus it is no surprise to find that males of polygynous species build more nests, have more elaborate courtship dies-plays and songs, and spend more time sing-in in the breeding season than do those of monogamous species.

These adaptations may be seen as the results of an evolutionary “arms race” driven over the millennia by a combination of male salesmanship and female sales resistance. Because they spend much time attempting to obtain mates male Common and Long-billed marsh wrens never incubate and only help feed their nestlings at the end of the season when, presumably, chances of achieving further productive coatings are negligible.

In complete contrast, the Cactus wren is monogamous and uses a cooperative system. Parents produce up to four broods a year, later ones being fed in the nest by both parents and they’re independent young from earlier broods. All members of these family groups assist the breeding mullein territorial defense against other families, but rather paradoxically all but recently fledged juveniles sleep alone in one of the many large nests dotted about in the action of their territory.