Grebes
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Horned grebe ( Podiceps auritus ) in magnificent dress |
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| Podicipedidae | ||||||||||||
| Bonaparte , 1831 |
The grebes (Podicipediformes, Podicipedidae) are an order and family of birds . In older literature they can also be found under the name tail feet . 23 species are known, two of which are certain and a third most likely to be extinct. The most famous species in Europe is the great crested grebe .
features
The grebes are a family of water-bound, diving birds. Although they are sometimes mistaken for ducks by laypeople , they are nothing like them. Apart from the different external shape, they are also much deeper in the water; this is due to the low level of pneumatization of the skeleton , that is, the bones of the grebes are not as hollow and filled with air as in many other birds.
The strong legs are positioned far back on the body. They provide the drive for swimming and diving and serve as oars. Unlike many other waterfowl, the toes are not webbed , but rather wear wide flaps. If the foot is pulled forward in the water, these fold together so that there is hardly any resistance. When returning, they open and push the body forward against the water. Three toes point forward, another pointing backward ( anisodactyly ).
When diving, the birds make a powerful leap forward, sometimes emerging with their entire body out of the water, before diving head and neck first. Through this jump, the birds dive at a steeper angle and reach greater depths. During the dive, the wings remain attached, so they are not used for propulsion, as is the case with penguins, for example . A dive usually takes 5 to 40 seconds, with the smaller species staying under water for less than the larger ones. Typically, a grebes can stay under water for about one minute, while a maximum of three minutes was measured for the ear diver . The diving depth is usually 1 to 4 m; however, a great crested grebe was found caught in a fishing net at a depth of 30 m . Grebes can cover long distances underwater in the horizontal direction.
While the legs, which are set far back, are an excellent adaptation to life in the water, they are largely unusable for locomotion on land. As a rule, grebes only leave the water to rest or at the nest. It is also relatively difficult for the grebes to fly up: in order to lift their relatively heavy bodies into the air, a grebe runs a long distance with flapping wings on the surface of the water before taking off. Therefore, grebes evade dangerous situations by diving rather than by being blown. However, once in the air, grebes can fly long distances and some species are migratory birds . Three species are flightless: the titicaca diver , the atitlán diver and the puna diver . These three species are not closely related to each other and each have airborne sister species .
The soft, dense plumage is water-repellent. Each grebe has more than 20,000 feathers. In the course of the year there are noticeable changes in the plumage of most species. The brood plumage is often characterized by bright colors on the neck and head, and there are also conspicuous bonnets, crests or tufts of ears. In the plain dress, however, gray and brown colors dominate. There is no conspicuous gender dimorphism ; Males sometimes have a slightly brighter color and are on average a little larger than females, but these differences are usually not enough to distinguish the sexes in a field ornithological manner.
The body length and weight of the grebes vary between 24 centimeters and 80 centimeters and between 130 grams and 1700 grams. There are two basic types: Long-necked divers with long, pointed beaks are primarily fish-eaters, while the insect-eating species have significantly shorter necks and beaks. The (extinct) Atlantean divers had a special position, their beak specialized for eating crustaceans .
voice
The vocalizations of the grebes vary considerably depending on the species. Some species have up to twelve different calls, others are largely silent. The various whistling, trill and screeching tones are mainly used in courtship, danger and aggression gestures.
Particularly characteristic of many species is the call issued before courtship, with which conspecifics of the opposite sex are to be made aware. In racing divers there are even individual differences in this courtship call; every bird here has its own call melody.
distribution and habitat
Grebes are common on all continents except Antarctica . They live in tropical , temperate and sub-polar regions. Only the eared grebe lives north of the Arctic Circle , the Inca and Rollandt divers are also found in the subantarctic . Unlike the loons, however, the grebes did not develop high arctic regions . Their worldwide distribution area also includes remote islands such as Madagascar or New Zealand .
All species live in inland waters during the breeding season, especially on shallow lakes with sandy bottoms and no currents. They are less common on slow-flowing rivers. Two species, the magellan diver and the racing diver, rarely breed in calm bays. Especially in South America, some species specialize in high alpine lakes in the Andes ; they breed at heights of up to 4000 m. The great crested grebe is the only species that has become a cultural follower in some places ; he also populated urban parks in Central Europe.
Some species only rest on the sea outside of the breeding season. While the magellan diver can also be seen a few kilometers from the coast on the open sea, the other species, if they have any contact with the sea, stay close to the coast.
The species of tropical and subtropical regions in particular are mostly resident birds that only migrate to neighboring lakes. Types of temperate zones are partial migrants or real migratory birds ; outside of the breeding season they are often found in large flocks. B. about 20,000 individuals of the great crested grebe at the IJsselmeer or 750,000 black-necked grebe at Mono Lake in California .
Way of life
activity
Grebes are mainly diurnal, but can still be active on bright full moon nights. Most species are solitary and live in pairs during the breeding season; some become more sociable in the winter quarters.
Seven species deviate from this and nest in colonies: the black-necked grebe, the pied-head diver, the golden-headed diver, the Inca diver, the Puna diver, the racing diver and the Clark diver.
nutrition
As described in the Characteristics section , there are two basic types of grebes who specialize in hunting fish and aquatic insects, respectively. The first include the great crested grebe and racing divers, the latter include the little grebe and the black-necked grebe. However, the specialization only means that fish or insects make up the main part of the diet of the respective species. The large species also eat insects as complementary food, and the insect-hunting grebes occasionally prey on a small fish.
The largest species can swallow fish with a maximum length of 20 cm and a height of up to 7.5 cm. The aquatic insects that are eaten by the smaller grebes include the larvae of dragonflies , mayflies and stone flies , water-dwelling bedbugs and swimming beetles . The prey spectrum also includes water snails , crustaceans , tadpoles and adult frogs .
Traces of aquatic plants are often found in the stomachs of grebes; mostly these are accidentally eaten. Small to medium-sized stones are also swallowed as gastroliths to crush food. What is unique is that grebes also swallow their own feathers , small feathers from the chest or abdomen. They decompose in the stomach to a greenish, felty mass, which is regurgitated regularly as bulges together with indigestible food residues . The benefit of this peculiar behavior is probably that the feather mass protects the stomach wall from injuries caused by sharp herringbones.
Reproduction
Courtship and mating
All grebes form monogamous pairs during the breeding season . A courtship ritual takes place to form pairs , which is more or less complex depending on the species. The comparative study of courtship rituals is of particular interest to phylogeneticists . The small species such as the little grebe and the grebe, but also the large, deviant magellan diver, have a very limited mating prelude. In contrast, highly complex rituals are found in most species of the genus Podiceps and in racing divers. Here the courtship ceremony is characterized by synchronous, dance-like movements of the partners. B. with the great crested grebe end with the mutual presentation of aquatic plants. The pairs of racing divers "run" synchronously with slightly S-shaped necks over the water before they submerge together.
Mating takes place on land. Then a phase begins in which the partners become territorial and defend the surroundings of the future nest against intruders of their own species, but also against foreign species such as ducks . The aggressive behavior is weakened in the seven colony-breeding species. Sometimes these breed not only with conspecifics, but also with other bird species; Examples in Europe are the black-headed gull and the whiskered tern . In such mixed colonies, the gulls and terns give early warning of approaching enemies.
Nest building and brood
The nest is built by both partners from aquatic plants, twigs and leaves. It is a floating nest floating on the water that is anchored to the adjacent vegetation, for example in the reed beds . The diameter of the nest is between 30 and 50 centimeters, in rare cases up to a meter. The smaller species also usually build smaller nests, but the nest size also depends on factors such as the swell or the plant material used.
The female lays two to seven eggs, which are initially solid white, yellow or light blue, but in the course of the breeding season they get brown spots due to the rotting moist nesting material. A grebe's egg is about 3 to 6% of the weight of a full-grown animal, making it relatively small; the absolute size varies between 3.4 × 2.3 cm ( black-headed divers ) and 5.8 × 3.9 cm ( racing divers ). With small species there are up to three broods per year, with large ones only one or two.
The breeding season is 20 to 30 days. Both sexes breed. In order not to draw attention to the nest, many species approach their nest by diving. Often both partners leave the nest for several hours; the embryos are extremely resistant to the resulting cooling down. The clutch is covered before leaving; possibly the rotting plant material of the swimming nests makes a small contribution to the warming of the eggs. In addition, the cover is a protection against egg robbers.
After hatching
Young rag divers are characterized by a typical striped pattern (exceptions: racing divers and Clark divers). These longitudinal stripes run initially over the whole body, later only over the neck and head. The boys can swim and dive independently from the start. Since they can only insufficiently regulate the body temperature and cool down quickly, they are usually transported on the back of the adult birds. One partner at a time carries the young, while the other dives for food and does the feeding. Freshly hatched young have bare skin on their head, which "glows" in color when excited and thus signals hunger or danger to the parents. The occasional claim that grebes have pockets under their wings to hide their pups when diving is wrong. An adult bird that transports young usually stays on the surface and does not dive.
Depending on the species, young grebes remain in their parents' care for between 44 and 79 days. From the moment siblings are hatched, there is competition for food in which the parents do not intervene. This usually results in the death of the weaker boys. The probability that a young bird will survive the first twenty days of its life is 40 to 60%.
Humans and grebes
Because their feathers were very popular as lining for textiles in the 19th century, the Holarctic species were hunted on a large scale. Great crested grebes and racing divers have been almost extinct in some regions - however, both species have recovered through protective measures over the course of the 20th century and are now quite common again. Today the rag-divers suffer from pollution of the waters and disturbances from boat traffic. Boats can endanger the sensitive swimming nests due to the waves they generate. Many grebes get caught in fishing nets and drown.
Two other species are now extinct: the Andean diver was widespread in the high swamps near Bogotá until 1977 and was eradicated by draining and contaminating the lakes with pesticides . The Atlantean diver lived only on Lake Atitlan in Guatemala ; Due to various causes (introduction of largemouth bass into the lake, removal of the reed belt, earthquake in 1976) it became extremely rare and has been considered extinct since 1986.
Who is threatened as much on the red list of IUCN the Alaotra Grebe Madagascar out; it has not been sighted since 1985, but because of its impassable and undeveloped range, it has not yet been declared extinct. The Puna diver, which occurs on a single Andean Sea, is also extremely endangered .
The titicaca diver, which was still quite common in the late 20th century, suffered a drastic population decline and is considered threatened.
Fossil history
Grebes are a very old group of birds. They have been fossilized since the Miocene by the genera Miobaptus and Thiornis . In the Pliocene , in addition to the fossil genus Pliolymbus, there were also representatives of the great crested grebe genus Podiceps , and from the Pleistocene there have been finds of species of two other more recent genera, Podilymbus and Aechmophorus .
Half of the species live in South America , so this could be the evolutionary origin of the grebes.
Systematics
External system
Grebes are not closely related to any other bird family. That is why they are listed as the only family in the Podicipediformes order.
Traditionally, the grebes are still placed near the loons (Gaviidae), with whom they have some things in common in their outward appearance and way of life. As early as 1758, Carl von Linné assigned both groups to a genus Colymbus in his Systema Naturae , which he classified under the Anseres, an order that included almost all waterfowl in his system. This was done by other zoologists, for example Johann Karl Wilhelm Illiger , who placed Colymbus together with the alks and penguins in the Pygopodidae family in 1811. At the end of the 19th century, rag and loons were first divided into two families, but were still considered related. Leon Gardner was the first zoologist in 1925 to question the relationship between sea and ragged divers.
According to later analysis, all similarities between sea and rag divers are due to convergent evolution ; A relationship between the two diving families is no longer assumed.
In 1990, Sibley and Monroe assigned the grebes to the Ciconiiformes (formerly known as walking birds in German) during their comprehensive redesign of the bird system, as did waders , plover-like , penguins , falcons and numerous other bird taxa. However, this large collective taxon was not generally recognized. The evidence of a relationship between the grebes and all these taxa is also very thin.
In 2003 Gerald Mayr formulated a new hypothesis, according to which the grebes could be in a sister group relationship to the flamingos . The assumption was later confirmed by several phylogenetic studies with the help of DNA sequencing .
Internal system
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The family comprises six genera and 22 species , two of which are considered extinct and a third is thought to be most likely extinct.
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Tachybaptus
- Little Grebe ( T. ruficollis )
- Australian Little Grebe ( T. novaehollandiae )
- Madagascar Little Grebe ( T. pelzelnii )
- Black-headed Grebe ( T. dominicus )
- † Delacour little grebe ( T. rufolavatus )
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Podilymbus
- † Atitlan divers ( P. gigas )
- Grebe ( P. podiceps )
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Poliocephalus
- Scorpionfish ( P. poliocephalus )
- Maori diver ( P. rufopectus )
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Rollandia
- Titicaca diver ( R. microptera )
- Rolland diver ( R. rolland )
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Aechmophorus
- Racing diver ( A. occidentalis )
- Clark's diver ( A. clarkii )
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Podiceps
- Red-necked grebe ( P. grisegena )
- Great crested grebe ( P. cristatus )
- Ear grebe ( P. auritus )
- Black-necked Grebe ( P. nigricollis )
- † Andean diver ( P. andinus )
- Puna diver ( P. taczanowskii )
- Golden log diver ( P. gallardoi )
- Inca diver ( P. occipitalis )
- Magellan Diver ( P. major )
The relationships between these genera are shown in the following cladogram (Fjeldså 2004):
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The generic name Colymbus , which was often used in the past , is older than Podiceps and goes back to Carl von Linné, but was used alternately for loons and ragged divers and in 1956 was declared invalid by the ICZN . The name Pygopodidae for the family is also invalid, as it is pre-assigned according to the priority rule for the pinnacles , a reptile family, and a scientific name cannot be applied to two different taxa.
literature
- Josep del Hoyo et al .: Handbook of the Birds of the World. Volume 1: Ostrich to Ducks. Lynx Edicions, 1992, ISBN 84-87334-10-5 .
- Jon Fjeldså: The Grebes . Oxford University Press, 2004, ISBN 0-19-850064-5
Individual evidence
- ↑ Hadoram Shirihai: A Complete Guide to Antarctic Wildlife - The Birds and Marine Mammals of the Antarctic Continent and Southern Ocean , Alula Press, Degerby 2002, ISBN 951-98947-0-5 , p. 237
- ↑ Gerald Mayr: Morphological evidence for sister group relationship between flamingos (Aves: Phoenicopteridae) and grebes (Podicipedidae) . In: Zoological Journal of the Linnean Society , 2004, Issue 140, pp. 157-169.
- ↑ Hackett et al .: A Phylogenomic Study of Birds Reveals Their Evolutionary History . Science 27 June 2008: Vol. 320. no. 5884, pp. 1763–1768 doi : 10.1126 / science.1157704
- ↑ Per GP Ericson et al .: Diversification of Neoaves: integration of molecular sequence data and fossils . Biol. Lett. doi : 10.1098 / rsbl.2006.0523
