grebes

Grebes () are aquatic diving birds in the order Podicipediformes . Grebes are widely distributed freshwater birds, with some species also found in marine habitats during migration and winter. Some flightless species exist as well, most notably in stable lakes. The order contains a single family, the Podicipedidae, which includes 22 species in six extant genera. Although, superficially, they resemble other diving birds such as loons and coots, they are most closely related to flamingos, as supported by morphological, molecular and paleontological data. Many species are monogamous and are known for their courtship displays, with the pair performing synchronized dances across the water's surface. The birds build floating vegetative nests where they lay several eggs. About a third of the world's grebes are listed at various levels of conservation concerns—the biggest threats including habitat loss, the introduction of invasive predatory fish and human poaching. As such, three species have gone extinct.

Field characteristics

Grebes are small to medium-large in size, have lobed toes, and are excellent swimmers and divers. Although they can run for a short distance, they are prone to falling over, since they have their feet placed far back on the body. Grebes have narrow wings, and some species are reluctant to fly; indeed, two South American species are completely flightless. They respond to danger by diving rather than flying, and are in any case much less wary than ducks. Extant species range in size from the least grebe, at 120 g (4.3 oz) and 23.5 cm (9.3 in), to the great grebe, at 1.7 kg (3.8 lbs) and 71 cm (28 in). Bills vary from short and thick to long and pointed depending on the diet. The feet are always large, with broad lobes on the toes and small webs connecting the front three toes. The hind toe also has a small lobe. Recent experimental work has shown that these lobes work like the hydrofoil blades of a propeller.

Grebes have unusual plumage. It is dense and waterproof, and on the underside the feathers are at right angles to the skin, sticking straight out to begin with and curling at the tip. By pressing their feathers against the body, grebes can adjust their buoyancy. Often, they swim low in the water with just the head and neck exposed. They swim by simultaneously spreading out the feet and bringing them inward, with the webbing expanded to produce the forward thrust in much the same way as frogs. In the nonbreeding season, grebes are plain-coloured in dark browns and whites. However, most have ornate and distinctive breeding plumages, often developing chestnut markings on the head area, and perform elaborate display rituals. The young, particularly those of the genus ''Podiceps'', are often striped and retain some of their juvenile plumage even after reaching full size. When preening, grebes eat their own feathers and feed them to their young. The function of this behaviour is uncertain, but it is believed to assist with pellet formation and to reduce their vulnerability to gastric parasites.

Systematics

The grebes are a radically distinct group of birds as regards their anatomy. Accordingly, they were at first believed to be related to the loons, which are also foot-propelled diving birds, and both families were once classified together under the order Colymbiformes. However, as early as the 1930s, this was determined to be an example of convergent evolution by the strong selective forces encountered by unrelated birds sharing the same lifestyle at different times and in different habitat. Grebes and loons are now separately classified orders of Podicipediformes and Gaviiformes, respectively. Recent molecular studies have suggested a relation with flamingos while morphological evidence also strongly supports a relationship between flamingos and grebes. They hold at least eleven morphological traits in common, which are not found in other birds. Many of these characteristics have been previously identified in flamingos, but not in grebes. One clade of fossil stem-flamingos the palaelodids can be considered evolutionarily, and ecologically, intermediate between flamingos and grebes. For the grebe-flamingo clade, the taxon Mirandornithes ("miraculous birds" due to their extreme divergence and apomorphies) has been proposed. Alternatively, they could be placed in one order, with Phoenocopteriformes taking priority.

Fossil grebes

The fossil record of grebes is incomplete; there are no transitional forms between more conventional birds and the highly derived grebes known from fossils, or at least none that can be placed in the relationships of the group with any certainty. The enigmatic waterbird genus ''Juncitarsus'', however, may be close to a common ancestor of flamingos and grebes.

The Early Cretaceous (Berriasian, around 143 mya) genus '' Eurolimnornis'' from NW Romania was initially believed to be a grebe. If it is indeed related to this lineage, it must represent a most basal form, as it almost certainly predates any grebe-flamingo split. On the other hand, the single bone fragment assigned to this taxon is not very diagnostic and may not be of a bird at all.

''Telmatornis'' from the Navesink Formation – also Late Cretaceous – is traditionally allied with the Charadriiformes and/or Gruiformes. However, a cladistic analysis of the forelimb skeleton found it highly similar to the great crested grebe and unlike the painted buttonquail (now known to be a basal charadriiform lineage), the black-necked stilt (a more advanced charadriiform), or the limpkin (a member of the Grui suborder of Gruiformes), namely in that its dorsal condyle of the humerus was not angled at 20°–30° away from long axis of the humerus. The analysis did not result in a phylogenetic pattern but rather grouped some birds with similar wing shapes together while others stood separate. It is thus unknown whether this apparent similarity to grebes represents an evolutionary relationship, or whether ''Telmatornis'' simply had a wing similar to that of grebes and moved it like they do.

True grebes suddenly appear in the fossil record in the Late Oligocene or Early Miocene, around 23–25 mya. While there are a few prehistoric genera that are now completely extinct; ''Thiornis'' (Late Miocene –? Early Pliocene of Libros, Spain) and ''Pliolymbus'' (Late Pliocene of WC USA – Early? Pleistocene of Chapala, Mexico) date from a time when most if not all extant genera were already present. Because grebes are evolutionarily isolated and they only started to appear in the Northern Hemisphere fossil record in the Early Miocene, they are likely to have originated in the Southern Hemisphere.

* Genus †''Miobaptus'' Švec, 1982
** †'' Miobaptus huzhiricus'' Zelenkov, 2015
** †'' Miobaptus walteri'' Švec, 1982 'Podiceps walteri'' (Švec, 1984) Mlíkovský, 2000* Genus †'' Miodytes'' Dimitreijevich, Gál & Kessler, 2002
** †'' Miodytes serbicus'' Dimitreijevich, Gál & Kessler, 2002
* Genus †''Pliolymbus'' Murray, 1967 'Piliolymbus'' (sic)** †'' Pliolymbus baryosteus'' Murray, 1967
* Genus †''Thiornis'' Navás, 1922
** †'' Thiornis sociata'' Navás, 1922 'Podiceps sociatus'' (Navás, 1922) Olson, 1995
A few more recent grebe fossils could not be assigned to modern or prehistoric genera:
*Podicipedidae gen. et sp. indet. (San Diego Late Pliocene of California) – formerly included in ''Podiceps parvus''
*Podicipedidae gen. et sp. indet. UMMP 49592, 52261, 51848, 52276, KUVP 4484 (Late Pliocene of WC USA)
*Podicipedidae gen. et sp. indet. (Glenns Ferry Late Pliocene/Early Pleistocene of Idaho, USA)

Phylogeny

To date there is no complete phylogeny of grebes based on molecular work. However there are comprehensive morphological work from Fjeldså (2004) and Ksepka et al. (2013) that has been done on the grebe genera.

Fjeldså (2004)

Ksepka et al. (2013)

Recent species listing

Compiled from the following websites: Extinct species assignment follows the Mikko's Phylogeny Archive and Paleofile.com websites. and subspecies names from '' English Names of Birds''.

Natural history

Habitat, distribution and migration

Grebes are a nearly cosmopolitan clade of waterbirds, found on every continent except Antarctica. They are absent from the Arctic circle and arid environments. They have successfully colonized and radiated into islands such as Madagascar and New Zealand. Some species such as the eared grebe (''Podiceps nigricollis'') and great crested grebe (''P. cristatus'') are found in multiple continents with regional subspecies or populations. A few species like the Junin grebe (''P. taczanowskii'') and the recently extinct Atitlan grebe (''Podilymbus gigas'') are lake endemics. During the warmer or breeding seasons, many species of grebes in the northern hemisphere reside in a variety of freshwater habitats like lakes and marshes. Once winter arrives many will migrate to marine environments usually along the coastlines. The species is most prevalent in the New World with almost half of the world's species living in this part of the world.

Feeding ecology

The feeding ecology of grebes is diverse within the group. Larger species such as those in the genus ''Aechmophorus'' have spear-like bills to catch mid-depth fish while smaller species such as those in the genera ''Tachybaptus'' and ''Podilymbus'' tend to be short and stout with a preference for catching small aquatic invertebrates. There are much more species of grebes that predate on aquatic invertebrates, with only a handful of large-bodied piscivores. The aforementioned ''Aechmophorus''is the most piscivorous of the grebes. Closely related species that overlap in their often avoid interspecific competition by having prey preferences and adaptations for it. In areas where there is just as a single species, they tend to have more generalized bill with more open preferences to different prey sources.

Breeding and reproduction

Grebes are perhaps best known for their elaborate courtship displays. Most species perform a duet together and many have their own synchronized rituals. Some like those species in the genus ''Podiceps'' do the iconic "penguin dance" where the male and female stand upright, breast posturing out and run along the water's surface. A similar ritual in other species is the "weed dance" in which both partners hold pieces of aquatic vegetation in their bills and positioned upright towards each other. Similarily there is the "weed rush" in which partners would swim towards each other, necks stretched out with weeds in their bill, and just before colliding do posture themselves upright and then swim in parallel. In the smaller and basal genera like ''Tachybaptus'' and ''Podilymbus'' there is some form of incorporation of aquatic vegetation in their courtship, but it is not as elaborate as the more derived and larger species. It has been hypothesized that such courtship displays between mates originated from intraspecific aggression that evolved in a way it strengthen pair bonds. Once these courtship rituals are completed, both partners would solicit copulation towards each other and would mount on these floating platforms of vegetation. Sometimes they become nests, although often they build a more well structured platform. Females lay two to seven eggs and incubation can last nearly a month. Chicks of the nest hatch asynchronously and once the whole nest has hatched, do the chicks begin to climb on one of their parent's backs. Both parents take care of rearing their young, and the duration of care is longer than those of waterfowl. This enables a greater success rate of survival for the chicks. One parent would dive while the other would watch the young on the surface.

Conservation

Thirty percent of the total extant species are considered to be threaten species by the IUCN. The handful of critically endangered and extinct species of grebe are those of lake endemics and nearly all of them are or were flightless. The extinct species consists of the Alaotra grebe (''Tachybaptus rufolavatus''), the Atitlán grebe, and the Colombian grebe (''Podiceps andinus''). These species went extinct due to anthropogenic changes, such as habitat loss, the introduction of invasive predatory fishes, and the usage of fishing nets that tangled birds in the lakes they once existed in. These are the same issues along with climate change that are happening to the Colombian grebe's closest relatives, the Junin grebe and hooded grebe (''Po. gallardoi'').

See also

*List of Podicipediformes by population

References

Further reading

*Konter, André (2001): ''Grebes of our world: visiting all species on 5 continents''. Lynx Edicions, Barcelona.
* Ogilvie, Malcolm & Rose, Chris (2003): ''Grebes of the World''. Bruce Coleman Books, Uxbridge, England.
* Sibley, Charles Gald & Monroe, Burt L. Jr. (1990): ''Distribution and taxonomy of the birds of the world: A Study in Molecular Evolution''. Yale University Press, New Haven, CT.

External links

Grebe videos and photos
on the Internet Bird Collection
Tree of Life Grebes

*openclipart.org
Grebes clip-art''Diving Birds of North America''
by Paul Johnsgard
Grebes Walk on Water
Documentary produced by Oregon Field Guide
Grebe Vocalization - The Songs of The Grebes
*

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Podicipedidae
Podicipediformes
Diving animals
Extant Chattian first appearances
Taxa named by Charles Lucien Bonaparte