Podicipediformesgrebes

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Podicipediformes comprises one family (Podicipedidae), six genera, and 22 species (two extinct: Podilymbus gigas, Podiceps andinus).

Grebes are distributed almost worldwide (absent from the Arctic, Antarctic, some oceanic islands).

Grebes inhabit freshwater ponds and lakes, and slow flowing rivers (up to 3000m altitude). Northern populations migrate to large inland bodies of water, estuaries, or coastal waters for the winter months.

Grebes are stocky to narrow-bodied, diving waterbirds. Plumage is counter-shaded: dorsal coloring brown or gray to black and ventral coloring white or light. Sexes are similar with females often smaller in body and bill size. Sexually dimorphic breeding plumage may include bright head or neck coloration or plumes. Grebes are medium to large (22-76 cm; 100-1600g), with short to moderately long bills, short wings (12 primaries; 15-21 secondaries), and rudimentary tail lacking stiff retrices. Iris may be scarlet, yellow or brown. Other traits include: schizognathous palate; diastaxic (fifth secondary absent); holorhinal (nostril entire, cleft shallow); pervious nares (nasal septum absent); 3-6 fused thoracic vertebrae and a bilobed tufted oil gland. The blue-black colored legs are placed far back on the body; tarsi are laterally compressed. The anterior three toes are individually lobed, elevated hallux is present in most species. Toenails are wide and flat; middle toenail is pectinate (comb-like).

Grebes prey on fish, aquatic insects, crustaceans, mollusks and small vertebrates. Prey fish include: eels (Anguilla anguilla), roach (Rutilus rutilus), tench (Tinca tinca), minnow (Phoxinus phoxinus), trout Salmo trutta), perch (Perca fluviatilis), herring (Clupea), pipefish (Signathus), blenny (Zoarces), goby (Gobius), and cod (Gadus). Insect prey items include: mayflies (Ephemoptera), stoneflies (Plecoptera), dragonflies (Odonata), waterbugs (Hemiptera) and beetles (Carabidae, Ditiscidae). Other invertebrate prey species include: mollluscs, snails (Lymnaea, Valvata), shrimp (Gammarus, Artemia) and crayfish (Astacus).

Predators of grebes include humans and ferrets (Mustela furo) gulls (Larus dominicanus), falcons, hawks, coots, terns, crows, bass (Micropterus spp.) and pike (Esox spp.)

Grebes are seasonally monogamous. Either sex may initiate courtship, which often entails elaborate water displays and distinctive advertising calls. Courtship may involve mirroring-like behavior including: swimming synchronously, rearing out of the water, or rushing side by side along the water surface. Pair-bonds may be initiated in winter or spring and are often seasonal.

Both sexes help construct the nest and incubate (21-30 days; beginning after one or two eggs are present). Upon leaving the nest adults often cover eggs with nest material. Both sexes feed, brood and carry chicks on their backs. Chicks are covered with dense down with longitudinally striped dorsal pattern (patterning retained until the post-juvenile molt). They have precocial mobility and sensory organ development, but depend on adults for food, warmth, and protection. Chicks show elaborate begging and appeasement behaviors.

Young become independent at fledging (6-12 weeks) and breed when one or two years old. Some grebes moult flight-feathers simultaneously prior to or at the end of the breeding season, while others have moult migrations.

The slightly concave nest of aquatic vegetation floats in shallow water while anchored to rooted aquatic plants. Nests of territorial grebes are usually well dispersed and concealed, while colonial breeders (up to several hundred pairs) may have nests one meter apart. Eggs are white or cream colored and clutch size ranges from two to seven eggs. Females lay one egg every one to two days until a clutch is complete. Some species raise two or three broods per season. Some breed year round while others breed seasonally (three to six month period). Seasonal breeding may synchronize with annual flooding or growth of emergent vegetation.

Grebes are excellent divers and powerful underwater swimmers (foot propelled). However, with legs placed far back on the body (enhancing pursuit diving), grebes are unable to move quickly on land. To become airborne, grebes rapidly beat their wings while running across the water. The Short-winged Grebe (Rollandia micropterum), and the two extinct grebes are flightless. Grebes may dive when alarmed and reappear a short distance away. Grebes swallow molted contour feathers, which may help in regular pellet formation, perhaps functioning to reduce gastric parasite loads.

During the breeding season grebes are found in pairs or family groups. Non-migratory species may form loose flocks during non-breeding season. Wintering migratory birds are gregarious, forming flocks of hundreds or thousands at migration stopovers and wintering areas.

While some grebes are highly vocal, others are nearly silent even during the breeding season. Many grebes have a repertoire of 10-12 calls. Vocalizations range from whistles and wails to beeps and bamps.

Today, grebes are hunted or trapped primarily for subsistence. At the turn of the century however, tens of thousands of grebes were shot for their "fur".

The evolutionary history of grebes remains unclear. Analyses of morphological characters have suggested that grebes may be related to divers (gaviids), and together form a group of foot-propelled divers related to Sphenisciformes (penguins). Contrastingly, DNA hybridization suggests that grebes are sister to a diverse assemblage of waterbirds comprising: tropicbirds, cormorants, pelicans, herons, New World Vultures, storks, penguins, loons, and tube-nosed birds. Other studies note that grebes have neck musculature and skull morphology similar to gruiform birds (cranes, rails, and allies).

Fossils of grebes described from the Tertiary period include: an early Miocene fossil from Oregon (Podiceps oligocaneus); a Miocene fossil from Spain (Thiornis sociata); and a lower Miocene fossil (Miobaptus walteri) from the former Czechoslovakia.

Carroll, R. 1988. Vertebrate Paleontology and Evolution. W.H. Freeman and Company, New York.

Cracraft, J. 1982. Phylogenetic relationships and monophyly of loons, grebes, and hesperornithiform birds, with comments on the early history of birds. Syst. Zool. 31:35-56.

Feduccia, A. 1999. The Origin and Evolution of Birds, 2nd edition. Yale University Press, New Haven.

Gill, F. B. 1995. Ornithology, 2nd edition. W.H. Freeman and Company, New York.

Proctor, N. and P, Lynch. 1993. Manual of Ornithology. Yale University Press, New, Haven.

O'Donnel, C. and J. FjeldsÂ. 1997. Grebes -- Status Survey and Conservation Action Plan. IUCN/SSC Grebe Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK.

Sibley, C. G. & J. E. Ahlquist. 1990. Phylogeny and Classification of Birds,A Study in Molecular Evolution. Yale Univ. Press.

Storer, R. W. 1979. Order Podicipediformes. Pp. 140-155 in Check-list of birds of the world, vol. 1 , 2nd edition. E. Mayr and G.W. Cottrell, eds. Mus. Comp. Zool., Cambridge, MA.

Storer, R.W. 2000. The systematic position of the Miocene grebe Thiornis sociata Nav·s. Ann. PalÉontol. 8

Contributors

Laura Howard (author), Animal Diversity Web.

Glossary

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

visual

uses sight to communicate