Podilymbus podiceps

A small, stocky, and poorly buoyant waterbird, 31-38 cm in length (Cramp 1977), with small, narrow wings, feet placed far back, and a blunt-ended posterior. During the non-breeding period, the bill is unmarked, the throat is white, and the white rear becomes more conspicuous. As adults, sexes are alike, whereas juveniles are distinguished by the lack of a white orbital ring, an unmarked bill, darker brown sides of the head and neck, and a whiter underbelly (Palmer 1962). Downy chicks have a striking, zebra-like pattern of black and white stripes, interspersed with reddish-brown spots, and a bare loral area (Palmer 1962, Storer 1967).

VOCALIZATIONS: During territorial defense, males emit a distinctive prolonged call, a loud "cow-cow-cow-cow-cow- cowp...cowp...cowp..." This call is reminiscent of cuckoos (Palmer 1962), and enables communication over several hundred meters in nesting habitats dominated by dense, visually restricting vegetation. A variety of other calls are also produced during the breeding season (see Palmer 1962), but during the non-breeding season generally silent.

NEST: Build sodden, floating nests of rotting and green plant material and mud. Often anchored to growing, emergent plants. In Iowa, 138 nests averaged 38 cm in diameter (Glover 1953).

EGGS: elliptical to subelliptical, approx. 44.3 x 30.1 mm, smooth and nonglossy (Harrison 1978, Arnold 1989). Although white or tinted bluish or buff when laid, eggs gather a heavy, brown stain from the wet, organic matter comprising the nest.

MOLT: Molt is poorly known. Palmer (1962) noted that a complete molt into basic plumage takes place in autumn, with considerable individual variation in its timing and duration (Cramp et al. 1977). Flight feathers are lost simultaneously, prior to loss of body feathers. Complete molt takes one to two months. Basic molt and late nesting may occur simultaneously (Otto and Strohmeyer 1985). A partial molt into alternate plumage occurs in spring (Palmer 1962).

A short, stout, chicken-like bill with a broad, black band in the middle, large head and elongated neck, white orbital ring, black throat patch and forehead, and drab, brownish plumage throughout except for white under the tail (Palmer 1962, Cramp 1977).

BREEDING: In eastern U.S., occurs in ponds, sloughs, and marshes, in marshy inlets and along edges of rivers, lakes, and reservoirs, and occasionally in estuarine wetlands (Palmer 1962, Chabreck 1963, Cramp et al. 1977, Andrle and Carroll 1988). Nests are typically built in shallow water surrounded by dense vegetation, especially cattail (TYPHA spp.) and bulrush (SCIRPUS spp.), and are farther from shore than from open water (Glover 1953, Stewart 1975, Faaborg 1976, Sealy 1978, Forbes et al. 1989). Wind and waves are major threats to floating nests and surrounding emergent vegetation acts as a wave break, anchors the nest, and conceals the nest from predators (Forbes et al. 1989). Because the direction of wind and waves shifts frequently during the nesting season, sheltered nesting sites can be limiting (Faaborg 1976). In Nova Scotia, avoided nesting on edges of stands of emergent vegetation that were exposed to wave action, and nest-site selection was related to structure but not type of vegetation available (Forbes et al. 1989). In comparison to randomly chosen marsh locations, nests were characterized by greater distance from shore, increased proximity to open water, and deeper water (Forbes et al. 1989).

Microhabitats at Manitoba wetlands included the densest and tallest stands of emergent vegetation available, particularly those in deeper portions of ponds (Nudds 1982). In Iowa, always associated with dense stands of emergent, littoral vegetation, and avoided wetlands with 100% open water (Faaborg 1976). On moist-soil impoundments in Missouri, habitat use was associated with water > 25 cm deep and vegetative cover characterized as "open, sparse, or short" (Fredrickson and Reid 1986). Grebe use was not associated with shallower waters or "dense" or "rank" emergent vegetative cover (Fredrickson and Reid 1986).

NON-BREEDING: Habitats in winter and migration similar to breeding areas (Cramp 1977), but many shift to more exposed areas on brackish, estuarine waters or sheltered inlets on large lakes, rivers, and salt water (Palmer 1962). Root (1988) noted that the densest overwintering populations occur on wide rivers and large lakes.

Residents in pairs or family groups; more gregarious in winter (Stiles and Skutch 1989), with groups commonly including 100 or more. Generally one nesting pair on ponds up to 4 ha, but sometimes many more. One study recorded defended area of 46 m radius around nest, though nests sometimes closer than this (Johnsgard 1987). Mean distance between successful nests 55 meters (n=96; Chabreck 1963). Average home range 1.3 hectares (n=44; Glover 1953), but some as large as 35 hectares (Muller 1995).

Little is known about sources of mortality. Avian botulism, avian cholera, and gizzard worms are known to occur in grebes (Friend 1987). Predators include cottonmouths (AGKISTRODON PISCIVORUS) (Leavitt 1957), peregrine falcons (FALCO PEREGRINUS) (Buckalew 1948), and American alligators (ALLIGATOR MISSISSIPPIENSIS) (Delany 1986). Coulter (1957) reported substantial predation by snapping turtles in Maine.

Sometimes associate with other birds to enhance foraging opportunities. Have been observed in mutualistic foraging associations with snowy egrets (EGRETTA THULA) in Virginia (Leck 1971), snowy egrets and tricolored herons (HYDRANASSA TRICOLOR) in North Carolina (Mueller et al. 1972), and boat-tailed grackles (QUISCALUS MAJOR) in Mississippi (Jackson 1985).

Single- or double-brooded and lay two to ten eggs, usually six to eight eggs per clutch (Sealy 1978, Forbes et al. 1989). Eggs are laid daily. Incubation is initiated after the fourth egg is laid, and occurs during about 90% of a given day (Forbes and Ankney 1988). Incubation is shared equally between sexes during laying and post-laying periods, although females spend more time incubating around hatching (Forbes and Ankney 1988), which occurs at about 23 days (Palmer 1962). Begin incubation before completing the clutch (Cramp et al. 1977) leading to considerable hatching asynchrony among the brood. Two to four eggs generally hatch on the first day of hatching, and the remaining eggs hatch daily over a period of three to seven days (Forbes and Ankney 1987, 1988). A detailed description of the hatching muscle and its development is given by Fisher (1961). The first eggs laid are about 8% lighter than subsequent eggs within clutches (Forbes and Ankney 1988), but variation in egg weight probably has little effect on the vigor of individual hatchlings (Arnold 1989).

Adults usually divide brood. Age at first flight has been estimated at 35 days (Kirby 1976, Forbes and Ankney 1987). Age at first breeding may be as early as 13 months (MacVean 1990). Solitary nesters and defend relatively small territories of as little as less than two ha (Glover 1953) that provide food, cover, and nest sites. Territorial birds also sometimes forage outside their defended areas. Highly territorial and usually only one pair nests at a wetland (Faaborg 1976, Sealy 1978). Wetlands more than five ha, however, may support more than one pair (Palmer 1962, Faaborg 1976), and large marshes with suitable habitat support multiple pairs (Chabreck 1963).

NESTING PERIOD: Initiation of nesting activity varies throughout the range, occurring as early as April and as late as June, and peaking in May in most areas. Examples of nesting periods are 3 May to 10 September for 107 nests in Louisiana (Chabreck 1963), 2 May to 8 August for 138 nests in Iowa (Glover 1953), and 3 May to 22 August in Ontario (Johnsgard 1987). Although some pairing may occur on wintering areas (Palmer 1962), courtship begins soon after ice-out following arrival at nesting areas. Courtship behavior is mutual and less formalized than other species of grebes (Palmer 1962).

NESTS AND EGGS: Both sexes build nests and may add plant material and mud as the season progresses and as nests slowly sink (Fjeldsa 1975). Air-pockets in green plants and trapped gases generated by the fermenting and rotting vegetation give the nest buoyancy. The floating, rotting nest generates substantial quantities of heat, and many aspects of reproduction may be related to their use of a warm, humid nest (Davis et al. 1985). Nests have a hollow to hold the eggs, and may extend 90 cm below the surface but only eight cm above (Glover 1953). The eggs have a threefold increase in pore density, compared to other birds' eggs, which enables the eggs to lose sufficient water within the humid confines of the nest prior to hatching (Davis et al. 1985). When leaving the nest, adults cover their eggs with plant material, and the rotting nest, where temperatures may remain 11-13 degrees Celsius higher than the surrounding water, can provide enough heat to incubate the eggs in the adults' absence (Davis et al. 1985). Time constraints imposed by incubation may thereby be lessened, providing adults with more time for foraging and territory defense.

NESTING SUCCESS: High nest success has been reported in many areas: 70% of 138 nests in Iowa (Glover 1953), 77% of 150 nests in Wisconsin (Otto 1983), 90% of 107 nests in Louisiana (Chabreck 1963), and 90% of 115 nests in Nova Scotia (Forbes et al. 1989) hatched one egg or less. Wind and high waves, fluctuating water levels, and predation can be significant sources of nest loss. Of 42 nests in Manitoba, 69% failed, mostly due to flooding from high waves (Sealy 1978). Half of total nest loss in Iowa was due to wave action or water level fluctuation and 25% to raccoon (PROCYON LOTOR) predation (Glover 1953). In Nova Scotia, nest loss resulted from predation, including crows (CORVUS BRACHYRHYNCHOS) and poor weather (Forbes et al. 1989). In comparison to clutch sizes, observations of relatively small broods, e.g., averaging 4.4 (Chabreck 1963) and 2.9 (Yocum et al. 1958), suggest that substantial chick mortality occurs. Snapping turtles (CHELYDRA SERPENTINA) may represent important predators of young (Coulter 1957). Females are indeterminate layers (Fugle and Rothstein 1977), and frequently replace lost clutches, usually renesting within 50 m of destroyed nests (Glover 1953, Forbes et al. 1989).

CHICK REARING: Adults usually divide broods and provision chicks with a variety of small-sized prey, including dragonfly naiads, dytiscid beetle larvae, leeches, and salamanders (Forbes and Ankney 1987). Chicks usually remain near parents, and frequently ride on the backs of adults, even during foraging dives (Forbush 1925). Initial size disparities of chicks, due to asynchronous hatching, influence food allocation within broods. Aggression among chicks is high when rates of food-delivery by adults are low, and larger chicks win more disputes over food than smaller chicks (Forbes and Ankney 1987). The bare loral area of chicks changes from dull-colored to bright crimson in hungry chicks, however, and adults may use this indicator of nutritional status to allocate food among members of a brood (Forbes and Ankney 1987). Two unusual forms of chick provisioning occur: for unknown reasons, chicks are occasionally fed by adults other than pair members (Forbes 1987), and young grebes from first broods may feed young from second broods (Cramp et al. 1977).