Species: Podilymbus podiceps

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). <br><br>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). <br><br>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). <br><br>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. <br><br>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). <br><br>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).