By far the most abundant member of its family in North America and, indeed, in the world, the Eared Grebe breeds widely through the interior of the western United States and Canada, preferring shallow alkaline lakes and ponds, where it feeds primarily on small invertebrates. Highly social, its colonies may number into the low thousands. Like other grebes, it uses distinct advertising calls for sex recognition and to initiate elaborate breeding displays. Both sexes work together to construct sodden platforms and nests of vegetation heaped into a floating mound and attached to aquatic vegetation. Incubation starts with the first egg, resulting in asynchronous hatching of chicks. Parents take turns carrying young on their back while the free parent brings food until the young can swim and forage on their own. Parents divide surviving members of the brood after about 10 days and usually cease parental care by 20 days after hatching.
Despite a general similarity to other grebes, the Eared Grebe is highly distinctive and ranks among the most remarkable members of the North American avifauna. For most of the year, it vies with Wilson's Phalarope (Phalaropus tricolor) as the species most closely associated with highly saline environments. Immediately after the breeding season, most of the population, adults and juveniles, moves either to Mono Lake, CA, or Great Salt Lake, UT, to exploit the superabundant crops of brine shrimp and alkali flies that thrive in those waters. The predictable use of these permanent hypersaline lakes in the Great Basin contrasts with the frequent and often unpredictable shifts in location of Eared Grebe breeding colonies.
While at fall molting/staging lakes, adults more than double their arrival mass and allow the pectoral muscles to atrophy below a size that allows flight. Then, during a hyperphagic period, they greatly increase the size of the organs involved in digestion and food storage. These changes in size and proportions—the most extreme yet known for any bird—are then reversed during a brief predeparture period, when the birds catabolize much of their just-deposited fat, increase heart size, and reduce digestive organ mass to perhaps 25% of peak in preparation for a nonstop flight to wintering areas. The function of the predeparture events is to reduce wing-loading while maximizing flight range and performance. Migration occurs shortly after food supplies run out, typically in December–January. As a result, the Eared Grebe is the latest of the North American migrants to move to its winter stations. Because a similar atrophy/hypertrophy cycle is repeated 3–6 times each year, the Eared Grebe has the longest flightless period of any volant bird in the world, perhaps totaling 9–10 months over the course of a year. At fall staging areas alone, flightless periods average 3–4 months for adults and may reach 8 months or more in nonbreeders.
Many aspects of Eared Grebe biology have been elucidated: breeding biology (McAllister Mcallister 1963 , Mcallister 1958 , Breault 1990 , Boe Boe 1992b , Boe 1993 , Boe 1994 , Burger and Gochfeld 1995 , Cullen 1998 ); molt ( Storer and Jehl 1985 ); postbreeding biology ( Jehl 1988 ); migration and energetics (Jehl Jehl 1993 , Jehl 1994d , Jehl 1997 , Jehl et al. 1999 ); mortality (Jehl Jehl 1993 , Jehl 1996 ); body composition ( Jehl 1997 ); breeding behavior and vocalizations ( Nuechterlein and Buitron 1992 , Brua 1993 ); parasites ( Stock 1985 , Storer 2000 ); systematics (Storer Storer 1963a , Storer 1967b ; Fjeldså Fjeldså 1981 , Fjeldså 1982a ); and population size ( Boyd and Jehl 1998 , J. Jehl, S. Boyd, D. Paul unpubl.). Information from the wintering grounds remains sparse. The species is much less common in the Old World, where it breeds widely in the Palearctic, with isolated populations in Asia and Africa. As elsewhere, it prefers hypersaline conditions in the nonbreeding season ( O'Donnel and Fjeldsa 1997 ). Whether it undergoes a molt migration, as in North America, remains to be determined.