1.6 million grebes and counting
by Sean Boyd
The Canadian Wildlife Service and Simon Fraser University have been studying the breeding biology of the Eared Grebe, Podiceps nigricolis, at Riske Creek (near Williams Lake), British Columbia, for the last 10-12 years. Work by Dr. Joseph Jehl Jr. suggested that up to three quarters of a million Eared Grebes staged and molted on Mono Lake each fall. In an attempt to determine to what extent our grebes used Mono Lake, I marked birds with radio transmitters at Riske Creek in the summers of 1995 and 1996 and found that about 50% of these marked birds were on Mono Lake each fall.
While tracking birds at Mono Lake, I noticed that the grebes were easy to photograph from a small aircraft. So, I developed a cost-effective protocol to estimate total abundance; this involves flying east-west transects across the lake at about 1200 ft above the lake and taking vertical photos every 10 seconds. The protocol results in a good estimate of abundance with little error.
I have conducted photo surveys in conjunction with Dr. Jehl every October since 1996. The number of Eared Grebes on the surface has varied from 0.8 million to 1.6 million birds. Data from radio-marked birds suggested that the birds spend about 15-20% of their time foraging under water in October so total abundance on Mono Lake has varied between 1-2 million birds. We estimate that Mono Lake and Great Salt Lake together support more than 90% of all North American grebes in fall. A large die-off of grebes apparently occurred in Mexico during the winter of 1998, and our counts at Mono Lake (and Great Salt Lake) may reflect this die-off. Hence, consistent counts at Mono Lake may be a good way to track the health of a large proportion of the North American population of Eared Grebes.
Sean Boyd works out of the Pacific Wildlife Research Centre in British Columbia with the Canadian Wildlife Service.