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Common Loons: Threats and Migration

Mike Yates and a Common Loon

Mike Yates and a Common Loon

ABSTRACT

Walker Lake, in west central Nevada, is a troubled desert terminal lake with an uncertain future. There is significant mercury contamination of the migrating common loons using the lake each spring and fall, and of the principal native fish species on which these loons feed. Sampling conducted in the Walker River Basin indicates that historic gold mining activities within the basin may be the principal source of contaminants. Furthermore, decreased water flows, largely caused by diversion for agricultural use over the past hundred years, have greatly reduced the volume of Walker Lake and degraded its water quality.  This, in turn, has reduced available food resources at the lake for loons and is seriously threatening the lake’s fishery. Mercury contamination and the possible loss of Walker Lake’s fishery could have a devastating effect on bird species that rely upon the food resources provided by the lake; the former is also a potentially serious threat to public health. Each spring and fall, large numbers of common loons have historically migrated to Walker Lake, spending up to a month resting and feeding before continuing their journey. Although we have begun to define the breeding areas of the loons that use Walker Lake, our initial efforts to define their wintering grounds were unsuccessful. Thus, we cannot presently estimate the effects that contamination and the loss of these food resources would have on the western North American loon population.  Likewise, planning management strategies that might mitigate the effects of contamination and loss of food is impractical without first determining the importance of Walker Lake in the ecology of western loons.  By capturing and radio marking sentinel individuals with satellite-received telemetry, we have begun to study the movements of loons and thus provide managers information to address the implications of the loss of this at-risk migratory cohort. Loons, in turn, will serve as a model for considering the potential effects of these threats to the other migratory birds that use Walker Lake. We have, to date, tracked four radio marked loons to presumed breeding grounds in west central Saskatchewan. Mean blood mercury levels in 104 loons we captured at Walker Lake in 1998-2004 are the highest documented anywhere in the United States. Inquiries to date suggest mercury levels in fish at Walker Lake likely contribute to the contamination in the loons. [Loon Final Report Summary]