Summary

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Evers, D.C. 1993. Northern great lakes common loon monitoring program. Final report submitted to the Minnesota Department of Natural Resources. 61+ pp.

Summary:

There were multiple objectives in this study of the common loon in the Great Lakes including investigation of environmental contaminants, capture and banding of Gavia immer, study of biology of Gavia immer, and study of ecology of Gavia immer. A standardized population methodology was used to conduct counts during the spring and fall migrations at various study locations in Minnesota, Wisconsin, Northern Wisconsin and Michigan. A collaborative study occurred in New Hampshire and Ontario. Since 1987, a span of 7 years, a total of 572 Common Loons have been banded under this program.

There is a disparity in the sex ratio for recaptured loons (n=46), nearly two times the number of males are captured from a pool of a relatively equal number of banded females and males. Loon pairs exhibit a strong sexual dimorphism, averaging 931 +/- 382 grams (n=121).

The main purpose for this program was to investigate the population ecology of the Common Loon using a landscape ecology approach: the adult minimum survival rate or annual return rate as a key component. 1993 marked the second year with a significant sample of potential returnees. Results showed an overall 75% (246 of 328) return rate for marked adults. In nearly all cases, the non-returning mate or pair are replaced by another adult. The quick replacement process may be due to a high degree of inter- and intra- year mate switching; a possible reproductive strategy that occurs whether or not a mate returns. The trigger for mate switching in loons appears to be nest failure. Generally, an established pair produces fledged chicks every third year. Family units typically remain intact through mid-September or until the juveniles can fly. Breeding pairs that are unsuccessful begin to wander from their territories beginning in mid July.

Most adult loons move to the Great Lakes and congregate in flocks until early November, although some remain through the winter (most of these are probably injured). Loons continue to move around the Great Lakes sometimes forming movement patterns that peak in September. The general pattern is an east to west one. Young-of-the-year begin to arrive in early September, most leave their natal lakes in mid October; all are by the beginning of November. Most of the Great Lakes population migrates east to Chesapeake Bay, arriving in late October and early November. By late November, loons are moving south and dispersing along the southern Atlantic shoreline.

In late January, the adult loons begin a complete molt including a simultaneous remigial molt; it lasts through early March. Adults maintain territories during their flightless period while subadults (1/2 to 2 1/2 year olds) are not as territorial; subadults do not start the complete molt until June.

Adult loons move from the shores of Alabama and Florida in mid March arriving in the southern Great Lakes Region at ice off in mid to late March. Adults arrive on territory in mid to late April; loons arrive as soon as the ice clears. The annual return rate for adult loons is 75%. Nearly all loons (98%) return to their same territory (there are only 4 known cases of territory switching, n=197 territories with banded loons). Upon arrival, 2-3 weeks are spent foraging and establishing territorial and pair bonds. Each year the subadult returns it remains on its breeding territory longer, and spends less time on the Great Lakes (400-500 loons are killed each year by commercial fishery nets). At 5-7 years, it is reproductively ready to defend a territory, pair bond, and attempt to nest.

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