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The common loon in Minnesota

, : The common loon in Minnesota. Occas Papers Minnesota Mus Nat Hist 5: 1-76

Gavia immer chicks have 2 downy plumages which, in 8-10 weeks, are replaced by a juvenal plumage similar to the winter plumage of adults. All loons migrating northward through Minn. were in adult breeding plumage. The postnuptial molt occurs on the breeding grounds in late Aug., Sept., and early Oct. No evidence of the loss of flight feathers or of flightless birds was seen during this period. The prenuptial molt occurs in late March and April on the wintering grounds and apparently includes a flightless period. The measurements and weights of 17 loons taken in Minn. show considerable variation, which makes subspecific detn. doubtful. The measurements of 15 loon chicks and 30 eggs are reported. Characteristic attitudes, such as the "rolling preen" and "peering" under water, are descr. The chicks performed all of these when only a few hours old. The usual dive seldom lasts more than a minute, although submergences of several minutes have been noted. Extreme dives of 200 feet and over in depth have been recorded. Four basic types of calls (tremolo, yodel, wail, and talking) are descr. In addition to registering alarm, annoyance, greeting and summons, calling was an integral part of displays. The young gave a single note, "peep," during early life, but acquired the adult calls by early fall. Diurnal displays stem from courtship, territorialism, and defense of the young. Some displays appear to be used for more than one purpose. The intensity of display varied with individuals, especially in relation to defense of the young. A peculiar flight display was observed later during the summer. Territorialism is exhibited by pairs as evidenced by defense of a certain area constantly inhabited during the nesting season. 52 such territories were located in the study area. Territories varied in size from a small bays of 15-20 acres up to entire lakes of over 100 acres. The territories were gradually abandoned late in the summer as the cruising radius of the young increased. Unsuccessful pairs usually left their territories after midsummer. Fifty out of 54 nest sites were found on small islands, only 4 on the mainland. The sites varied from muskeg to rock ledge and were usually well sheltered from prevailing winds. All nests found were originally placed as close as possible to the water's edge, a majority of them under some form of cover. Nest sites may be used over several years. As incubation progressed, nests were built up gradually, the extent of nest construction varying with the availability of materials in the vicinity of the nest site. The clutch size was 2 in 26 nests and one in 21 nests. Since most of the latter were destroyed, a majority of them may represent incomplete clutches. Nest establishment reached its max. between June 15 and June 28, 1950. During the 29-day incubation period, both birds of a pair shared the incubation. Attachment for the eggs increased as incubation progressed. Thirty-five of 57 nests failed during the 1950 season. Predation and desertion were the most important decimating factors. Twelve of 27 pairs which lost eggs renested at least once, 3 pairs renested twice. Fifteen 2 egg nests and a single 1-egg nest were known to have hatched July 11-26, the peak of the hatch occuring July 20-24. One or both adults were always found with the young The young were fed by the parents for at least 4-6 weeks. The chicks learned to dive and swim well within 2 weeks, the spectacular defense display display of the adults lessening as greater dependence was placed on the chick's diving ability for escape. Flight was achieved 10-11 weeks after hatching. Four chicks died while hatching, 4 disappeared during the first week, and 2 more within 2 weeks. No further mortality was noted. Only 1 brood was known to fail completely, 7 remained intact, and 8 lost one of a set of twins. Out of a theoretical possible production of 84 young, 42 nesting pairs of loons successfully reared only 21 young. The low productive potential, with the high rate of nest failure and the mortality during hatching and the week following being important direct sources of loss. In the 60-sq.-mi. study area the population during the nesting season was estimated to be between 109 and 114 adults (42 nesting pairs, 10 non-nesting pairs, and 5-10 single loons). During the brood season this rose to between 128 and 143 individuals, or slightly over 2 per square mile. By mid-Oct. 20-25 adults and 7-10 young remained in the area.

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