eWestern Birds

The Quarterly Journal of Western Field Ornithologists

Vol. 34, No. 4
December 2003
Western Field Ornithologists


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Molt, Plumage, Body Mass, and Morphometrics of a Population of the White-throated Swift in Southern California
Manuel Marín

ABSTRACT: Over a 10-year period, I examined molt patterns, plumage, and body-mass changes in a population of White-throated Swifts in the Mecca Hills, Riverside County, California, by sampling birds at regular roosts in each month from February through November. The swifts used the roosts year round but numbers at them declined in March then increased in June and July, after breeding. As a result of plumage wear, the juvenal plumage can be categorized in three “phases”: prior to fledging, after fledging, and prior to the first prebasic molt. I found no sexual dimorphism in wingspan, wing length, tarsus length, tail length, culmen length, or body mass. I found an average difference between the sexes in the depth of the tail fork, but because of much overlap it cannot be used to determine the sex of individuals. The maximum difference between highest and lowest body mass was 28.7%, less than reported for other species. Testis size began to increase in March, reaching a peak in April. Primary molt lasted 6–7 months, from May through November. Tail molt lasted about 2.5-3 months, from June through August. Breeding began at this desert site one to two months earlier than at a coastal site. The overlap of breeding and molt in this desert population appears to be less than reported for other species of swifts.

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