Effects of a Severe Drought on the Population Biology of a Tropical Hummingbird

Abstract

At Finca La Selva, in the Caribbean lowlands of Costa Rica, the Long—tailed Hermit (Phaethornis superciliosus) has a long breeding season (December or January through August or September) during which flowers are abundant, followed by a severe but predictable season of flower scarcity in late October—November, during which most of the annual mortality normally occurs. In 1973, the third of a 4—yr population study of marked lek males, an unusual drought caused a severe flower shortage at the height of the breeding season. During the latter part of the breeding season lek activity of males was reduced, successful breeding by females was drastically curtailed, and masses of both sexes dropped to levels lower than those of the lean season. Interrupted molt, an indicator of energy stress normally seen only in a minority of breeding females, was observed in 25% of adult males, and 50% of females, 1973. Survivorship of lek males through the breeding season dropped from 90% in normal years to °60%, and the total lek population at the start of the 1974 breeding season was one—third lower than before the drought in 1973; the cohort of yearling males was most strongly affected. Recovery of numbers and age structure of the lek male populations to pre—1973 levels required 3—4 yr. Jolly—Seber analyses showed close agreement with survivorship as determined by direct observation (possible because of the lek social system), and permitted estimation of other parameters of interest: populations of marked and unmarked males, probability of recapture of marked males, and recruitment. The average annual survivorship of male P. superciliosus (in normal years) is near the mean value reported for a series of tropical species. A simple model of the reproductive output of female P. superciliosus, assuming no sexual differences in survivorship, suggests that a female breeding only in the prime breeding season (roughly February—July, when over 80% of the fledglings are normally produced) will no more than replace herself in the population; breeding throughout 8—9 mo will permit her to increase her reproductive output by up to 10%, which agrees well with the 3+ yr required by the (male) population to recover from the drought. Possible implications of these findings for tropical bird demography and conservation are discussed.