Demography of Gregariously Roosting Populations of the Nymphaline Butterfly Marpesia Berania in Costa Rica

Abstract

Populations of the daggerwing butterfly, Marpesia berania (Hewitson) (Nymphalidae; Nymphalinae), assemble nightly in roosts of up to 68 individuals in the tropical rain forest of the Osa Peninsula, Costa Rica. Gregarious nocturnal roosting in butterflies in uncommon, and reported almost exclusively in supposedly distasteful species. We studied two roosts, using a marking technique that permitted visual censusing. The butterflies maintained roosting sites in the same general locations and regularly returned to them before evening, during life spans of over 5 months. The sex ratio was unity, and the rates of population recruitment and mortality were apparently the same for both sexes. The instantaneous mortality rates were approximately 0.0126 (on a daily basis) over the study period, and the recruitment rates for the two roosts were 0.907 and near 0.15 butterfly per day for March and April, 1968 (the last month of the dry season and the first month of the wet season). In the following 3 months of the wet season, recruitment decreased to near zero. The empirically determined rates of recruitment and mortality accurately predicted the initial population size, indicating that the adult population was under equilibrium conditions during the dry season and the first month of the wet season.