CLUSTERING PATTERNS OF MONARCH BUTTERFLIES (LEPIDOPTERA, DANAIDAE) AT 2 CALIFORNIA CENTRAL COAST OVERWINTERING SITES

The behavioral ecology of monarch butterflies, Danaus plexippus (L.), was studied during the 1988-1989 and 1989-1990 overwintering seasons at two central coastal sites in San Luis Obispo County, Calif., that differ in stand composition. In both seasons, patterns of tree use were similar at both sites; fewer trees supported clustering individuals as the season progressed, even though population levels remained constant. Tree use was not random, and as few as 3% of the available trees accounted for 97% of the clustering. A method of describing the location of clusters on specific trees was developed. Roosting individuals were consistently found on the southern exposures of trees, and cluster locations varied neither diurnally nor seasonally. Microclimate variables such as solar radiation, temperature, and wind were not good predictors of clustering position on trees within sites.