Temporal allopatry of models and Batesian mimics is made possible by differences in the longevities and life histories of mimics, models, and insectivorous birds, as well as by the birds' long retention of the learned avoidance response to aposematic insects. Thus, rather than coinciding with the peak abundance of their models in midsummer, mimics may occur when resources such as oviposition sites and nectar and/or pollen plants are more abundant. It has also been hypothesized that mimics can gain a selective advantage by avoiding the threat in midsummer from naive fledgling birds that are just learning to avoid the models. In a mesophytic and a xerophytic forest in central Illinois, the great majority of high-fidelity mimics of stinging Hymenoptera clustered in the spring, whereas most of the rest clustered late in the summer. Their models were least abundant in spring, but very few young birds had as yet fledged then. The models were on the decline late in the summer, but most young birds had fledged by...