Phenological relationships of wasps, bumblebees, their mimics and insectivorous birs in northern Michigan

. 1. In northern Michigan 72% of the high fidelity dipterous Batesian mimics of bumblebees and vespoid wasps occurred in spring while they still outnumbered their models but before the great majority of fledgling birds left their nests to begin foraging for insects on their own. 2. This extends the known occurrence of this sort of phenology to several additional mimetic species and to an area which is radically different climatically and ecologically from the central Illinois areas in which this sort of phenology was first noted. 3. Our observations confirm the hypothesis that Batesian mimics may be selected to avoid the midsummer period when newly fledged insectivorous birds are abundant but have not yet learned to shun their stinging hymenopteran models. 4. The shortness of the warm season in Michigan reduces opportunities for temporal separation and may have forced the other 28% of the high fidelity mimics to occur when naive birds are abundant. The threat from naive birds was, however, presumably somewhat ameliorated by the abundance of models at that time.