Seasonal Variation in Body Size:
A Clue to Migration?
Although the Red Admiral (Vanessa atalanta) does not vary much geographically, it does have distinct summer and winter forms (Opler and Krizek, 1984). The summer form is distinctly larger than the winter form. The summer form may also appear more strongly colored, with an interrupted forewing band which may distinguish it from the duller winter form (Opler and Krizek, 1984).
The Red Admiral has two broods per year throughout most of its range (Opler and Krizek, 1984). Only one generation per year may occur in the northernmost United States and Canada, while three may be possible in the southern states (Opler and Krizek, 1984). Scott (1986) says that four or more broods may be active nearly all year in south Florida, South Texas, and lowland California.
In Iowa, where there are two broods each year, the smaller winter and larger summer forms alternate. The smaller winter butterflies emerge beginning in late August, but are then non-reproductive. They typically spend September and October nectaring from flowers during the day, storing fat reserves to prepare to migrate south or hibernate. (The proportions of Red Admirals which migrate from vs. hibernate in particular areas are not clearly known.) After overwintering, the winter forms re-appear in early April to mid-May, when the weather has warmed enough for them to become active. Then they become reproductively mature, and the males begin perching and territorial behavior. Eggs from these females soon hatch into larvae, and eventually become the larger summer brood, which typically emerges in mid-June. These butterflies stay active until late August or early September, when territorial and mating behavior ends for the year. Their offspring become the next brood of winter forms.
What happens in other areas where there is an odd number of broods? In Britain, for example, there is a single reproductive brood of smaller individuals from March through September, followed by a non-reproductive fall brood of larger individuals which , evidence suggests, migrate south (Baker, 1968, pp. 428-430). One might suspect that their descendants eventually migrate back north to repopulate Britain next year. But the butterflies found in North Africa during March and April are even smaller than those occurring in Britain in May and June. Baker (1968) therefore said that the British spring butterflies could not be from this same population, and thus probably did not migrate northward from North Africa. Instead, he said, they probably hibernated.
This interpretation has its problems, too. The non-reproductive fall butterflies are larger than the butterflies seen in Britain the following spring. It seems unlikely that these are the same individuals either. Might the British spring butterflies have migrated back from an intermediate location?
According to Baker (1968, p. 429), Red Admirals in North Africa are free-flying and reproductive throughout the winter. In southern Europe they overwinter as non-reproductive adults that roost during cold weather but appear to feed during warmer weather....
More to be added to this page....