How To Identify Common Species of Migratory Butterflies
Many species of migratory butterflies have distinctive patterns and are easy to
identify. Others are not particularly distinctive and can be confused with similar
migratory or non-migratory species. Easily identified species include buckeyes, mourning
cloaks, and red admirals. 
Problem migratory species, with many look-a-likes, include cloudless
sulphurs, pipevine swallowtails, most of the angle wings, and fritillaries. Ironically,
the single best know and most studied species, the monarch butterfly is also a problem
species with two look-a-likes, the queen butterfly (migratory), and the viceroy butterfly
(non-migratory). Because viceroy butterflies do not migrate and are not particularly
abundant when monarch butterflies are making their spring and fall migrations, viceroys
are unlikely to be picked up by our sampling techniques. Fortunately, monarch butterflies
are usually much more abundant than queen butterflies during the migration period,
minimizing the risk of errors due to mistaken identification. In any case, it is essential
for researchers in this project to get a good field guide, a pair of binoculars, and a
butterfly net (which can double as a wind sock), and practice identification in the field.
Some species with highly distinctive markings can be identified by viewing them through
binoculars. However, most species will have to netted and examined at close range. As
noted above, monarchs fall into the second category. Over the years I have netted
'monarchs' flying among vegetation that turned out upon closer examination to be viceroys,
queens, and on one occasion, a large gulf fritillary.
The need for practice in species identification brings up another topic. How can a field
researcher convince others that their identifications are accurate? After all, everyone
makes mistakes (see above). I've even received photographs of a tiger swallowtail from
someone who believed that they had photographed a monarch butterfly. Voucher specimens
and/or good quality photographs are the only way around this problem. In my opinion,
voucher specimens are the better of the two options. Photographs can ensure that an
identification was correct but present practical problems. Monarch butterflies are
abundant, large, showy, tend to roost in clusters, and while nectaring, almost seem to
pose for the camera. Most other migratory butterfly species are less cooporative. They are
usually less abundant than monarchs, less brightly colored, smaller, solitary, and
camera-shy, tending to fly off as soon as you approach. Finally, because the top and
bottom surfaces of the wings can be equally important for identification, at least two
high quality photographs are required for many species. On the other hand, preserved
voucher specimens can be examined at leisure and may even serves as resource material for
future DNA analysis. The last point is important. Migratory and non-migratory populations
of some species may eventually prove to have significant genetic differences. If there is
a change in the distributions of the two types over the years, voucher specimes could
indicate how things once were 'back then'. In in case, if you decide to collect voucher
specimens, several individuals of each species should be plenty.