December brings us both good news and bad. The bad news is that we are subjected to the shortest days of the year. The good news is that on Dec 21, the days start increasing again. Interestingly, the latest sunrise and earliest sunsets do not occur on the same day. In fact, the earliest sunsets occur in early December and the latest sunrises occur in early January. This year, the earliest sunsets in Washington, DC occur at 4:45 pm from Dec. 5 – 11, while the latest sunrises occur at 7:26 am from Dec. 30 – Jan. 13. Between these two extremes, the shortest day is about Dec. 21.
Winter is definitely for the birds; at least, the migratory ones. There are about 200 species of migratory birds that spend the winter in warmer southern climes, or, approximately half of our 390 known species that can be found in our Appalachian mountains. Scientists use the term Neotropical migratory birds to describe our migrants. Neotropical migrants are the species in which the majority of individuals breeds north of the Tropic of Cancer and winters south of that same latitude. This latitude is 23° north of the equator, and is found somewhere in the Gulf of Mexico, just north of the Yucatan Peninsula. The majority are songbirds (such as warblers, thrushes, tanagers, and vireos). There are also many shorebirds (sandpipers, plovers, and terns), some raptors (hawks, kites, and vultures), and some waterfowl (teal).
Migration isn’t limited to birds, however. A number of butterflies migrate. We all are familiar with the monarch butterfly, known to fly over 1,800 miles from Toronto, Canada to the nine high mountain sites in fir forests of central Mexico. Not surprisingly, while they have several broods during the summer, the last brood, which will migrate, is different. The energy available in the chrysalis is shunted toward the development of reinforced, robust wings, necessary for the long journey. What’s sacrificed? Energy is taken from the reproductive parts of the butterfly. In fact, unlike the earlier seasonal broods, the reproductive parts will not mature until March!
And how does this monarch know which way to travel? Research supports the belief that monarchs have an internal magnetic compass that reorients its direction one degree clockwise per day. With 360 degrees roughly equaling the 365 days in a year, such a theory fits the clockwise migration documented in the monarch’s annual cycle.
Other butterflies migrate between our mid-Atlantic states and the frost-free coastal zones of the southern Gulf States. These include the Common Buckeye, Red Admiral, Painted Lady, Mourning Cloak, Gulf Fritillary, Question Mark, Cloudless Giant Sulphur, Pipevine Swallowtail, Dwarf Yellow, Mexican Yellow, and the Sleepy Orange.
Dragonflies migrate as well! At least, some do. The known migrants who overwinter in southern Florida include the green darner, and various species of skimmers, saddlebags, and gliders. Like birds (and butterflies), sometimes the fall migration takes the form of spectacular mass flights that can involve hundreds of thousands, even millions of individuals. However, not all green darners migrate. Two different populations of green darners live in Canada and the U.S. The resident population breeds in the north over the summer. They lay their eggs in northern waters, and the nymphs spend the winter in that cold water beneath a thick layer of ice. In spring, they emerge from the water and spend the summer as adults.
The other population of green darners is migratory. They arrive from southern regions each spring to breed in the north. Their young emerge in late summer of that same year, and migrate south during August and September. Not only that, but apparently the migratory population alternates generations between breeding in the north and breeding in the south.
Who else has left us for the warmer southern climes? It’s hard to imagine mammals migrating, but let’s not forget the bats. Of our 12 Appalachian species, three are migratory. These are the summer tree-dwellers, including the red, hoary, and silver-tipped bats. The other bat species are hibernators, who take up residence in caves or hollow trees – or buildings. The summer and winter roosts are normally different, but within a few miles of each other. Actually, all of our hibernating bats species have some members that migrate and all of our three migrating species have some members that hibernate in their summer grounds.
Wintering habits of these three migratory bats are quite distinct and variable within each species. Apparently, male and female red bats migrate at different times and have different winter and summer ranges, gathering in fall for mating. Among the hoary bats, both sexes overwinter in southeastern US (southern Georgia and Alabama and northern
Florida). In spring, the females travel north of the Mason-Dixon line, while the males either stay in the winter grounds or travel west to SW US, including southern California! And the female silver-tipped bats summer north of Pennsylvania and winter in Virginia and places to the south, along with the males, who don’t tend to migrate north very much (There are no known summer records of males or females in Virginia.).
With all this separation of sexes among the migratory bats, they have evolved a very interesting way of ensuring the propagation of the species. Called “delayed fertilization”, females mate with the males (normally) in the fall, storing the sperm internally until the following spring, when they will inseminate themselves, enabling birthing at the appropriate time. This is different from ‘delayed implantation, a function in which the egg is fertilized immediately, but soon the egg is separated from the placenta, with all further growth arrested for several months until the egg is again united with the placenta, allowing the embryo’s growth and birthing at the appropriate time. Delayed implantation is practiced by bear, most of the weasel family, and those crazy armadillos.
One tagged butterfly was tracked along a 1,870-mile route. Originally tagged on September 18, 1957 in Highland Creek, Ontario, it was spotted again in San Luis Potosi, Mexico, four months later.
Up to 100 million monarch butterflies migrate either to California or to Mexico each year. (This isn't the entire population. Some monarchs never make the migration.) There are more than 25 winter roosting sites along the Californian coast and about a dozen known sites in the Sierra Madre Oriental Mountains of Mexico.