May is the month of the spring bird migration. Best known of these migrants are the wood warblers, or just warblers. The warblers are a family of birds within the larger group of passerine birds, or perching birds. The most advanced of all birds, this Order Passeriformes includes flycatchers, swallows, titmice, wrens, thrushes, vireos, wood warblers, orioles and sparrows, among others. (At the other extreme, the most primitive of all birds is the loon; so primitive, it still retains the solid bones of its ancestors. No wonder this large bird is such a low swimmer!)
These warblers have just finished a roughly six-week trip from Central and South America, where they wintered since last November. Following the greening procession of the forests northward, these birds consume the newly hatched insects for their fuel, while the insects, in turn, turn the leaves into their fuel. Migrants come through a given area in waves, corresponding to the weather. The ideal day to observe the spring migration is the first warm, sunny day after a prolonged cold and wet period. A warm front from the south is ideal for the migrants, not only because the birds can take advantage of the prevailing winds, but also because it stimulates insect development. While some warblers reach Maryland as early as late March (black and white, prothonotary, and northern parula), the main wave hits our region in mid to late April. By the first week of May, nearly all the summer breeding birds have arrived in our region.
Migration is initiated by the increasing day length. The production of discrete hormones by the pituitary gland stimulates the birds to react with specified instincts, such as migration and mating. Birds migrate either by day or by night. Many songbirds, such as the thrushes, warblers, orioles, tanagers, and finches migrate at night. Nocturnal migration enables these birds to feed during the day, their normal foraging time. Diurnal migrants include swifts and swallow, which feed on airborne insects, and hawks, which utilize daytime thermal currents to achieve energy-efficient soaring flight.
Male wood warblers arrive at their nesting site five to ten days before the females. Each male needs a territory of some twelve to twenty-five acres of forest with ample under story brush. Returning males will return to the exact same territory for its two or three year life. This ‘site fidelity’ allows the warblers to minimize conflicts with other returning birds. Even if a returning bird had a marginal site the previous season, the benefits of accepting its place overrides the battle of securing a new, and perhaps, better quality site. The birds have established boundaries and maintain a détente year after year by becoming, what evolutionary biologists term, ‘dear enemies’. Such agreements are maintained through the ability to recognize and remember a neighbor’s songs from one year to the next. More on that next month.
While worldwide, only 15% of all bird species migrate, in North America, of the 650 breeding bird species, 332 (51%), winter in the American tropics. Many others migrate to the southern states, while still others migrate locally, such as some juncos and chickadees that migrate to lower elevations, and grackles, that travel west to east from the Blue Ridge to the Dismal Swamp. Some, like robins, have a shift in populations of several hundred miles, so that we here in the Mid-Atlantic States actually always have robins around, just not the same birds in winter as in summer. Severe winter weather patterns may see the robins travel another hundred miles or more south to avoid the low-pressure system, but they’ll return as soon as it passes. We may not see robins in winter, since they stay in the deeper woods, where their food sources and shelter can be found. However, come March, with the warming of our yards and the surfacing of worms and other insects, back come the robins. In fact, the Native Americans called the full moon of March the ‘Worm Moon’, to signify the importance of this event.
The Artic tern holds the record for migration, covering 11,000 miles twice a year. The blackpoll warbler is a close second, traveling 10,000 miles between the Hudson Bay and Venezuela. However, the majority of our wood warblers travel about 2,000 miles. Even hummingbirds regularly migrate 1,000 miles.
It’s interesting to note that site fidelity applies to the wintering grounds as well. Studies have shown that wintering migrants with a specific territory have a much better chance of survival than floaters; birds with no specific winter territory. This helps to explain the impact of loss of habitat, with many of the territorial birds having to become floaters. In a final nod to ‘dear enemies’, the year-round residents of the tropics do not take over the vacated territories of their winter guests, rather these sites are left unoccupied, an apparent acknowledgement of both the benefits of avoiding competition with the fall returning migrants, and the quality (and quantity) of available habitat. Once again, with the continuing reduction of habitat, it will certainly have an effect on this natural balance between the residents and migrants.
It’s a busy and difficult time for birders, with the early weeks of May revealing many northern breeding birds passing through our area, and many species presenting themselves in habitats where they normally wouldn’t reside. But a birder wouldn’t want it to be any other time of the year.
Even if you don’t know the birds, it’s the best month to be out, bar none. Whether you love the birds, the flowers, the frogs or snakes, May is the month that life explodes in our Appalachian natural world.