Male Yellow-Rumped Warbler (Photos by Don Reimer)
Male Yellow-Rumped Warbler (Photos by Don Reimer)
Some birders who keep detailed records of spring bird arrivals like to post FOY (first one seen for this year) sightings on the Maine Birds Googlegroup website. For instance, the initial spring arrival of Ospreys at the roadside nesting platform at Bath’s Taste of Maine Restaurant was reported on April 3. Other early-spring arrivals, such as Red-Winged Blackbirds and American Woodcock, had occurred several weeks earlier. As I write, thousands of American Robins currently dot the local landscapes, and hordes of Song Sparrows are establishing local residences as others press north toward Canada.

We humans use various types of calendars to schedule the timing of our personal activities, but how do wild birds plan their travel itineraries to coincide with the cadences of the changing seasons? With minor variations among individual species, two basic categories of migration account for standardized arrival times for most species. These two categories aren’t ironclad though — many birds fall somewhere between these extremes.

A large number of species are obligate migrants. This means they follow genetically “hard-wired” instincts that dictate seasonal travel within specific time frames of the calendar. Birds in this category are naturally programmed to move about the same time each year, regardless of weather or other conditions farther north.

This group includes birds that winter in the tropics of Central and South America: Wood Thrushes that winter in Costa Rica would have no way of judging the current weather conditions here in Maine. Tundra-bound Semi-Palmated Sandpipers currently occupying the shores of northern South America and Bobolinks in the Argentinian grasslands also use this instinctive signal for spring departure dates. A host of eastern warblers such as Canada and Blackburnian are also part of this group. By contrast, Pine, Palm and Yellow-Rumped Warblers that winter principally within the southeastern U.S. arrive here earlier than most of the neotropical warblers.

The second main category of migrants is the facultative migrants. This fancy term denotes a degree of flexibility and individuality in their migratory behavior. Blue Jays are good examples of this tendency during the winter season. Under certain conditions, such as periodic food shortages, a portion of jay populations wander outside their normal range. This past winter, thousands of jays abandoned Canadian forests in pursuit of Maine acorns.

Most facultative species winter relatively close to Maine and, accordingly, are more attuned to the regional conditions of the moment. While these species have a standard period for migration, they are able to tweak migration by days or even weeks depending on ambient temperatures and regional weather patterns.

Is there a perfect time for a bird to arrive on its breeding grounds? That depends on some incidental factors. Early arrival provides distinct advantages in claiming prime nesting territories. By mid-March we see scattered male Red-Winged Blackbirds perched and singing energetically from spires of frozen cattail. Ignoring the cold temperatures and the fact that no females are present, these hardy individuals persist. Successful early males will reap the bonus of gaining multiple adjacent nesting territories and possibly acquiring two or more mates.

On the other hand, premature arrival has potential perils. Birds that cannot find adequate food must temporarily struggle for survival. As Maine’s earliest swallow species, a few Tree Swallows have already straggled into the state. Swallows are dedicated insectivores that capture flying prey on the wing. Since flying insects are scarce in early spring, you may soon notice swallows dipping low over ponds and lakes on cold or rainy mornings. Why? That’s the aerial zone inhabited by emerging insects. Adapting their foraging behavior in this manner can boost the swallows’ odds of survival.