Bird of the Month - January 2004
The Wintering
Semi-hardy Landbird
The temperate West Coast in the colder months hosts millions of birds. Away from the higher mountains, habitats for several hundred species remain available—or become available. Blackbirds, sparrows, and other birds inhabit a mosaic of common edge habitats. Waterfowl from the Far North resort to lagoons, ponds, rivers, refuges, estuarine and seacoast habitats, and sodden pastures. Generalist species continue to exploit opportunity, while specialists stick with what works. Scattered thinly among these millions which have placed themselves in the right place at the right time are those that have not. This phenomenon is well-noted among our landbirds.
For, say, every fifty Orange-crowned Warblers, it seems there’s a Black and White making do north of its principal winter range in the tropics. We have learned that Sharp-tailed and Le Conte’s Sparrows, each very rare here, might be found once in a great while through searches in tidelands and ‘decadent thatch’. Rose-breasted and Black-headed grosbeaks, various orioles and swallows, and from one to many representatives of 25 warbler species not expected here have lingered in the Redwood Region after the close of the main fall migration period. These are the wintering semi-hardy landbirds, called such for the varying response they exhibit to cold weather.
Fall migration in birds is spurred by changes in hormones caused by decreasing daylight after the summer solstice. In late fall, having migrated some distance, an off-course waif in the midst of a flock of similar birds--and with its needs met for the present--may elect to remain where it is, the urge to continue migrating quelled. Clay-colored Sparrows run with ‘crowned’ sparrows. Nashville Warblers flock with chickadees and kinglets. Such a lingerer will have found itself in a somewhat exotic winter environment, in circumstances its innate biology has not specifically prepared it for, in the company of a suite of wintering birds likely far different than those species it might rub coverts with on the main wintering ground. If it has what it takes, it shall know the pleasures of Ferndale in March.
Winter tests birds, picking from the nations of water- and landbirds each of those individuals less fit to survive. The survivors find appropriate food and habitat, maintain their metabolism, and keep out of harm’s way. Most of the wintering semi-hardy landbirds are encountered among flocks of routine species. To be associated with a flock surely confers greater survivorship on such birds: the individual is less frequently the lone target of predators, is alerted to them more readily, benefits from discovery of food resources by the group, and may roost with them at night.
Cold is typically the greatest test faced by a proto-wintering warbler, swallow, or Empidonax flycatcher in our area. During a normal year, the near-coastal lowlands where most of these individuals persist do not experience hard freezes. This allows adult and larval insects to remain active. Intense cold continued over days removes this food source, leaving the obligate ‘big-bug-getters’ to find alternative food, find food through alternative methods, leave, or starve. With fat reserves low or absent as a result of their daily struggle, most of these birds may not have the option to continue to migrate far. Indicating that some of our regular wintering landbirds actually are only a few notches above semi-hardy is that the Siberian Express winter about fifteen years ago was thought to have killed 90% of the Townsend’s Warblers in Marin County. The same massive freeze left Western Oregon with essentially no living Ruby-crowned Kinglets.