Most of the last big pushes of spring migration will occur this week. Light to moderate movements will occur in many precipitation-free areas of the West. The Great Plains will experience moderate to heavy movements in many areas early in the period, but a substantial low pressure system will shut down migrants with rain later in the week. Highly variable conditions in the Upper Midwest and Northeast mean equally variable movements of birds, with a high likelihood of heavier movements in rain-free areas. The Gulf Coast and Southeast continue to experience widespread moderate to heavy movements in western areas of the region, and lighter, more widely scattered movements farther east, although increasing possibility of precipitation across the region should have birders on the lookout for fallouts of late migrants. Here is last week’s animation.
Willow Flycatcher is a common late spring migrant, departing from central and northern South American wintering grounds by late April and appearing in the US and Canada by mid-May. The species breeds primarily in open, wet, shrubby habitats across a large portion of North America, from the southern Canadian Maritime provinces to southern British Columbia, south into the southern Appalachians and central Great Plains and through the western US to Arizona and southern California. As a circum-Gulf migrant, it largely or entirely skips the Southeast and is an extreme rarity in Florida, for example. Like other Empidonax, Willow Flycatcher is often misidentified, with incautious birders sometimes reporting singing Eastern Phoebes or other species ofEmpidonax as Willow Flyatcher. Understanding migration timing is critical to knowing when to be especially cautious with the identification of Willow Flycatcher. Any Willow Flycatcher before 1 May should be identified with extreme caution and is likely possible only in Texas and other southern states. In most areas, Willow Flycatchers do not appear until mid-May or even late May, and birds are still arriving on territory in early June. With four very similar subspecies in three groups, Willow Flycatcher shows significant variation in arrival timing, with many western individuals yet to arrive on territory, exemplifying a striking pattern that we have discussed previously in species with eastern and western breeding populations. Some populations–particularly those in the Southwest (Empidonax traillii extimus)–are declining, largely because of widespread degradation of riparian habitats.
Continued favorable conditions for widespread moderate movements begin the weekend from southern California east through the Four Corners. However, scattered precipitation in portions of the Great Basin and northern Rockies, as well as unfavorable winds aloft, will keep migration to a minimum in most northern areas. Monday will see marginally favorable conditions across much of the region, including some scattered precipitation, and migration will be light and scattered at best in most areas. Over the course of the week, movements will become more widespread and light to moderate despite the marginal or unfavorable forecast winds. Exceptions will be areas where precipitation shuts down movements, particularly in the Pacific Northwest toward the end of the forecast period.
As the forecast period begins, southerly flow will spawn moderate to heavy movements in Oklahoma and Kansas, but marginal easterly flow farther north will not be as conducive to such large movements. By early Sunday, precipitation moving across the Dakotas and Nebraska will shut down movements where they occur. However, southerly flow farther south will continue, and migration will be moderate to heavy. Birders from the Dakotas south and east through Iowa should be on the lookout for fallout conditions in areas where birds and rain meet. Shorebirds may compose the bulk of fallouts if they occur. This pattern should continue through Tuesday, when a substantial low pressure center will move through the region and shut down movements in all but the most southerly areas. In this low pressure center’s wake conditions will be marginal (unfavorable winds and scattered precipitation) for the remainder of the forecast period. Note that in areas where precipitation does not fall, movements will be moderate to heavy, though local, as migrants seek to arrive at breeding areas despite unfavorable winds aloft.
Upper Midwest and Northeast
A strong low pressure center east of Hudson Bay will bring favorable southerly and southwesterly flow to New England and coastal New York and New Jersey to begin the weekend, facilitating moderate to heavy movements. Farther south and west across the region, areas without precipitation will experience mostly moderate movements in primarily westerly winds. Note, however, the threat of precipitation across the eastern Great Lakes south and west through the Mid-Atlantic states suggests that birders should watch for fallout potential, particularly given the westerly component to the winds and the position of high pressure. As this system passes and the next one approaches, conditions at the beginning of the week will become increasingly favorable for moderate to heavy movements in the Mississippi and Ohio River valleys, and less favorable east of the Appalachians into New England. As this next system moves through the region, favorable winds aloft coupled with scattered precipitation will make local fallouts a possibility in many areas. Where precipitation does not occur, movements may be locally heavy. By midweek through the end of the forecast period, northerly and westerly flow combined with scattered precipitation will spread across the region. Migration may continue to be moderate in many areas free of rain. Note: conditions in the coastal Northeast may be more favorable for locally heavy movements to occur, including in coastal areas where concentrations may occur. Birders on Long Island north and east through Cape Cod and coastal Maine should check migrant traps for late migrant songbirds.
Gulf Coast and Southeast
Texas will continue to experience moderate to heavy movements to begin the period, with these movements diminishing to scattered light movements farther east in largely unfavorable winds. This pattern continues through the early part of the week, although more moderate and locally heavy movements will spread eastward as more favorable winds aloft spread east. Given forecast precipitation over the Gulf of Mexico during this time, birders in Texas should watch for small fallouts in coastal areas – these could include late migrant passerines and shorebirds. The remainder of the forecast period will see an increasing threat for precipitation in many areas and less favorable winds. However, moderate to heavy movements are likely to persist in areas free of rain, given the date. Birders across the region should watch precipitation, as the potential exists for local fallouts extending from eastern Texas through the Carolinas and south into Florida. Of particular interest is the forecast for rain and favorable winds aloft over Florida and portions of the coastal bend at the end of the period. Despite the late date, birders along the east coast of Florida and the Florida Keys should watch for fallouts of late passerine migrants.