Common Nighthawk. Carl Winstead/Macauly Library. 19 May 2016. eBird S29791783
A period of generally favorable migration conditions featuring light to moderate flights of Common Nighthawk, Broad-tailed Hummingbird, Willow Flycatcher, Common Yellowthroat, Grasshopper Sparrow, and Western Tanager is in store for much of the West, particularly in the Rockies, while an unfavorable cool and wet start to the East gives way to later week moderate to heavy flights of White-rumped Sandpiper, Dunlin, Red-necked Phalarope, Black Tern, Black-billed Cuckoo, Red-eyed Vireo, Mourning Warbler, and Scarlet Tanager.
Arrows show wind speed and direction (arrow points in the direction to which wind is blowing) 100 m above ground level. Areas with southerly winds are colored red; northerly winds colored blue. Accumulated precipitation (in 6 hour intervals) is green, outlined by white. Broadly speaking, areas of the map in red will experience conditions that are favorable for migration, and areas where red and green (and red and blue) intersect and overlap may experience migrant concentrations and fallouts as migrants interact with precipitation.
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Increasingly unfavorable migration conditions will build into the region over the weekend, with moderate to heavy movements likely only in scattered areas that do not experience precipitation. Once the disturbance responsible for these unfavorable conditions departs off the Atlantic Coast on Monday, southerly winds gradually return. Western reaches will experience moderate to heavy flights on Monday night, with more widespread flights occurring across much of the region by Tuesday night. By Wednesday, an approaching frontal boundary keeps movements chugging east of the Appalachians, but shuts down flights farther west with its associated precipitation. These conditions continue through the end of the period, as the frontal boundary becomes stationary. During this period, areas free of precipitation will continue to see moderate to heavy flights and areas experiencing nocturnal precipitation will see little to no movement.
Henslow’s Sparrow. Kurt Schwarz/Macaulay Library. 19 May 2016. eBird S29778887
Unsettled conditions deliver a mixed bag of migration events to the region for the weekend. The combination of precipitation in coastal areas, marginal conditions for exodus from migrant origins to the south of the Gulf, and the late season date make extensive concentrations and fallouts unlikely; however, birders in coastal zones should watch carefully in stopover habitats along the entirety of the Gulf Coast for such events. As low pressure responsible for these conditions moves east, slowly, more favorable conditions for late season flights arrive. Monday night sees moderate to very heavy flights west of the Mississippi River, with similar conditions and similar migration intensities expanding east through the remainder of the period. This window of time will see some of the last of the big nocturnal flights of the period, as late May and early June begins a new period of greatly diminished numbers of migrant birds aloft and greatly increased numbers of bats and insects.
Mourning Warbler. Carl Winstead/Macaulay Library. 19 May 2016. eBird S29775850
Moderate to very heavy flights are likely across the region for the weekend, as favorable winds and localized rather than regional precipitation make for generally favorable flying conditions. The beginning of the work week sees a slight change to this pattern of movements, as disturbances in the northern and central Plains dampen (literally) movements; however, moderate to heavy flights will continue in the southern Plains. As the disturbance becomes slightly more organized and extensive toward the end of the week, conditions for migration will become significantly less favorable across more of the region, restricting movements and their intensities.
Grasshopper Sparrow. John Mackin/Macaulay Library. 18 May 2016. eBird S29767203
Favorable migration conditions will prevail across much of the Desert Southwest and the Rockies for the weekend and into the early part of the work week. Light to moderate flights are likely, with some locally heavy flights occurring east of the Rockies. Much less favorable conditions are in place for the Pacific Northwest during this time, where migration will be local and light if it occurs at all when precipitation is not falling. Such unfavorable conditions are associated with the significant change currently in place and forecast for this part of the region. This west-east contrast in favorable and unfavorable conditions intensifies by Wednesday, when favorable conditions and the light to moderate nocturnal movements taking place will be restricted primarily to the Desert Southwest and southern Rockies, and continues through the remainder of the period.
Willow Flycatcher. Curtis Marantz/Macaulay Library. 6 May 2016. eBird S29545941