This week saw widespread arrivals of a diversity of species in many areas–far too many to mention in full. In general, most Neotropical migrants have now reached the southern U.S. A few very late species such as Willow Flycatcher, Yellow-bellied Flycatcher, and western Yellow-billed Cuckoos and Common Nighthawks, are still well to the south of the United States, with their peak migration not until May (late May for the cuckoos and nighthawks). Our BirdCast for the week of 31 March to 5 April discusses the concept of “fallout”, how it occurs, and what to watch for to try to witness one. This week had several noteworthy events: a classic Texas fallout, good migration in the West, and a great hawk day on the Great Lakes. We’ll touch on each of those this week.
To see this radar animation of this system from 15-17 April, click here
On the evening of 15 April, good migration conditions occurred east of a stalled frontal boundary in central Texas. As it grew dark, nocturnal migrants took off in the areas ahead of the front with good warm, southerly flow signaling good migration conditions. Radar here shows moderate to heavy movements. As the night wore on, the front intensified and moved east, effectively putting a stop to migration as it passed. As dawn began to break, most nocturnal migrants landed in safe areas to feed and refuel. However, a large movement of birds had also departed on a flight across the Gulf of Mexico, presumably departing from the Yucatan Peninsula and possibly other points to the south. Unfortunately for the birds, the front reached the coast just after sunrise, bringing with it a strong line of thunderstorms that progressed offshore. The heavy precipitation and northerly winds offshore created very dangerous conditions for those birds on a trans-Gulf migratory flight—essentially their worst-case scenario. As migrants encountered the wind and rain, they presumably dropped low to the surface of the Gulf, in an effort to fly under the wind and continue to make headway toward the land that they knew was ahead of them (birds flying this low won’t be picked up by radar). Fortunately, in this case conditions had been very good for movement before, so it seems that a large number of birds reached land and observed mortality was not overly high. These types of fallouts can be devastating at times, killing tens or hundreds of thousands of birds. Birders at coastal Texas fallouts noticed a great variety of birds dropping into migrant traps all day long. Once safely in the trees, these birds fed vigorously. As the front moved away to the east, good migratory conditions resumed and the radar from the same night shows the huge arrival of grounded migrants making an equally huge exodus from areas that became free of precipitation.
The drama of trans-Gulf migration takes place every spring and fall. Birds departing the Yucatan in spring tend to do so on good migratory conditions, but it can be an 18 hour flight until their next landfall, and the greatest risk for these birds is that the weather changes in the middle of their journey. In recent warm and dry years, strong and wet cold fronts have rarely penetrated into the Gulf of Mexico during peak migration in April. However, this year is proving to be an exception to that recent pattern, with several fronts now affecting the trans-Gulf migration and triggering small to moderate fallouts. It will be interesting to watch how the rest of the spring plays out.
These types of events are always conflicting for the birder. For those who witness them, it can be incredible to see so many confiding and colorful warblers in one tiny patch of woods. On the other hand, any time this happens it comes at great cost, since for every beautiful warbler feeding in the live oaks at High Island, there are surely others that did not make it. When fallouts of songbirds occur over land, the birds are much safer, since they are able to ride out the weather in sheltered woodlots or brushy hedgerows. Shorebirds are usually resilient enough to use a lake edge, farm field, or grassy area until the storm is over. Waterbirds are usually able to find somewhere to land too, although on rare occasion fallouts will occur with waterbirds dropping down in parking lots and driveways (especially common with grebes). Migration can be dangerous for all birds, but the payoffs for getting back to the breeding grounds early are strong, so natural selection works in both directions simultaneously favoring the lucky birds and penalizing the unlucky. As our climate changes, we can almost certainly expect some shifts in which migratory strategies are most successful.
Great Lakes Hawks
The system discussed above stretched from Mexico to Canada and while our focus is on the fallouts along the Gulf Coast, the warm southerly flow ahead of the system propelled massive migration. The combination of three weeks of unfavorable conditions timed with this warm air produced exceptional migration. Among the highlights was an incredible movement of Broad-winged Hawks with some 34,000 counted (along with 3000 other raptors) at Braddock Bay on 16 April. These birds were even quite obvious on radar, and Tom Carrolan shared news of how hawk counters can now use smart phones to watch hawk flights approaching on radar even before they are visible through binoculars.
This impressive flight wasn’t limited to raptors. Check out Dave Tetlow’s count of 90 species in 4 hours that morning at West Spit including over 1740 Rusty Blackbirds and 223 Northern Flickers. Special thanks to Tom Carrolan for a great email alerting us to this flight.
Migration in the West
Predicting and understanding migration in the West is much more complicated than in the East. The extremely varied topography, habitat, and wind vectors (a consequence of the topography) make it much harder to understand how and where bird migration is occurring. Clearly there are some good movements along riparian corridors, and last week’s BirdCast predicted potentially good migration in the desert Southwest.
The lower Colorado River Valley is a shadow of what it once was thanks to upstream dams, extensive irrigation, overgrazing, and settlement. Still, it provides one of the best north-south corridors for bird movement and it does seem that many birds likely funnel along the river itself. This season a team of ace birders are conducting surveys along the river and were able to provide some observations from the field this week.
Tom Johnson provides this account from the weekend: “Landbird migrants were excellent the 16-17 April along desert washes north of Yuma, Arizona, primarily involving Yellow-rumped (Audubon’s) and Orange-crowned Warblers with good numbers of Nashville, Wilson’s, and smaller numbers of Black-throated Gray and MacGillivray’s. Black-headed Grosbeaks “hit” in earnest 16 April, for the first time this spring. Empids took a noticeable uptick on 15 April in the Lower Colorado River Valley, with Hammond’s, Pacific-slope/”Western”, and Gray dominating. Swallow migration continues to be impressive, with 75-100,000 Trees going in to roost at the north end of Lake Havasu and 110-125,000 at Martinez Lake (near Imperial Dam) on the night of 15 April. The morning of 17 April was the first morning I’ve had on the Lower Colorado River this spring with flocks of White-winged Doves moving in the mornings.”
This of course is just one window on migration in the West, but it is an area that has had near constant coverage this spring, so it is fascinating to hear these reports.
Below are our weekly summaries of weather and migration activity, from our BirdCast radar analysis. See the radar animation here to follow along with the week’s movements and migration.
West – Although widely scattered precipitation occurred across the region during the first few days, some scattered light migration occurred, particularly in the Desert Southwest. As a frontal boundary moved through the region over the weekend, this pattern continued, intensifying to some locally moderate movements in northern California. The beginning of the week in California saw more widespread light to moderate movements, some even locally heavy. This pattern continued into midweek and expanded south and east into the Desert Southwest with high pressure over the Four Corners. With a couple exceptions, more northerly portions of the region did not experience much movement through midweek. However, the Pacific Northwest did have some light movements to begin the forecast period and on Tuesday, and portions of the far northern Rockies saw some scattered light movements early Wednesday morning. At the end of the forecast period, light to moderate migration occurred from the Desert Southwest west and north along the coast and Central Valley north into the Pacific Northwest.
Great Plains – Northern reaches of the Plains states saw largely unfavorable conditions for movements during this period. Light movements occurred in the Canadian border states early on Sunday and Thursday, but otherwise precipitation and cooler, more northerly flow kept many birds grounded. Southern portions of the region experienced moderate migration, including some locally heavy movements through the weekend as advancing low pressure brought favorable southerly flow. The passage of a frontal boundary to begin the week all but shut down movements, but by Tuesday light migration reappeared in calm winds associated with a high pressure ridge. By Wednesday and Thursday, high pressure over Texas brought conditions favorable for more widespread moderate movements, with scattered heavy movements in the central Plains states.
Upper Midwest and Northeast – Beginning on Friday morning, this region finally began to some good migration conditions return, after almost three weeks of cooler temperatures. By the weekend, many areas experienced light to moderate movements, with scattered locally heavy movements. Despite the appearance of precipitation by Sunday in a number of areas, light to moderate movements continued in precipitation free areas in continued favorable southerly flow. By early Monday, a strong frontal boundary approached the region, spawning a line of strong storms as well as light to moderate movements in many areas and heavy migration in New York state. As this front pushed east, by early Tuesday migration had slowed to widely scattered light movements in more inland areas; coastal portions of the region continued to experience light to moderate movements. By Wednesday, light to moderate movements occurred southwest of building high pressure over the eastern Great Lakes, although much of the Northeast saw only scattered light movements. However, as a front moved into the northwestern portions of the region on Thursday morning, light migration occurred in most areas away from the southern New England and Mid-Atlantic coasts, with scattered moderate to heavy movements close to the frontal boundary in Michigan and Illinois. Note that Maine experienced a heavy movement on Tuesday, with a large influx of migrants into its airspace, and light movements at the end of the forecast period.
Gulf Coast and Southeast – Migration was moderate with locally heavy movements across much of the region to begin the period. This pattern largely continued through the weekend, with some areas of Texas, Louisiana, and Georgia seeing heavy migration. As strong storms associated with a frontal boundary moved across Texas, fallouts occurred in many coastal areas, whereas moderate migration, with some locally heavy movements, continued in the southerly flow ahead of the front. Although the front continued to affect the region on Tuesday, effectively bisecting it, light to moderate movements persisted, with some continued locally heavy movements in Texas, Louisiana, Georgia and Florida. As the system pushed farther to the south and east, moderate to heavy movements continued in portions of Florida, Georgia, and the coastal Carolinas, whereas precipitation from the southern Appalachians to the coast marked a line of much diminished movements near the Mississippi River valley. Midweek in Texas saw more moderate movement, with scattered heavy movements in the Panhandle and south coastal Texas. This pattern expanded and intensified by early Thursday morning, though areas east of the Mississippi River Valley experienced increasingly lessened migration. Note that the Florida Keys received several light to moderate pulses of migrants from the Caribbean during the period.
Please note: With warmer conditions becoming pervasive across the country, bird movements detected on radar will increasingly contain other migrant and dispersing animals, including bats and fast-flying and drifting insects. Without doubt, these non-avian targets contribute to the reflectivity values we see in radar mosaics and imagery and bias the interpretation of migration amounts.
Posted 20 April 2012 by Marshall Iliff, Andrew Farnsworth, Christopher Wood, and Brian Sullivan of the Cornell Lab of Ornithology on behalf of Team eBird and BirdCast. Special thanks to David Nicosia, our partner at NOAA, for the weather maps and analysis in our BirdCast predictions.