Transatlantic Transport

As the 2012 Atlantic hurricane season spins its way into autumn, tropical systems that strafe the Atlantic Coast of North America sometimes, eventually, make their way across the pond to Europe. Although not terribly regular, when these systems arrive on European shores they may bear avian gifts along with generally wet and messy conditions. They arrive with varying degrees of intensity, from the most scattered remnants of a tropical depression to nearly completely intact hurricanes. With their arrival in the eastern Atlantic, birders should hope for any of a potpourri of megas that could get entrained in these storms.

A tropical system is moving between Iceland and northwest Europe now and associated winds could well be favorable for trans-Atlantic vagrancy. With North American migration at its peak for many species, conditions this weekend could well bring some Yank vagrants to the U.K., Ireland, or France. Below we review a couple past examples of similar phenomena and go out on a limb with a few things for European vagrant hunters to think about for this coming weekend. We draw on eBird–a free, global system for entering and sharing bird sightings–to help illustrate some of the patterns, past and present, that we discuss.

Migrant birds flying over the Atlantic may get caught up in these fast-moving tropical systems and may survive aloft long enough to reach European shores. Many of these birds intentionally depart from the Atlantic Coast of North America on long non-stop flights to the Caribbean or coastal South America, including waders such as American Golden-Plover, Hudsonian Godwit, Hudsonian Whimbrel, Pectoral, White-rumped and Semipalmated Sandpipers, Willet (the probably specifically-distinct ‘Eastern’ subspecies T. s. semipalmata), and several others, Black-billed and Yellow-billed Cuckoos, Red-eyed Vireo, wood-warblers (especially Blackpoll, but also Connecticut, American Redstart, Northern Parula and others), Bobolink, and a number of other passerines. These intentional over-water flights are probably more common than has been generally appreciated, and, not surprisingly, many of the migrants that do this are among the more common vagrants to Europe.

Wind drift offshore is also common off the eastern U.S. and Canada, especially following the passage of strong cold fronts with northwesterlies. Sometimes these frontal boundaries carry birds directly into the heart of a tropical system. The list of species that could end up offshore could theoretically include almost anything that is on the move in North America right now. A glance at eBird’s seasonal histograms for Cape May County, New Jersey, gives a good sense for the landscape of migration off eastern North America right now (try clicking on the map links and the species’ names for more info).

SOME HURRICANE HISTORY

Under similar conditions, some past results have been quite spectacular from a vagrant bird perspective. In early October 2011, for example, observers in New England, eastern Canada, and Bermuda experienced a combination of effects from both continental and marine systems, as a strong low-pressure system moving eastward interacted with a Category 4 hurricane in the Atlantic. While Hurricane Ophelia passed east of Bermuda and churned the western North Atlantic into a frenzy, a strong low pressure system over the southeastern United States catapulted birds off the Atlantic Coast of the US. What occurred is affectionately known in American English as a “slingshot” effect, in which southern species appear at sites far to the north of their expected ranges and destinations (e.g., McLaren 1981, McLaren et al. 2000). Tremendous numbers (up to 1000!) of Yellow-billed Cuckoos dropped out in Bermuda, and a suite of Caribbean and Central American winterers were deposited in New England and the Canadian Maritimes (Farnsworth and Iliff, in press). Unfortunately, this system did not reach the Western Palearctic in any organized way, dissociating long before reaching any continental land masses.

Perhaps the best Western Palearctic vagrant load in a hurricane to date came compliments of Hurricane Wilma in 2005. Dinsmore and Farnsworth (2005) wrote of the effects of Hurricane Wilma from a North American perspective. Wilma had the lowest pressure ever recorded for an Atlantic hurricane (882 mb) and reached the strongest category 5 status. Forming off Jamaica on 15 October, Wilma reached Tropical Storm status two days later and hurricane status by 18 October. The next day, she was a powerful and dangerous Category 5 storm that slammed into the Yucatan Peninsula. Weakened by landfall in Mexico, she then took moved east through the Gulf of Mexico, crossed Florida 24 October, and regained category 3 strength off the East Coast of the U.S. and moved rapidly northeastward. It was during this period that birds were probably entrained in the storm and in the transatlantic wind fields that formed due to her interaction with other systems. On October 26 she was an extratropical cyclone southeast of Nova Scotia that finally hauled across the Atlantic toward after merging with the remnants of Tropical Storm Alpha and with another strong, continental low-pressure system as it moved northeastward away from North America. These three systems merged off the coast of Atlantic Canada, south of Greenland, further intensifying an already exceptional low pressure system. Of primary importance in this scenario was the union of these storms off the coast – the strong continental front pushed any birds already aloft over the ocean farther east, birds already entrained over water in the tropical systems stayed entrained, and the entire system of three storms became more cohesive over water, not to see land again until European soils. For those that follow meteorological extremes, this combination of three systems will sound reminiscent of the so-called “Perfect Storm” from 1991. This weather site lets you step through the progression of these lows and how they moved across the ocean. Note that several strong low pressure systems arrive even before Wilma, which can be seen on 25 October 2005 imagery as a 991 and 985 milllibars of mercury (mb) low pressure systems west and north of the Azores; the arrival of these low pressure systems certainly aided movement of additional Nearctic/Neotropical migrants to the Palearctic.

The “Wilma” systems finally passed north of the Azores as an extra-tropical depression late on 27 October 2005 (e.g. at that URL above, the 966 millibar low pressure system). The ensuring Nearctic fallout was perhaps most memorable on Corvo in the Azores, where Peter Alfrey described his epic birding in Birding World (Alfrey 2005) and in Birdwatch (Alfrey 2006). A partial list of his highlights on Corvo included 27 Chimney Swifts at once, Lapland Bunting, Scarlet Tanager, Buff-bellied Pipit, Rose-breasted Grosbeak, Laughing Gull, and Forster’s Tern. Other observers elsewhere in the Azores had Tree Swallow, Indigo Bunting, Greater Yellowlegs, Upland Sandpiper, and a Palearctic first American Barn Swallow. A strong low pressure system two days before Hurricane Wilma had also deposited the first Palearctic Hooded Warbler, multiple Indigo Buntings, Philadelphia and Red-eyed Vireos, a Bobolink, and a White-crowned Sparrow.

Wilma brought great vagrants to mainland Europe too, and some probably remember mad twitching adventures, during the aftermath of that tropical system: multiple Magnificent Frigatebirds in Spain, Portugal, France, and England, Great Blue Heron, American Bittern, Pied-billed Grebe, Sora, Yellow-billed Cuckoo, many Chimney Swifts in multiple countries (more than 80 among Ireland, England, Spain, France and the Azores), Gray-cheeked Thrush, Tree Swallow, Red-eyed Vireo, Black-throated Blue, Yellow-rumped, Blackpoll, and Hooded Warblers, Ovenbird, Indigo Bunting, and Rose-breasted Grosbeak. Clearly a whole host of migrants were “caught offshore” after the major low-pressure system had moved out over the Atlantic, including species probably making intentional jumps for South America (Bobolink, Red-eyed Vireo) and others that were simply displaced offshore by wind drift in associated with frontal passage (American Bittern, Tree Swallow, etc.). A combination of systems of this magnitude clearly means a potential bounty for areas outside of the typical island vagrant traps. And in terms of timing, some of the aerialists (e.g. Chimney Swift) arrived within 24 hours of passage of the Wilma system. Some, like Indigo Bunting, arrived as it was passing or even pre-passage, suggesting additional entrainment perhaps in advance of Wilma within another strong system (see the weather site referenced above to examine additional strong systems passing in advance of Wilma). Clearly it seems at the least that within 24 hours of passage, regular rechecks of known migrant traps are essential! A good accounting of the timing of vagrants, in 2005 and otherwise, can be found here and here (note in particular pages 306-307).

WEEKEND BIRDING — TROPICAL STORM LESLIE

Though not at the scale of Hurricane Wilma, or even the Ophelia event, several tropical systems are presently working their way across the Atlantic. As Tropical Storm Leslie moves across the North Atlantic, followed closely by Hurricane Michael or its remnants, birds that departed the coast of the US and Canada on 10-11 September with the passage of a strong cold front could well have been entrained in winds associated with these tropical systems. The figure below shows a mosaic of data collected on the network of Weather Surveillance Radars operating in the continental US. Additional, synoptic weather data are shown, including a frontal boundary in the Atlantic, low pressure off the Atlantic coast, high pressure just west of the Appalachian Mountains and over the upper Mississippi River valley, and surface wind direction and speed (the purple arrows). The uniform patterns visible in the radar data (the stippled blues and greens) represent migrating birds (some bats and insects as well). These data were collected at approximately 11 p.m. Eastern time on the night of 10-11 September, and heavy coastal movements of birds may well have moved offshore in Northwesterly and Northerly winds in the New England region. Migration conditions since then have remained good, but flight have not been as massive as the number of birds aloft that night.

With the passage of this strong front, it is possible that numbers of birds moving between continents, departing from Canada and the US for destinations in South America, also moved far offshore. The strong northerly and northwesterly winds that facilitated movements of birds on these trans-continental flights may push them right into the swirl of at least one of these storms. Along with species that may be drifted offshore in the front, South American over-water migrants like some waders (see list above), are probably the best bets. Most North American terns, along with Laughing Gulls, are also on the move now, and strong flyers that could well occur in this system. Passerines and near-passerines (swifts, cuckoos) of course would be most exciting, and although mid-September is earlier than most European records, Connecticut and Blackpoll Warblers, Red-eyed Vireo, Chimney Swift, Yellow-billed Cuckoo, all have reasonable potential to be entrained in the systems moving east across the Atlantic. Of course not all species have the same probabilities of successfully completing the flight to Europe, with the larger, long-winged species with better fat reserves (i.e., those already “planning” a multi-day flight to South America) being the most likely to reach Europe alive. This system may not be fast moving enough to hurtle the Western Palearctic’s first Ruby-throated Hummingbird across the Atlantic, but according to eBird observations this week is when most birds are leaving eastern Canada and the Northeastern United States and streaming south, so one can dare to dream…

Given the timing forecast for the arrivals of the remnants of the tropical systems in the Northeast Atlantic, it seems plausible to hope for North American waifs to appear in Europe some time between 15-18 September. Note the strong westerly winds that begin to overspread the UK by 14-15 September in the forecast image to the left. Since any excuse to go birding is a good one, why not check some western coastlines (or better yet, islands) in England, Wales, Scotland, Ireland, or northwestern France this weekend and see if you can prove our predictions correct!

Icelandic birders yesterday (13 September 2012; fide Yann Kolbeinsson) reported Pectoral Sandpiper, White-rumped Sandpiper, and American Golden-Plover, and the wind fields that reached Iceland may not have been as favorable as those that should reach Ireland and the northern United Kingdom. So perhaps the best is yet to come!

REFERENCES:

Alfrey, P. 2005. American vagrants on the island of Corvo, Azores, in October 2005. Birding World 18(11): 465-474.

Alfrey. P. 2006. Eye of the Storm. Birdwatch (October 2006), p. 37-40.

Dinsmore, S. J. and Farnworth, A. 2005. The Changing Seasons: Weatherbirds. North American Birds 60 (1): 14-26.

Farnsworth, A. and Iliff, M. J. In press. The Changing Seasons: Driven. North American Birds 66 (1): 16-67.

McLaren, I. A. 1981. The incidence of vagrant landbirds on Nova Scotian islands. Auk 98: 243-257.

McLaren, I., B. Maybank, K. Kiddy, P. D. Taylor, and T. Fitzgerald. 2000. A notable autumn arrival of reverse-migrants in southern Nova Scotia. North American Birds 54: 4-10.

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About Team eBird

Five of us (Jessie Barry, Andy Farnsworth, Marshall Iliff, Brian Sullivan, and Chris Wood) work for the Cornell Lab on the eBird bird-recording scheme .
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