Cave Swallow is a species from Mexico and the Caribbean, much resembling our local Cliff Swallow (Petrochelidon pyrrhonota), which has been showing an increasing and unusual pattern of vagrancy: birds from the Texas/Mexico population have been appearing regularly in small numbers in the northeast, particularly on the coast (Cape May, etc) and on the Great Lakes. With an exception that I'll get to soon, reports appear to have been increasing, but I don't know what this is due to - the old question of actual increased vagrancy vs. increased observer effort/awareness. Certainly, since Cave Swallows were first found moving along the southern (NY) shore of Lake Ontario in Nov 1999, they have been seen every fall since. For one example of such reports, see the Cave Swallows on the NYSARC 2002 report, including quite good pictures.
I feel it is now widely known in the birding community that these birds should be actively looked for in late Oct/Nov, and that virtually any swallow sighted this late is likely to be a Cave Swallow. Much like hummingbirds, actually - keep your feeders up, because by this point any hummingbird is practically guaranteed to be a western vagrant rather than a late Ruby-throat.
The exceptional case to Cave Swallow vagrancy I alluded to above is the massive flight of November 2005. I was not present for this flight, unfortunately. As recounted by Bob Spahn and Dave Tetlow (2006), and the geneseebirds-l archives, the events unfolded at the Hamlin Beach Lakewatch as follows:
Thursday, November 3th - observers tallied 28 Cave Swallows in six separate groups moving west into stiff winds along the Lake Ontario Shore. This was incredible, as this tally was not only a new high, but higher than the sum of all previous records for the region! (See Table 1 in Spahn & Tetlow 2006). They didn't expect what was to come:
ALL birds were passing East to West into today's strong winds. In the past nearly all of our Cave Swallows have been birds headed West to East. This suggests the possibility that tomorrow, with the winds down a bit, the whole bunch plus possibly others may pass again headed east. These birds generally fly past between the shelter and the edge of the lake, so viewing is close range.
(Bob Spahn, email to geneseebirds-l, Nov-3 2005)
November 4th - Weather conditions were not suitable in the morning, but when the skies cleared and the winds turned sharply southwest in the afternoon, the movement exploded, with 112 Cave Swallows being tallied before dark! All were heading west into the wind:
Saturday, November 5th - Many more observers were on the shore of southern Lake Ontario, but the flight of swallows was weaker than yesterday, with a mere 42.By 4:45 when the flight stopped, they had 112 CAVE SWALLOWS for the afternoon, ALL headed west, all probably Mexican race birds. The biggest single group was 20. Tomorrow?????? Seems like worth watching up to dark or the storm front. Might even be worth driving west before dawn and hoping to catch some of the tail of the flock between here and Wilson. Last fall he found 4 on a barn in Wayne County early 1 morning, so maybe they will be around structures near the lakeshore before it warms up and they move on.(Bob Spahn, email to geneseebirds-l, Nov-4 2005)
November 6th - The big day. A thunderstorm line built up and passed over, with a corresponding increase in winds. Before the rain ended the day, an overwhelming 579 Cave Swallows were recorded flying into the front to the west from near Hamlin Beach.
That's right, I said 579 Cave Swallows.
The following days after the front passed were a mere trickle, with observers actively searching and finding small numbers clinging to the marshes and barns alongside the lake. Scattered reports also came in during this time from elsewhere in the Great Lakes region and on the Atlantic Coast. One specimen even came from the far north in Algonquin Park, Ontario.
So the obvious question remains: why are swallows from Texas and Mexico showing up in the Northeast regularly? The speculative answer, from Spahn & Tetlow (2006) and others, is the coupling of rapid range expansion and population growth of this species since at least the 70's, with strong storm movement from the southwest to northeast.
Part of this may be dispersal of hatch year birds, which are known to be more widely vagrant than adults. The birds seen recently this fall on Cayuga Lake, including the bird I saw today, are considered probably hatch-year, because they show molt in the wings that adults should not at this time of year. Note that these birds do not show juvenile plumage as shown in Sibley, but closer to adult: photos of an individual from Tuesday by Mike Harvey here. The bird I saw today was very similar but molt was a bit more readily apparent.
The take-home message is: get out there and look for swallows!
Robert Spahn and David Tetlow. 2006. Observations of the Cave Swallow incursion of November 2005. The Kingbird 56(3). (pdf here)
Geneseebirds-l archives available here to list members.