Map quiz 7 - answer

Our Map 7 was a little different in that you were given two maps: one showed Nov-Dec 2008 and the other Nov-Dec 2009. It should have been noticeable that the prevalence of this bird around the Great Lakes and on the East Coast was quite different in those two years.

Map 7a: Nov-Dec 2008

So here's what we know. This is a bird with lots of records in central and
south Texas in November and December, and scattered records north and east of there in two years. For some reason, in 2008 the bird was seen all along the East Coast from New Hampshire to North Carolina, whereas in 2009 it looks like there is a dot for Connecticut and one for New Jersey, but no others north of southern North Carolina. Similarly, the 2008 map shows multiples spread along Lake Erie and Lake Ontario, but in 2009 there are just a couple of dots.

Map 7b: Nov-Dec 2009

And by now, you probably either know this one, or don't. Yes, Cave Swallow
is the answer. This breeder from Texas has established a strong pattern of
starying to the Northeast during November and December, and 2008 was a great year for them. However, 2009 was oddly depauperate for Cave Swallows. It was a great year for other western vagrants, with things like Sage Thrasher, multiple White-winged Doves, two Allen's Hummingbirds, three MacGillivray's Warblers from Massachusetts to Maine, Bell's Vireo in New Hampshire, and many other exciting birds turning up. But Cave Swallows were missing--why?

Although the fall has lots of days of southwesterly winds, which are thought
to bring birds like Cave Swallows (and many others) to the Northeast, what
it lacked was strong cold fronts. The typical pattern is to have a few days
of strong southwesterlies in November followed by strong Northwesterlies the
following day. In this pattern, Cave Swallows may "drift" northeastward (why
they let themselves "drift" is another question altogether...), and then
northwesterlies force them to the coast where they become easy to find. In
this fall's pattern, it is quite likely that many Cave Swallows drifted to
the northeast, but that without strong cold fronts to bring northwesterly
winds, they were never concentrated in areas where birders tend to find
them. We know that this is true partly because observers on Lake Ontario saw
73 (!!) at Hamlin Beach, NY, on southerly winds. (The Lake Ontario shore is
one of few places where fall southerlies can help find Caves). If present,
this year's Caves may have fed high in the air on insects borne by the same
warm southwesterly winds and may have been largely missed by birders
accustomed to finding Cave Swallows on the coast. Without cold temperatures
forcing them to roost and feed by water, they may have been harder to detect
as well.

In any event, the net result this year was fewer Cave Swallow reports, resulting in these two very different looking maps.