A Quantitative Study of the Nocturnal Migration of BirdsLowery, George H., Jr.
Science
A Quantitative Study of the Nocturnal Migration of Birds
Lowery, George H., Jr.
Birds -- Migration
It was my opinion that evidence then available proved conclusively
that birds traverse the Gulf frequently and intentionally; that the
same evidence suggested trans-Gulf flights of sufficient magnitude to
come within the meaning of migration; that great numbers of birds move
overland around the eastern and western edges of the Gulf; that it was
too early to say whether the coastal or trans-Gulf route was the more
important, but that enough birds cross the water from Yucatán to
account for transient migration in the extreme lower Mississippi
Valley; and, that, in fair weather, most trans-Gulf migrants continue
on inland for some distance before coming to land, creating an area of
"hiatus" that is usually devoid of transient species. I tried to make
it emphatically clear that I realized that many birds come into Texas
from Mexico overland, that I did not think the hordes of migrants
normally seen on the Texas coast in spring were by any means all
trans-Gulf migrants. I stated (1946: 206): "Proving that birds migrate
in numbers across the Gulf does not prove that others do not make the
journey by the coastal routes. But that is exactly the point. No one
has ever pretended that it does." Although some ornithologists seem to
have gained the impression that I endorse only the trans-Gulf route,
this is far from the truth. I have long held that the migrations
overland through eastern Mexico and southern Texas on one hand, and
the over-water flights on the other, are each part of the broad
movement of transients northward into the United States. There are
three avenues of approach by which birds making up the tremendous
concentrations on the Texas coast may have reached there: by a
continental pathway from a wintering ground in eastern and southern
Mexico; by the over-water route from Yucatán and points to the
southward; and, finally, by an overland route from Central America via
the western edge of the Gulf. As a result of Louisiana State
University's four-year study of the avifauna in eastern Mexico, I
know that migrants reach Texas from the first source. As a consequence
of my studies in Yucatán of nocturnal flight densities and their
directional trends, I strongly believe that migrants reach Texas from
this second source. As for the third source, I have never expressed an
opinion. I am not prepared to do so now, for the reason that today, as
three years ago, there is no dependable evidence on which to base a
judgment one way or another.
TABLE 6.--Computed Hourly Densities at Tampico, Tamps.,
in Spring of 1948
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