House Sparrows are declining over much of their native and introduced range.
In many areas, the decline has been precipitous over the last couple of
decades or so. (While we might be cheering in the US, remember this species
in native in Europe. )

In Britain, House Sparrows have been estimated to have declined by 50% to
60% between the 1970s and the late 1990s, from between 12 and 15 million
breeding pairs in the early 1970s to about 6 million breeding pairs in the
late 1990s.

(Reference: Crick, HQP, Robinson, R. A, and Siriwardena, G. M. 2002. Causes
of the population decline: summary and recommendations. In H. Q. P. Crick,
R. A. Robinson, G. F. Appleton, N. A. Clark, and A. D. Rickard (eds.)
Investigation into the causes of the decline of Starlings and House Sparrows
in Great Britain. B.T.O. Research Report No. 290. pp 265-292. DEFRA,
Bristol.)

Because the declines in these species are so widespread in the developed
world, they suggest widespread changes. There are several hypotheses, two
of which are probably most likely. One is that building construction has
changed so that there are fewer cavities for cavity nesters. The other is
that insect populations have declined. House Sparrows must have insects
during the breeding season as a source of protein for their youngsters.

Insects, as Dennis noted, appear to be declining in developed parts of the
world. (Insects are not as well documented as birds, of course.) The
postulated causes for insect declines range from an unleaded gasoline
additive toxic to insects (starts with an M; I can't recall the name
off-hand); to insecticides applied everywhere, to "clean" cities (roadside
and edge vegetation cut; fanatic removal of fallen leaves, standing dead
vegetation and other plant detritus). Many other factors have probably also
contributed.

The possible connection with Nighthawks is that House Sparrows might be a
leading indicator of insect declines. House Sparrows don't migrate, so
their decline cannot be blamed on land management in southern regions. Of
course, there are plenty of House Sparrows still around; insects have not
vanished. Still, if the food base shrinks, you would expect predator
species to start dropping off. The decline is not likely to be evenly
distributed, so the rise in Purple Martins is not necessarily a sign that
nothing is wrong.

Kelly Cassidy