Hi --

Robert Ostrowski made the excellent point that coverage is probably the main
reason certain areas are "best warbler" (or should we say "migrant"?) spots.
However there is more to it than that.

For example, Rock Creek Park DC was first identified as a major migrant trap
by Greg Gough and others in about 1990. The unique topography and location
of the park is what makes it consistently good both spring and autumn. RCP
runs directly north and south completely through urban Washington, which has
in the last few decades been losing tree cover. Recent aerial photos show
the park as an obvious green corridor in an otherwise rather developed
landscape, (with the exception of the Anacostia River corridor, also good
for migrants). The park also sits along a high N/S ridge and catches the sun
first. When we started birding Rock Creek Park in the early 1990s even on
weekend there might be, at most, 6 birders there. But some of the numbers
recorded were impressive -- e.g. on 9/11/93: 16 spp of warbler including 3
Wilson's, 25+ Magnolia, 12 B-T Green, 5 Bay-breasted, 5 Cape May (!), 2
Blue-wings, 8 Blackburnians, 10 Nashville, 3 Canada, 15 Redstarts, etc. This
with 6 birders present and only the Ridge and Yard covered. On 9/30 the same
year: 13 spp. including 50+ B-T Green (!), 20+ Magnolia, 12 Bay-breasted
(!), 8 Chestnut-sided, etc.

Unfortunately, although birder coverage has exploded, the numbers of birds
seen has continued to decline (although there are still some "big" days.)
This may be due to overall decreases in neotropical migrant populations but
also has been affected by habitat changes within the park itself (invasive
plants, loss of native food plants and understory due to deer, over-zealous
mowing and trimming) and the huge expansion of developed areas outside the
city which may affect regional migration patterns.

Birders looking for a good local "patch" (as the Brits say) should take
topography, vegetation and orientation into account. Availability of water
and shelter is also key. At my old house, 2 acres of woods and brush
immediately bordering upper Sligo Creek Park, with an ever-flowing spring
(one of the creek's sources) I would have large numbers of migrants each
season, attracted by the water and food supply.

Gail Mackiernan
Colesville, MD