Warbler Guy: Migrating warblers cross the Gulf? Or do warblers migrate by land? Both ways?

Great question, Eric.

Answer: both, as the majority of breeding eastern and midwestern USA wood-warblers migrate north over the Gulf of Mexico (and, hence, are often referred to as "Trans-Gulf Migrants."

A couple of land-migrating only wood-warblers that avoid the 500-mile over-water excursion include Nashville Warbler and Mourning Warbler.

A qualification: western USA breeding wood-warbler species may travel different flyway routes that never require them to travel over large bodies of water such as the Gulf.

(ABOVE: Blackpoll Warbler autumn migration route (right-most arrows) and spring migration routes (arrows shown in middle of map. Non-breeding range = blue color in S. America; breeding territory range = orange color.)

More on this subject shall appear as an upcoming new post here soon. Please check back.

Please note:

A fine, general overview of this migration phenomena is accessed at:
http://alabamaforeverwild.com/index.php/experience-bird-migration-along-alabamas-coast/

A more refined, scientific treatise on this subject is available at:
http://www.int-ornith-union.org/files/proceedings/durban/Symposium/S33/S33.1.htm

Plus, here's a species-by-species map of wood-warbler migration tendencies:
http://www.pwrc.usgs.gov/birds/migration/migrant.html