May 1998

by Kimball L. Garrett

For two springs now (21 April 1997 and 3 May 1998), Kathy Molina and I have run into a passage of landbird migrants that is pretty impressive by our standards along the eastern edge of the Colorado River Delta near El Doctor, Sonora.  I was curious if there might be other sites along the n.  coast of Sonora that match or exceed this kind of passage.

If one looks on a map, the NW-trending east side of the Gulf of California would seem a natural route for spring migrants coming out of W Mexico.  At El Doctor the habitat is extremely xeric, with only low, scattered desert shrubs.  But about 0.5 km west of the highway there is a 10-20 meter bluff that drops down to the level of the Colorado River delta (which is now, of course, mainly barren mud/salt flats).  Along the base of the bluff is an artesian-fed mesquite and palo verde woodland (now co-dominated by tamarisks); this woodland is linear -- just 10-50 m wide, but follows the base of the bluffs for many km.  This area is locally known as the "pozos", and is part of the Cienega de Santa Clara system.

On 3 May these trees were "filled" with passerine migrants.  But even more impressive was the morning flight of migrants along the bluff.  By sampling for a couple of ten-minute periods, we estimated a very conservative minimum of 400 passerines per hour passing a fixed point of the bluff.  Counts would probably be higher earlier in the morning (we were there around 0900).  This site offers a great opportunity for practicing flight identification of warblers, buntings, Empidonax, vireos, etc., and for learning their flight calls.  Since passing birds will often stop briefly in the isolated trees along the bluff, one often gets a chance to confirm identifications.  This is quite reminiscent of the edge of the canal at Higbee Beach, Cape May, New Jersey, were locals have perfected the art of in-flight identification of warblers.

Because the habitat at El Doctor is so linear, and since the birds seem to prefer a specific route at eye-level along the top of the bluff (energetic advantage?), I think migration is probably easier to quantify here than any other mainland situation I can think of.  I think that a team of ornithologists, with appropriate Mexican permits, could conduct some very interesting quantitative studies of migration patterns here through mist-netting and other monitoring techniques.

I would be interested in hearing of other obvious concentrations/ movements of spring passerine migrants through Sonora.  I imagine there is probably a large movement northward up into the river systems (San Pedro, Santa Cruz) of southeastern Arizona, but I wonder how the magnitude of birds moving up the Sonoran coast and Colorado River delta compares.  Before the destruction of the Colorado River delta woodlands, this must have been an extremely important "refueling" stop for northbound migrants that have crossed over the Sonoran desert.

If anyone is interested, the best sites are between Km 77 and Km 82, just south of the police checkpoint at El Doctor, on the good paved highway to El Golfo de Santa Clara.  This is about an hour south of San Luis, the border crossing near Yuma.  There is a network of dirt tracks leading from the highway to the edge of the bluffs, all quite passable in any vehicle.  Again, the bluffs are only about 0.5 km west of the highway.

If you're continuing on to El Golfo de Santa Clara, it is about another 25 minutes down the highway.  The trees in town also had lots of migrants, and the isolation of this town, combined with its extensive plantings, make it an excellent migrant trap.  In the past 2 years we've seen a number of interesting landbirds (for Sonora) here, including White-eyed and Red-eyed vireos, Red-breasted Nuthatch, Rose-breasted Grosbeak, and Red-breasted Sapsucker.

Kimball Garrett

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        Kimball L. Garrett
        Ornithology Collections Manager
        Natural History Museum of Los Angeles County
        900 Exposition Blvd., Los Angeles CA 90007 USA
        213/763-3368 phone; 213/746-2999 FAX
        kgarrett@nhm.org
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