21 MAY 1998. MEXICO (GULF OF CALIFONIA): SEABIRDS & LANDBIRDS

"Although sea surface temperatures are not currently exceptionally high in relation to normal (and may in fact be returning to normal), ENSO conditions during the previous winter of 1997-98 have apparently resulted in significant effects on the breeding and wintering birds of the Sea of Cortez and off the West Coast of Baja California. In preliminary surveys, Daniel Anderson, James Keith, Eduardo Palacios , and others (University of California, Davis) have found that brown pelican nesting effort in 1998 compared to "normal" was roughly as follows:
 
 
COLONY NORMAL NUMBERS (Nests) 1998 NUMBERS (Nests)
    Mar/Apr May
Isla San Pedro Martir
4,000-5,000
?
0
San Lorenzo area
10,000-20,000
280
0
Isla Piojo
500-1,000
20
0
Puerto Refugio
1,000-3,000
300
0
Isla San Luis
5,000-10,000
150
0
Laguna San Ignacio(on the Pacific side)
500-1,000
500
4
 

This is the most extensive El Nino related nesting failure seen in Baja California brown pelicans since studies began in 1970. Brown and blue-footed boobies were also present on the Midriff islands and northern Gulf of California in large numbers in the Spring of 1998, at a time of the year when they should be concentrated at nesting colonies such as Isla San Pedro Martir and Isla San Jorge (Erik Mellink from CICESE reports little nesting on the colony that normally has thousands of pairs, and there were no nesting boobies on San Pedro Martir in early-May). Storm petrels (least and black) were very scarce in the Midriff in March and April this year when they would normally be by far the most abundant birds seen on pelagic surveys; better numbers were seen to the south near Isla San Pedro Martir, and by May, increasing numbers were seen in offshore pelagic surveys in the Midriff area. Heermann's gulls at Isla Rasa were present in large numbers in late-March, but 4-5 weeks behind in their normal phenology and still widely dispersed, as were elegant and royal terns. By May, HEEG were beginning nesting at almost normal numbers on Isla Rasa (still 4-5 weeks later than normal), but nesting tern numbers (ELTE and ROTE) continued to be depressed. There were Brant's cormorants at two colonies with known histories that began nesting in normal numbers in March and April, but by May these two colonies were abandoned; and double-crested cormorants were abandoning nests at the same time (one small colony at Isla Gemelos east started with about 35 nests that were down to about 5 nests in May, and a small 30-50 nest colony at Isla San Luis was completely abandoned by May). Yellow-footed gulls were more dispersed and pelagic than normally found at this time of year, as well, and few nesting attempts were seen (one brood of newly hatched young on Isla Smith was the only seen in 1998).

Osprey, the most resident of the breeding "seabirds" in the study area, have during past El Nino events shown reduced but still somewhat successful breeding efforts. These birds feed largely on resident, local species of fish. In 1998, about two-thirds to one-half of the original nesting attempts in the Bahia de los Angeles area were abandoned by May, the lowest yet recorded in long-term studies since 1971. Yet, the osprey was the most successful of the nesting birds of the offshore islands in the Sea of Cortez, even in this exceptionally strong El Nino year of 1998, producing roughly 0.3 fledglings per original nest attempt.

So far, there has been little or no widespread high mortality detected in breeding species such as brown pelicans but some adult blue-footed boobies were showing up in beach walks in some local areas (we are still trying to locate areas of increased mortality for pelicans in Baja California and found one area from roughly Bahia Magdalena north to Laguna San Ignacio where higher numbers of carcasses were found; and an unusually high proportion of those carcasses were adult-plumaged birds. The state of the carcasses indicated that this mortality had occurred during the winter of 1998 in January and February-and at the same time we received reports of an active dieoff in those areas from cooperators who responded to our earlier e-mail requests for information). But wintering species such as eared grebes, pacific loons, and common loons were known to be dying at higher than normal rates over a larger area, and they were emaciated and showed delayed basic to alternate plumage development.

Preliminary results so far are just as predicted for this exceptionally strong El Nino. Long-term studies of seabird populations in the Gulf of California by D. W. Anderson have now continuously covered four major and three less-intense ENSO events. "-- Dan Anderson <fzdanand@mailbox.ucdavis.edu>.

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