NESTING SUCCESS OF CALIFORNIA LEAST TERNS 
AT THE GUERRERO NEGRO SALTWORKS, 
BAJA CALIFORNIA SUR, MEXICO, 2005 
ANTONIO GUTIERREZ-AGUILAR, Exportadora de Sal, S. A. de C. V., Guerrero 
Negro, Baja California Sur, Mexico 
ROBERTO CARMONA, Universidad Autonoma de Baja California Sur, Departa- 
mento de Biologia Marina, Apdo. Postal 19-B, La Paz, Baja California Sur, 23080, 
Mexico; beauty @uabcs.mx 
ANDREA CUELLAR, Centro Intercultural de Estudios de Desiertos y Oceanos, A. C. 
Conservacion Costera, Apdo. Postal 53, Puerto Penasco, Sonora 83550, Mexico 
ABSTRACT: We studied a nesting colony of the Least Tern in the saltworks at 
Guerrero Negro, Baja California Sur, between 31 May and 23 July 2005. We marked 
the nests as they appeared and counted nests, eggs, and chicks daily. In a comparison 
of two periods of laying (31 May-13 June and 17 June-19 July) average clutch size 
of 15 nests (2.06 and 2.00 eggs/nest, respectively) did not differ, suggesting that in 
the year of our study food was not a limiting resource. We observed 36 chicks, 31 
from the first period and five from the second. Incubation in the second period was 
interrupted by natural predation of nine nests (eight by coyotes and one by ravens). 
The hatching and fledging success of eggs laid during the first period were 100%, 
higher than for those laid in the second period (16% and 26%, respectively); the low 
values of the second period are attributable to a longer time during which the colony 
was exposed to predation. Though in an artificial habitat, the colony was not affected 
by human disturbance and enjoyed a high success rate overall. The problem of coyote 
depredation could be prevented with a fence, increasing the terns’ success further. 
The California Least Tern ( Sternula antillarum browni ) breeds from 
central California to the southern Baja California Peninsula (Thompson et al. 
1997). Prior to the 1970s, Least Tern populations in California decreased 
because of loss of nesting habitat on beaches to human development and rec- 
reation (Massey 1974). Listing of S. a. browni under the United States and 
California endangered species acts in the early 1970s led to management 
such as nest-site protection and predator control, and these have yielded 
population increases in California (Thompson et al. 1997). In California, 
many aspects of Least Tern reproductive biology, such as site fidelity, habi- 
tat requirements, and reproductive success (e. g., Massey 1974, Atwood 
and Massey 1988) have been studied, as well as the species’ response to 
management (Swickard 1974, Massey and Atwood 1981). 
On the Baja California Peninsula, even though most of the coastline 
remains unaltered (Kramer and Migoya 1989), habitat loss and disturbance 
have caused partial or total reproductive failure of the California Least Tern 
in some areas (Palacios 1992, Mendoza 1994, Ibarra and Carmona 1998). 
Information on the birds’ reproductive success, however, is available for 
only the northern and southern extremes of the peninsula (Palacios 1988, 
1992, Mendoza 1994, Ibarra and Carmona 1998, Cuellar 2003). Although 
Least Tern breeding on the peninsula has been reported since 1927, the 
only data available for the entire peninsula are locations of nesting sites 
(Bancroft 1927, Lamb 1927, Massey 1977, Massey and Palacios 1994, 
Palacios and Mellink 1996). 
Western Birds 40:225-229, 2009 
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