NESTING SUCCESS OF CALIFORNIA LEAST TERNS 
The Least Tern’s nesting at Ojo de Liebre (Scammon’s) Lagoon has been 
known since 1927 (Bancroft 1927). But its reproductive success in the central 
peninsula had not been studied until we addressed the question in the Guerrero 
Negro saltworks (central Pacific coast of the peninsula) in 2005 nesting. 
STUDY AREA AND METHODS 
The Guerrero Negro saltworks are a part of the El Vizcaino Biosphere 
Reserve and are also operated by Exportadora de Sal (ESSA); they are closed 
to the public. Salt production involves pumping seawater from the Ojo de Lie- 
bre Lagoon along a system of connected ponds covering 27,773 ha. Water 
levels and salinity are kept fairly stable at all times, as a requirement of the 
industrial process. The first ponds of the system (Sl-A and pond 1) receive 
water pumped from the lagoon, and their salinities are similar to those of the 
lagoon (ca. 33 practical salinity units); salinity values then ascend gradually 
from one pond to the next. The brine yielded by evaporation is pumped to 
a series of crystallization ponds where the salt precipitates and is collected. 
Although salt is produced year round, salt is collected from each pond for only 
one week every six months, At other times the ponds are undisturbed. 
The colony we studied settled on a sand substrate with patches of salt 
flats. The colony is surrounded by low dunes and a margin of crystallization 
ponds (pond number 26: 27° 56' 42" N and 114° 02' 24" W). There is no 
vegetation on the nesting area, although patches of alkali heath (. Frankenia 
spp.) and saltbush [Atriplex spp.) grow nearby and were used by the chicks 
to hide and to protect themselves from the sun and wind. The colony is 
approximately 4 km from the closest fishing areas. 
Once we located the colony, we searched for nests daily (between 07:00 
and 08:00), from 31 May until 23 July 2005. We counted the number of 
eggs in each nest and placed numbered wooden markers alongside each nest. 
We noted each egg’s date of hatching and, in case of failure, the probable 
causes on the basis of nearby tracks. Using these data, we calculated two 
indicators of reproductive success: hatching success (proportion of chicks 
hatched out of the total number of eggs laid) and nesting success (propor- 
tion of nests that produced at least one chick; Erwin and Custer 1982). We 
also compared differences in clutch size and reproductive success between 
the two nesting periods by using a Kruskal-Wallis nonparametric test (a = 
0.05 in all cases; Zar 1999). 
RESULTS 
The reproductive season ran from 31 May until 23 July 2005. Most eggs 
(31, 51%) were laid during the first two weeks of the season (31 May-13 
June), in 15 nests. Subsequently, laying was less concentrated, with 30 eggs 
laid over six weeks (17 June-23 July). In the first period, average clutch size 
was 2.06 eggs per nest, in the second, 2.00 eggs per nest; thus the periods 
did not differ significantly in clutch size (Table 1). 
We observed 36 chicks, 31 hatched from eggs laid during the first period, 
five from eggs laid during the second (Table 1). The second hatching period 
was interrupted by predation of nine nests, eight of those, judged from nearby 
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