NESTING SUCCESS OF CALIFORNIA LEAST TERNS 
Table 1 Breeding of the California Least Tern at the Guerrero Negro 
Saltworks, Baja California Sur, Mexico, 2005 
Variables 
measured 
First 
period 
Second 
period 
Both 
periods 
combined 
Kruskal- 
Wallis 
value 
Probability 
Number of nests 
15 
15 
30 
Eggs laid 
31 
30 
61 
(31 May- 
(17 June- 
13 June) 
19 July) 
Chicks 
31 
5 
36 
(20 June- 
(8 July- 
3 July) 
18 July ) 
Clutch size 
2.06 
2.00 
2.03 
0.17 
0.67 
Hatching success 
100% 
16% 
59% 
43.05 
<0.001 
Nesting success 
100% 
26% 
63% 
16.79 
<0.001 
tracks, by coyotes ( Canis Iatrans), and the remaining nest apparently was 
raided by Common Ravens ( Coruus corax). 
For both periods combined, the hatching success was 59% and the nest- 
ing success was 63% (Table 1). During the first period hatching success and 
nest success were 100%, but during the second period they were 16% and 
26%, respectively (Table 1). 
DISCUSSION 
In 2005 Least Terns nested at the Guerrero Negro saltworks during the 
same interval as observed at Laguna Percebu at the extreme north of the 
Gulf of California (Palacios 1992) and in some years at Ensenada de la Paz 
in the far south (Cuellar 2003 for 1999), with the first laying at the end of 
May. Laying began, however, two weeks later than is typical for colonies in 
California (Massey and Atwood 1981) and as observed at Ensenada de La 
Paz in other years (Cuellar 2003 for 2000). Therefore the timing of repro- 
duction at a given site apparently depends more on the specific conditions 
during a given season than on the geographic location. 
At various sites on the peninsula, one of the main reasons Least Tern nests 
have failed has been human disturbance (Palacios 1988, 1992, Mendoza 
1994). Within the Guerrero Negro saltworks, disturbance is at a minimum 
(Carmona and Danemann 1998), as probably reflected in the high success 
values we recorded, especially for the first period. Another factor that com- 
monly diminishes success on the peninsula is tidal flooding (Palacios 1992, 
Ibarra and Carmona 1998), but the nests we studied were all outside the 
range of the tides. 
At Guerrero Negro, nest failure was due largely to predation, with 60% of 
eggs laid after 17 June being lost to predation. Increased predation later in 
the season may be the result of the length of time of the colony is exposed; 
it has been noted that as more time passes, the probability that predators 
will locate the nests increases (Brunton 1999). 
Predation observed in the saltworks was by native species (coyotes and 
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