Segura el al. • MICROHABITAT EFFECT ON NEST SURVIVAL 
509 
TABLE 2. Estimated coefficients and precision for the top additive model (Table 1 
crested Cardinal nests. 
) explaining daily survival of Red- 
95% confidence interval 
Parameter 
Estimate (p) ± SE 
Lower 
Upper 
Intercept 
Date throughout the season 
Cover above nests 
1.161 ± 0.444 
-0.008 ± 0.004 
1.058 ± 0.319 
0.291 
-0.228 
0.431 
2.032 
-0.002 
1.685 
concealment helps hide and protect nests front 
predators, and earlier nesting attempts within 
the reproductive season are more successtul than 
those later in the season. 
Daily nest survival rates improved significantly 
in relation to increased nest cover above, possibly 
because of lower accessibility and less visibility 
of eggs and chicks. Vegetation above the nest had 
a significant impact and we speculate the main 
predators access nests from above (e.g., aerial 
predators such as the Guira Cuckoo, Chi mango 
Caracara, or Narrow-billed Woodcreeper). and not 
from the tree trunk li.e., terrestrial predators such 
as mammals or snakes). The main key for aerial 
nest predators to find nests is probably visual, and 
the more visual barriers, the less likely a predator 
might detect a nest (Watts 1989). 
Dense tree-canopy seems to provide more 
protection and Coronillo trees should offer high 
quality nesting sites and he chosen more fre¬ 
quently than Tala trees, which have less leafy 
canopies, However. Red-crested Cardinals nested 
more frequently in Tala than Coronillo trees 
Segura and Arturi 2009). Some authors have 
suggested that open nesting birds should have a 
balance between the advantages of high conceal¬ 
ment and the need to maintain visibility from the 
nest i.e,. the advantages to avoid predators by 
covering the nest and disadvantages associated 
with too much concealment (Gotmark ct al. 1995. 
Wilson and Cooper 1998). Red-crested Cardinals 
appear to use this balance, selecting sites with 
higher cover above the nest to avoid predators 
and. at the same time avoiding building nests in 
too concealed sites that may reduce visibility from 
the nest when predators (or conspecifics) ap¬ 
proach the nest. Another possible disadvantage is 
that nest sites that arc too covered may have low 
ventilation and luminosity, which may increase 
the frequency of ectoparasites (Love and Carroll 
1998. O'Connor et al. 2010). 
The date within the breeding season also had a 
significant effect on nest survival, indicating the 
earliest nests (initiated in Oct/Nov) are less likely 
to fail than those initiated in January and 
February. These results are consistent with studies 
which also found seasonal variation in Passer¬ 
iformes (Hochachka 1990, Jehle et al. 2004. Grant 
et al. 2005. Moreno et al 2005: but see Burhans 
et al. 2002). The decline in nest success with date, 
as Grant et al. (2005) suggested, could be the 
result of an increase in predator abundance and 
movement later in the reproductive season by 
post-reproductive adults and dispersing juveniles. 
The cumulative probability of nest survival was 
0.17 for a nest initiated in the middle of the 
breeding season (1 Dec) in a site with 50% cover 
above the nest. The Red-crested Cardinal has 
higher nest predation rates than north temperate 
birds (Martin 1993a), but similar predation rates 
to south temperate buds (Merino/ and Reboreda 
1998, Mezquida and Marone 2001. Astie and 
Reboreda 2006, Delhey el al. 2010. De Marsico 
and Reboreda 2010, Di Giacomo et al. 2011). Nest 
predation appears to be higher in several South 
American than North American locations, and 
predation might have had a stronger influence on 
the evolution of birds’ life-history traits in South 
America. South temperate birds should have 
strategics that allow them to maximize their 
reproductive fitness given high predation rates. 
Segura (2011) reported that, after a predation 
event, each pair of Red-crested Cardinals at this 
same study site rapidly made another reproductive 
attempt in the same territory. Nest intervals in 
Red-crested Cardinals are as short as 6 days from 
nest loss to initiation of the next clutch (Segura 
2011). and the breeding season is long, from 
October to February. This allows Red-crested 
Cardinals to have at least 6-8 reproductive 
attempts in a single season (Segura 2011). 
This is the first study to evaluate the effect of 
microhabitat vegetation cover on daily nest 
survival rates for a south temperate passerine. 
Little is known regarding breeding biology and 
bird predator communities in south temperate 
