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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 3, September 2011 
colored Thrush support this prediction although 
those for White-throated Thrush indicate that 
local-scale nest site characteristics were greater 
influences on nest success than land-cover type. 
White-throated Thrush ground nests were safer 
than those in vegetation in either land-cover type. 
Ground nests at Las Alturas were typically built 
into steep banks, which we assume protected them 
from being stepped on by livestock. DMR 
increased with increasing nest height, even when 
ground nests were excluded from analyses, i.e., 
nests low in the vegetation were safer than nests 
higher in the vegetation. 
Higher DMRs for While-throated Thrush nest¬ 
ing attempts that were above the ground, higher in 
the vegetation, and with low levels of conceal¬ 
ment suggest that predatory birds may be an 
important influence on nest success. Hawks and 
toucans were frequently observed perching in the 
overstory ol the coffee, apparently scanning for 
movement, and a toucan was observed eating 
nestlings (Cohen and Lindell 2004; C. A. Lindell, 
pers. obs.). Snakes were likely important nest 
predators as well (Cohen and Lindell 2004). Our 
results indicate our measurements of concealment 
were positively correlated with the way some nest 
predators perceived nests. 
Nest initiation date was not in the final model 
for either species, similar to a previous study that 
found no seasonal variation in nest predation 
(Young 1994). This suggests a possible difference 
in seasonal effects on nest success between 
tropical and temperate-nesting populations (Nur 
et al. 2004). 
Overall White-throated and Clay-colored thrush 
nest success rates at Las Alturas (16.9 and 10.2%, 
respectively) are comparable to some of the lower 
rates that have been reported for neotropical 
species. The nest success rate for 43 species 
combined in tropical lowland forest in Costa Rica 
was 24.4% (Young et al. 2008) and ranged from 
8.3 to 71.6% for 10 species in lowland forest in 
Panama (Robinson et al. 2000). In contrast, 
success rates for both thrush species in the active 
coffee plantation at Rio Negro were high (98.5 
and 67.5% for White-throated and Clay-colored 
thrushes, respectively). Only three of 24 thrush 
nesting attempts at Rio Negro were documented 
as failures. We suggest these results indicate two 
points deserving further investigation. First, all 
but one of the Rio Negro nests were in the highest 
concealment category. The coffee plants at this 
site were under uetive cultivation and tended to be 
more leafy and compact than those at Las Alturas. 
providing more concealment for nests. This 
pattern, along with the inclusion of concealment 
in the White-throated Thrush nest success model, 
suggests concealment may be a particularly 
important influence on tropical thrush nest 
success. Second, the area around Rio Negm is 
much less forested and has a greater human 
population density than Las Alturas. These two 
factors potentially result in reduced habitat for 
predatory birds and snakes, and greater hunting 
pressure on these species, diminishing predation 
pressure on nesting birds. 
CONSERVATION IMPLICATIONS 
Conservation recommendations based on land- 
cover type would be relatively easy if we could 
rank land-covers as to the quality of habitat they 
provide for target species and if rankings were 
consistent across species. Our results indicate 
these conditions are not met for these species. 
Clay-colored Thrush nest success was lower in 
abandoned coffee than pasture at our primary 
study site, but nest success was high at an active 
coffee plantation; a simple dichotomy of coffee 
and pasture could not predict nest success. White- 
throated Thrush nest success varied substantially 
with local-scale nest-site characteristics. Thus, for 
this species, variation within a land-cover type 
influenced nest success more than variation 
among land-cover types. In addition, hunting 
pressure lor predatory birds and snakes, and 
landscape features such as forest cover potentially 
influenced nest success. 
White-throated Thrush nests that were well- 
concealed, either because of vegetation near the 
nest or because of an inconspicuous location 
w ithin a bank, had higher probabilities of success 
than less-concealed nests. Maintaining features 
that help conceal nests, in the form of dense 
foliage and the existence of vines and moss (Stiles 
and Skutch 1989). and or the existence of steep 
banks, should contribute to White-throated Thrush 
nest success more than land-cover type. 
We suggest focusing resources in three ways to 
make progress in understanding the factors that 
influence nesting success of tropical birds and in 
providing conservation recommendations to trop¬ 
ical property owners and land managers. 
I. Focus on species of conservation significance 
w'ith nests that can be found and monitored in 
reasonable numbers. 
