Undell et al • NEST SUCCESS OF NEOTROPICAL THRUSHES 
505 
TABLE 2. Daily mortality rates (exposure days) of White-throated Thrush nests at Las Alturas by substrate, 
concealment, height, and year and of Clay-colored Thrush nests by land-cover type. Only DMRs for those variables in final 
Cox regression models are included. 
Year 
Daily mentality rale r SE 
(White-throated Thrash) 
Nesl success rate 
<%) 
Daily mortality rate ± SE 
(Clay-colored Thrush) 
Nest success rate 
(%) 
Ground 
0.058 ± 0.005 (1.955) 
21.2 
Vegetation 
0.076 ± 0.007 (1,520) 
12.8 
Concealment low 
0.090 ± 0.017 (288) 
8.6 
Concealment medium 
0.077 ± 0.009 (941) 
12.5 
Concealment high 
0.058 ± 0.005 (2.246) 
21.2 
Height low 
0.059 ± 0.004 (2,889.5) a 
20.6 
Height medium 
0.100 ± 0.013 (501) 
6.5 
Height high 
0.118 ± 0.035 (84.5) 
3.8 
1999 
0.037 ± 0.018 (109) 
37.5 
2000 
0.049 ± 0.008 (809.5) 
27.1 
2001 
0.077 ± 0.008 (11,121.5) 
12.5 
2002 
0.068 ± 0.007 (1,435) 
16.0 
Coffee 
0.096 ± 0.012 (560.5) 
7.3 
Pasture 
0.054 ± 0.014 (275.5) 
23.6 
The low heijrhl category include* both ground ncsls and those above the ground up to 1-5 in. When ground nests are excluded from calculations, the DMR for 
low nests is still lower than those lor higltcr height categories (DMR - 0,061 — 0.008. n - 934 5 exposure days). 
attempts in a lower concealment category had a 
27.3% greater rate of failure than those in the next 
higher concealment category, (3) increasing nest 
height from one category to the next resulted in a 
68 .6% greater occurrence of nest failure, and (4) 
nesting attempts in 1999 had a 44.8% higher rate 
ol success than those in other years (Tabic 1). 
Overall daily mortality rate (DMR) for all White- 
throated Thrush nests at Las Alturas over all years 
was 0.066 ± 0.004 (n = 3,475 exposure days), for 
an expected nest success rate of 16.9%. DMR for 
categories of White-throated Thrush nesting 
attempts varied (Table 2). 
The final Clay-colored Thrush model included 
only land-cover type, which was a marginally 
significant influence on nest survival (P = 
0.0567. p = -0.4714, % 2 = 3.6308; Table 2). 
Tile hazard ratio for the land-cover variable was 
0'624, indicating nesting attempts in coffee had a 
38.6% greater rate of failing than those in pasture; 
consequently, DMRs for nests in coffee were 
higher than in pasture (Table 2). The overall 
DMR for Clay-colored Thrush nests at Las 
Alturas was 0.084 ± 0.010 (/t - 847 exposure 
days), or an expected nest success rate of 10.2%. 
Daily mortality rates at Rio Negro were 
approximately an order of magnitude lower than 
at Las Alturas; 0.006 ± 0.004 for White-throated 
Thrush (n = 18 nests and 318.5 exposure days) 
'°r an expected nest success rate of 98.5%, and 
0-015 ± 0.015 for Clay-colored Thrush (n = 6 
nesting attempts and 67 exposure days) for an 
expected nest success rate of 67.5%. All White- 
throated Thrush nesting attempts at Rio Negro 
were in vegetation rather than on the ground and 
all nests where concealment was measured were 
in the highest concealment category (n = 15). 
Sixty-seven percent of the attempts were in the 
lowest height category and the other 33% were in 
the medium height category (n = 15; 3 nests were 
not measured because attempts were still occur¬ 
ring when the field season ended). Clay-colored 
Thrush nesting attempts were all in vegetation, 
five of six were in the highest concealment 
category, and five of six were in the medium 
height category. 
DISCUSSION 
Our first objective was to compare nest success 
of Clay-colored and White-throated thrushes in 
two land-cover types, coffee and pasture, that 
cover large areas of the Neotropics. Clear 
differences in nest success would facilitate 
ranking the relative value of these land-covers 
as nesting habitat for these species. Our results 
indicated that nest success was site-specific and 
varied with nest-site characteristics, making 
conservation recommendations based on land- 
cover type elusive, and underscoring the necessity 
of investigating environmental influences on nest 
success at multiple scales. 
We expected daily mortality rates (DMRs) 
would be greater for nests in coffee than pasture 
(Lindell and Smith 2003). The results for Clay- 
