SHORT COMMUNICATIONS 
613 
1980. Rothstein 1993. Shaw and Hauber 2009, 
Schielzeth and Bolund 2010). The first hypoth¬ 
esis is less capable of explaining IBP in our study 
population as the Pale-breasted Thrush is plastic 
in types of sites they use for nesting; they are 
capable of nesting in human-built structures, such 
as on rafters and pillars, as well as in natural 
sites, such as trees in Cerrado areas or forest 
borders. Thirty-three (69%) of 48 nests that we 
monitored on the study area were in trees or 
bushes. 14 (29%) were in man-made structures 
(eaves, rafters or on the top of pillars), and one 
was in a niche in a bank. Thus, unlike cavity 
nesters that may exhibit IBP due to limited nest 
site availability (Yom-Tov 1980, Rohwer and 
Freeman 1989), this parameter is unlikely to 
shape Pale-breasted Thrush reproductive strate¬ 
gics. The other hypotheses are plausible lor 
explaining IBP in this species. Overall nesting 
success estimated using the Mayfield (1961 1 
method was among the highest ever documented 
for thrushes (57%; 748 nest days, n = 37 nests) 
(PVD. unpubl. data), but many nests are depre¬ 
dated. Our study area is an isolated patch ot 
habitat in which a high number ot fledglings are 
produced each season, and territory availability 
may be limited, potentially resulting in a high 
number of unpaired and/or floater lemales. 
Testing these hypotheses would be worthwhile 
for future research. 
The IBP rate apparent from our data must be 
viewed with caution. The first and second obser¬ 
vations arc more reliable than the third and fourth, 
because it seems physiologically possible lor a 
female to lay an egg early or late but not to lay tw o 
eggs in 1 day (Yom-Tov 1980). We arbitrarily 
assumed in the third and fourth observations that an 
egg being laid 2 or more days betore nest com¬ 
pletion or 2 or more days after onset of incubation 
onset could be evidence for IBP. Females did not 
lay eggs more than 1 day alter beginning ot 
incubation except for the fourth nest, in which an 
egg was laid 6 days alter onset ot incubation. 1bus. 
6 days is a large deviation from the norm lLatii 
et al. 2006). We observed other cases of potential 
IBP in addition to the records we considered as 
evidence for IBP. We observed a 1-day gap be¬ 
tween the first and second egg in another nest, and 
there was a 1 -day gap between the second and third 
egg in two additional nests. 
Our observational methods could not identify 
parasitic eggs laid the day after clutch completion 
or the day before clutch initiation, as well as eggs 
that could have been ejected (Latif et al. 2006). 
The IBP rate of the Pale-breastcd Thrush might be 
higher, if our estimate is low, than that lound lor 
the neotropical Blue-black Grassquit (Volatinia 
jacarina) (27.3%) (Carvalho et al. 2006). Both are 
remarkably higher compared with cavity or open 
nesting songbirds from temperate regions: 0- 
7.1% for European Pied Flycatchers (Ficedula 
hxpoletiea). 4.6-5% for Cave Swallows (Petru- 
chelidon fulva), and 10.7-11.9% for American 
Cliff Swallows (P. pyrrlumota) (Yom-Tov 2000. 
Weaver and Brown 2004). 
Onr data document a potential case where 
limited territory availability or nest loss duiing 
laying periods' may be inducing IBP. Further 
researeh is needed to test these and other potential 
explanations for IBP in our study population. 
Only a few cases of IBP have been reported for 
neotropical songbirds, they include representa¬ 
tives of different families: White-throated Mag- 
pic-Jav ( Calocitia formosa) (Corvidae) (Langen 
1996)! Vermilion Flycatcher (Pyrocephalus ntbi- 
nus) (Tyrannidae) (Rfos-Chelen et al. 2008), 
Clay-colored Robin ( Turd"s grayi) (Muscicapi- 
dae) (Dyrcz 1983), and Blue-black Grassquit 
(Emberizidae) (Carvalho et al. 2006), supporting 
the evidence this strategy can be more widespread 
than previously thought (Yom-Tov 2001). Our 
study provides new evidences for IBP among 
neotropical thrushes. 
ACKNOWLEDGMENTS 
We are grateful to Faculdade de Engenharia de Sorocaba 
(FACF.NS) for authorizing field work in die campus. PVD 
was supported by Universidade Federal de Sao Carlos 
(PIADRD fellowship programe), and LSO received a 
fellowship from Conselho Nacional de Desenvolvimento 
CicnuTico e Tecnologico (PIBIC/CNPq). We especially 
lhank A. J. Piratelli. M. N. Schilindwein, A. V. Christiamm. 
and two anonymous referees for important comments on 
previous versions of this manuscript. 
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