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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 1. March 2012 
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Laying Early Inc. Late Inc. Early Nest. Late Nest. 
HG. 2. Daily nest mortality (mean ± SE) during laying, early incubation, late incubation, early nestling ami lute 
nestling periods of the Creamy-hcllied 1 brush. The number of nests included was 39 in laying, 50 in early incubation. 53 in 
late incubation, 50 in early nestling, and 24 in late nestling periods. 
(Fig. 3) and suggests that visual predators were 
involved. This could be related to attraction of 
visual predators when parental activity increased 
around the nest (Skutch 1949, Martin et al. 2000, 
Martin et al. 2011). An experimental approach 
would be needed to rule out other alternative 
hypothesis (e.g., older nesllings/nests may pro¬ 
duce stronger odors that may attract the attention 
of predators). We observed that 33% of the eggs 
disappeared during the incubation stage. This 
could be caused by partial predation or by brood 
parasitism, as Shiny Cowbirds puncture eggs in 
parasitized and in unparasitized nests (Astid and 
Reboreda 2006 ). 
Hatching success, and egg and fledgling 
survival in successful nests (nests that were not 
predated or deserted) was high (74, 67, and 87%. 
respectively). Hatching success and egg survival 
were not related to number of eggs present in tin 
nest. There was a proportion of eggs that did no 
hatch (0.26) and this may be caused by infertilit\ 
or by death of the embryo caused by insufficien 
incubation (Davies and Brooke 1988. Svenssoi 
et al. 2007). We found that number of egg: 
present in the nest had no effect on hatching 
success and it is unlikely that insufficien 
incubation was the main cause. Nestlings dice 
rom starvation more often when they shared the 
“rr lh T ° ne Sib,ing - siting that 
food competition between nestlings was strong 
Most of the nestlings survived when only one or 
two eggs hatched due to previous egg loss or 
hatching failure. However, when all eggs hatched, 
at least one nestling died. The brood reduction 
hypothesis (Lack 1947, Ricklefs 1965) suggests 
that some species lay more eggs than they can rear 
nestlings because food supply may vary unpre- 
dictably. Thus, parents may lay a large clutch 
appropriate for a food-rich year and. in food-poor 
years, nestling survival would be mediated by 
sibling competition. The brood reduction hypoth¬ 
esis could be a possible explanation for Creamy- 
hcllied Thrush nestling mortality. However, future 
studies should evaluate if nests with three 
nestlings improve nestling survival in food-good 
years. 
The clutch size of Creamy-bellied Thrushes at 
our study site was small (mean = 3 eggs) and 
typical of tropical and southern temperate birds 
There is a lack of information of the breeding 
biology of most thrushes, but the available data 
suggests this group has the same pattern as other 
passerines. The clutch size reported for thrusto' 
from north temperate areas is 4-5 eggs (Scbnao- 
1991, Arheimcr and Svensson 2008. Morton am- 
Pereyra 2010), and is 2-3 eggs for thrushes a 
tropic and south temperate areas (Lichtensteu 
1998. Sackmann and Reboreda 2003. Akinp- ■' 
2005, Astie and Reboreda 2005, Halupka and 
Greeney 2009). Ours is the first study presenting 
data from a Tardus species that inhabits an so 
temperate area as far as we know. We did not 
