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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 3. September 2012 
coarser scale than observed within one region. 
The extended incubation and high nest predation 
rates observed for Yellow-olive Flatbills suggests 
predation risk per se is unlikely to explain the 
observed large clutch size. There is also no 
obvious reason to expect adult mortality to be 
higher for this tropical species, which could 
explain investment in larger clutches. 
The nestling period of Yellow-olive Flatbills was 
longer than for tyrunnids with synchronous nesting 
cycles such as Lathrotriccus euleri (Aguilar et al. 
1999), Mionectes rufiveniris (Aguilar et al. 2000). 
or Leptopogon amaurocephalus (Aguilar 2001) in 
the same area as die present study. Breeding pairs 
of Yellow-olive Flatbills had more fledglings than 
other tyrannids in our study area (Aguilar et al. 
1999, 2000; Aguilar and Marini 2007). Fledglings 
had similar body masses, but were heavier relative 
to adult body mass than other synchronous breed¬ 
ing tyrannids (e.g., Pyrrhomxias cimamomeus, 
Collins and Ryan 1995; L cimaurocephalus . 
Aguilar 2001). Heavier body masses of Yellow- 
olive Flatbills fledglings may reflect asynchronous 
hatching in combination with the more synchro¬ 
nous fledgling events, which indicates older 
fledglings remain longer in nests, being fed by 
adults, at a low energy cost and accumulating body 
mass. Broods with irregular laying and asynchro¬ 
nous hatching are usually larger and produce 
smaller nestlings, as competition for food sets 
the balance between brood and fledgling sizes 
( Ragusa-Netto 1996. Stenning 1996). Our findings, 
however, suggest food resources are likely not a 
limiting factor for nestling growth in our study 
area. 
We observed nest provisioning only once, but it 
was not possible to confirm if it was food transfer 
to nestlings or mate feeding. Considering the nest 
was in the nestling phase, and that two adults Hew 
from the nest, it was possibly feeding nestlings, as 
both parents usually provision nestlings in tyrannids 
(Skutch 1960, 1967). However, the second adult 
attending the nest may also receive and eat the 
provisioned food instead of transferring it to the 
nestlings (Hannelly and Greeney 2008). 
Predation was the main cause of Yellow-olive 
Flatbill nest failure and was equally likely during 
incubation (53%) and nestling (49%) periods. 
Similar predation rates were observed across 19 
passerine species with closed nests (Oniki 1979, 
Martin 1995) and, in Empidona.x oberholseri. nest 
loss was as high as 72% of which 96% was due to 
predation (Liebczeit and George 2002). The 
species studied by Aguilar et al. (2000) and 
Aguilar and Marini (2007) experienced higher 
rates of nest predation during the nestling phase 
than during incubation, possibly due to increased 
parental activity at the nest site, which increased 
attraction of predators during this phase (Rodri¬ 
gues and Crick 1997. Conway and Martin 2000. 
Galhambor and Martin 2000). 
The higher nest predation rates observed for 
Yellow-olive Flatbills in comparison to other 
tyrannids with closed nests likely resulted from 
longer exposure of a larger and asynchronous 
clutch, and higher activity levels at the nest site 
caused by more nestlings. Evolutionary theory 
would predict higher predation rates would act 
to reduce exposure and broods (Martin 1993). 
However, when opportunities to increase fitness 
are used, leading to production of more chicks on 
average, it may also increase predation probabil¬ 
ity. Increased clutch size and longer incubation 
periods of Yellow-olive Flatbills suggest the cost 
of increased exposure is overcome by fledgling 
productivity in the studied population. 
The overall survival of Yellow-olive Flatbill 
nests was lower than reported for passerines in 
temperate zones (Mayfield 1961, Sargent et al. 
1997) in agreement with the well documented 
pattern of higher predation rates in the tropics 
(reviewed by Robinson et al. 2000; but see Martin 
1996). This species also has the lowest nest 
survival rate among studied tyrannids from the 
study area. This observation is valid for spe¬ 
cies that build open nests ( Lathrotriccus euleri. 
Aguilar et al. 1999) or closed nests (Mionectes 
rufiveniris. Aguilar et al. 2000 and Leptopogon 
cimaurocephalus, Aguilar and Marini 200' 1 ). 
Yellow-olive Flatbill nest survival probabilities 
w ere similar to estimates of passerines common to 
open areas ( Pyrocephalus rubious and Zonoiri- 
chiti capensis. Mason 1985; Elaetiia chiriquensis 
albivertex, Medeiros and Marini 2007; Suiriri 
affinis. Lopes and Marini 2005: Elaenia cristata 
Marini et al. 2009), suggesting a possible effect ot 
high exposure from nesting in sites relatively clear 
of vegetation, such as rivers, streams, and dirt 
roads, where Yellow-olive Flatbills nest. Nest site 
selection is an important adaptive behavior as it 
may influence individual fecundity, survivorship, 
and breeding success as dense vegetation provides 
better concealment from predators (Martin 1993, 
1995; Howleu and Stutchbury 1996). Yellow- 
olive Flatbills apparently use old nests as cues tor 
