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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123. No. 4. December 2011 
FIG. 1. Mass of nestling Eastern Bluebirds that hatched 
early (gray) or late (white). The line within each box 
represents the median mass; the upper and lower borders of 
each box represent the 25 and 75% percentiles; the lower 
and upper bars are the 10 and 90% percentiles. 
siblings as larger body mass and wing length 
influence post-fledging survival in other species 
of altricial birds (RSbefg ct al. 2005). We failed to 
detect any interaction between hatch order and 
gender, and found no effect of hatch order on 
nestling corticosterone or plumage coloration. 
Lack of effect of hatch position on stress 
hormones and color suggest that late-hatched 
chicks may not have experienced extreme stress 
as a result of hatching position. 
Studies of other species of birds also found 
laying order can be a strong predictor of hatching 
order < Beissinger and Waltman 1991. Clotfelter el 
al. 2000, Saino et al. 2001). This relationship is 
expected when females commence incubation 
prior to laying of the final egg (Magrath 1990). 
Laying order affects hatching order, and female 
bluebirds could potentially strategize their invest¬ 
ment in eggs from different positions in the laying 
sequence to compensate for chick disparity caused 
by hatching asynchrony. Thus, although females 
could have differentially invested in eggs from 
different positions in the laying sequence, we 
found no evidence of a greater incidence of males 
hatching early. Our observations are similar to 
those of Lombardo (1982) for Eastern Bluebirds 
and Koenig and Dickinson (1996) for Western 
Bluebirds (Sicilia ntexicana), who also found no 
evidence ot sex ratio bias associated with 
environmental conditions. 
Early-hatched nestlings were significantly larg¬ 
er than their late-hatched siblings throughout the 
growth measurement period (up to day 14). The 
FIG. 2. Comparison of the begging intensity of nestling 
Eastern Bluebirds that hatched early and late, standardized 
by nest. The line within each box represents the median 
begging intensity score; the upper and lower borders of 
each box represent the 25 and 75% percentiles; the lower 
and upper bars are the 10 and 90% percentiles, 
early fledgling period is a lime of high mortality 
in passerine birds (Sullivan 1989). and all siblings 
in a brood tend to Hedge on the same day (L. 
Siefferman, pers. obs.); thus, nestlings with 
relatively shorter wings may be less able to 
survive the fledgling period. We found no 
evidence that bluebirds in our population use a 
strategy of brood reduction; nestlings that ap¬ 
peared to die of starvation were no more likely to 
have hatched early or late in the dutch. Bluebirds 
in our study population experienced low incidence 
of nestling starvation compared to birds in other 
studies (Kendeigh 1942, Pinkowski 1977). 
Hatching asynchrony created differences in 
begging rates among Eastern Bluebird nestlings: 
the late-hatched nestlings likely experienced 
greater need for food as they had lower mass 
and begged more than their early-hatched siblings. 
Begging has been shown to be an honest signal of 
need in other passerine species (Mondloch 1995. 
Price and Ydenburg 1995, Lotem 1998. Saino et 
al. 2000). Late-hatched nestlings did not have 
higher concentrations of circulating corticosterone 
at 8 days of age. Past experiments demonstrate 
that corticosterone promotes nestling begging 
(Kitaysky et al. 2001b, Loiseau et al. 2008). 
Corticosterone concentrations in those studies 
mimicked levels exhibited by nestlings during 
