Clutch Size in Eastern Bluebirds:

1
Clutch Size in Eastern Bluebirds No Support for
Individual Optima 
Austin Mercadante, Nichole Cudworth, Mark
Stanback Department of Biology, Davidson
College, Davidson, NC
Abstract
The Individual Optima Hypothesis proposes that
individual females lay a clutch that reflects the
quality of their breeding situation (the quality
of themselves, their mate, their territory, and
the season). Consequently, clutch size should be
a predictor of chick condition, since the latter
undoubtedly reflects the quality of the breeding
situation. However, when we examined factors
potentially affecting chick condition, clutch
size had no significant effect, whereas year,
nest date, and brood size all did. We then
compared broods of identical size and similar
nest date (within 2 days), but different clutch
sizes and found no difference in chick condition.
These data suggest that clutch size is not a
reliable indicator of breeder condition in
eastern bluebirds.
Thanks go to the many students who have collected
bluebird data over the years. 2005 funding
provided by a NSF REU Site Grant.
For our paired comparison, we used nests with
nearly identical initiation dates and compared
the condition of alpha chicks in broods of 4 from
nests in which 5 vs. 4 eggs were laid. Clutches
of 5 and 4 are the most common clutch sizes in
our population 4 is the modal brood size.
Moreover, we wanted to compare the condition of
chicks in nests where brood size clutch size
vs. chicks in nests in which brood size lt clutch
size. If clutch size accurately reflects the
breeding condition of females, we would expect
that when we control for year and nest date,
alpha chicks from clutches of 5 would be in
better nutritional condition than alpha chicks
from clutches of 4.
Introduction
Lack (1947) proposed that natural selection
would produce in each population a mean clutch
size that maximizes fitness. In order to explain
the variation seen within populations, Pettifor
et al (1988) hypothesized that individual females
choose a clutch size that reflects the quality of
their breeding situation (the quality of
themselves, their mate, their territory, and the
season). This Individual Optima Hypothesis
predicts that clutch size should be an accurate
predictor of chick condition. We tested this
prediction using reproductive data from eastern
bluebirds nesting near Davidson, NC.
In broods of 4 with similar nest initiation
dates, alpha chicks from clutches of 5 were in
similar condition to alpha chicks from clutches
of 4 (N 150 pairs).
Methods
Results
Conclusions
Reproductive data (nest initiation date, clutch
size, brood size, and chick condition) have been
collected weekly from approximately 400 nestboxes
in the Davidson area since 1999. Because chicks
were measured anytime between 8 and 14 days of
age, we quantified chick condition using the
residuals from a 3rd order polynomial regression
of mass and wing length. In this way we were
able to assess the nutritional condition of
nestlings of various ages. To avoid
pseudo-replication, we included only the
largest/longest-winged (alpha) chick in our
analyses. The Individual Optima Hypothesis
predicts that clutch size (as well as brood size)
should have a measurable effect on chick
condition.
Factor DF SS F P Year 6 257.06 11.12
lt0.001 Month 4 656.72 35.49 lt0.001 Clutch 4
14.33 0.77 0.542 Brood 5 160.09 6.92
lt0.001

• Chick condition was influenced by year, nest
  date, and brood size, but not clutch size.
• Comparing nests with similar initiation dates,
  we found no difference in chick condition between
  chicks in broods of 4 that hatched from clutches
  of 4 vs. 5.
• These results, along with a brood size
  manipulation experiment performed in 2004,
  suggest that clutch size is not a reliable
  predictor of breeder condition in eastern
  bluebirds.

Clutch size had no effect on chick condition,
whereas year, nest date, and brood size did.