Soley et al • HATCH ORDER OF EASTERN BLUEBIRDS 
775 
TABLE 1. Effects (estimates ± SE) of hatch order and gender on 
concentration, and plumage coloration traits of nestling Eastern Bluebirds. 
body size, begging behavior. 
corticosterone 
Trail 
Age(day) 
Factor 
Estimate r SE 
df 
F 
p 
Mass (g) 
Hatch order" 
1.24 ± 0.24 
1, 525 
26.4 
<0.001 
Gender*’ 
-0.18 ± 0.24 
1, 525 
0.58 
0.45 
Age 
2.09 ± 0.03 
1, 525 
5.734.90 
<0.001 
Brood size 
-0.34 ±0.11 
1. 525 
9.37 
0.002 
Tarsus length (mm) 
14 
Hatch order* 
0.46 ± 0.52 
1, 85.0 
0.77 
0.38 
Gender* 1 
0.43 ± 0.51 
1, 85.0 
0.72 
0.40 
Brood size 
0.30 ± 0.26 
1. 85.0 
1.27 
0.26 
Wing length (mm) 
14 
Hatch order* 
1.01 ± 0.42 
1, 65.4 
5.79 
0.02 
Gender* 1 
-0.65 ± 0.44 
1, 67.8 
2.23 
0.14 
Brood size 
0.20 ± 0.56 
1, 26.7 
0.14 
0.72 
Begging intensity 
5 
Hatch order* 
-2.90 ± 1.27 
1, 66.5 
5.26 
0.02 
Gender* 1 
-2.00 ± 1.31 
1. 72.2 
2.33 
0.13 
Brood size 
0.51 ± 1.08 
1, 27.7 
0.23 
0.64 
Con (ng/ml) 
8 
Hatch order* 
-0.03 ± 0.04 
1, 42.7 
0.42 
0.52 
Gender* 1 
0.04 ± 0.04 
1, 50.0 
0.85 
0.36 
Brood size 
0.01 ± 0.02 
1, 15.9 
0.07 
0.80 
Brightness (%) 
14 
Hatch order* 
0.15 ± 0.43 
1. 58.4 
0.12 
0.73 
Gender* 1 
-2.56 ± 0.45 
1. 66.3 
32.8 
<0.001 
Brood size 
-0.92 ± 0.35 
1. 24.1 
7.03 
0.01 
UV chroma (%) 
14 
Hatch order* 
-0.01 ± 0.01 
1, 58.8 
0.02 
0.89 
-0.05 ± 0.01 
1, 66.9 
159.7 
<0.001 
Brood size 
-0.01 ± 0.01 
1, 24.3 
11.0 
0.003 
b Climates are relative lo late-hatchcd nestlings. 
Estimates are relative to males. 
hatched nestlings. Eggs in 14 of the 31 clutches 
measured hatched within 24 hrs and 17 clutches 
exceeded a 24-hr hatching span. We found no 
effect of laying order on nestling gender ( Wald X" 
= 0.18,/, = 3|, p = o.67); 56% of the early-laid 
e ?S s were male and 54% of late-laid eggs were 
female. 
Effect of Hatch Order and Gender on Nestling 
$i:e. Begging Behavior, and Hormones. Early- 
hatched nestlings were significantly (P < 0.001) 
heavier than late-hatched nestlings throughout the 
nestling period (no significant age x hatch order 
interaction; P = 0.83), brood size negatively 
affected mass, and we found no effect of gender 
0,1 mass (Table 1; Fig. I). Late-hatched nestlings 
^ged significantly more vigorously than their 
e arly-hatched siblings, but neither brood size nor 
gender influenced begging rates (Table 1; Fig. 2). 
We found no effect of hatch order, gender, or 
“rood size on nestling corticosterone levels 
♦Table I). We also found no difference in length 
01 tarsi of early- and latc-hatched nestlings at 
14 days of age. but early-hatched nestlings had 
lon ger wings (Table 1). Hatch order did not 
significantly influence either brightness or chroma 
of plumage coloration. However, males were 
significantly more colorful than females and 
nestlings from smaller broods were more colorful 
than those from larger broods (Table l). 
DISCUSSION 
We were able to identify the hatching order of 
all eggs in only ~15% of the clutches despite 
close monitoring for hatching, demonstrating that 
many nestlings hatch during the night and many 
clutches hatch synchronously in this population of 
Eastern Bluebirds. Thus our data may represent 
nests with the greatest hatching asynchrony. 
Position in laying order was a strong predictor 
of position in hatching order, and hatching order 
influenced the morphology and behavior of 
nestlings. Late-hatched siblings had lower mass 
compared to their early-hatched siblings through¬ 
out the nestling period, and late-hatched nestlings 
begged more vigorously than early-hatched nest¬ 
lings at 5 days of age. Late-hatched nestlings were 
lighter in mass and had shorter wing length near 
the age of fledging. It is possible that late-hatched 
nestling bluebirds are more likely to experience 
post-fledging mortality compared to their older 
