Stambaugh et al. • INNATE IMMUNITY IN NESTLING TREE SWALLOWS 
783 
10 - 
0 . 8 - 
* 
0. 
u 
§ 
0 . 2 - 
° 0-H |-- i-- i -- I 
0.00 0.7 0.8 0.9 1.0 
Midparent MPK 
FIG. 2. The midnestling MPK vs. midparent MPK. v = 
0.92 - 0.32t, F Xi = 1.96, P = 0.22, r 2 = 0.28. 
(Fig. 2). The midnestling MPK-mother MPK 
regression estimate was h 2 = 0.41 ± 0.41 (SE). 
DISCUSSION 
The in vitro microbicidal ability of nestling Tree 
Swallow whole blood to kill E. coli increased 
during development (Fig. I A). The in vitro micro¬ 
bicidal ability of whole blood is time dependent 
Willett et al. 2007), and our data suggest the 
relative quickness of nestlings to respond to 
pathogens is low at ND6 and rises to about one- 
half of adult level at fledging (Fig. 1 A). We infer 
rom this pattern that the ability to mount an adull- 
hke innate immune response in Tree Swallows 
requires post-fledging development. 
The leve of in vitro microbicidal ability of whole 
blood of ND18 nestlings was about one-halt of 
‘‘dull levels (Fig. 1A) and is consistent with 
Palacios et al.’s (2009) findings that complement 
reediated cell lysis and levels of natural antibodies 
* ere not fully developed by time of fledging of 
Tree Swallows. In contrast, Moller and Haussy 
f2007) found no differences between ND12 
filing and adult Barn Swallows (Hirundo 
nwiea) in complement mediated lysis. Natural 
antibody levels of Great Tits (Pams major) were 
fu Hy developed by the end of the nestling period 
but complement activation was not detected until 
^r fledging (DeCoster et al. 2010). Additional 
comparative studies of the innate immune system 
^ needed to detect patterns of its development 
and the relationships between phylogeny, ecology, 
and innate immune responses. 
The relatively slow development of the micro¬ 
bicidal ability of Tree Swallow nestling whole 
blood contrasts sharply with the rapid develop¬ 
ment of the intestinal innate immune system of 
precocial Domestic Chicken (Galhis gallus ) 
hatchlings. Chicken hatchlings are immunologi¬ 
cal ly prepared for encountering bacteria (Bar- 
Shira and Friedman 2006) as would be expected 
given their precocial condition at hatching (Ardia 
and Schat 2008). Comparative studies of the 
development of the innate immune system across 
the precocial and altricial spectrum in wild species 
should delineate differences in design and func¬ 
tion of the immune system that reflect differences 
in their respective ecological niches and patterns 
of development (Ardia and Schat 2008). 
The in vitro microbicidal ability of blood, body 
mass, and wing chord length all developed at 
different rates in nestling Tree Swallows (Fig. 1). 
Development of in vitro microbicidal ability of 
whole blood was relatively slow compared to that 
of body mass and wing chord length. Typically, 
Tree Swallow fledglings weigh about as much as 
adults but their wing feathers are about 80-85% of 
adult length (Robertson et al. 1992; Fig. IB, C). In 
contrast, the in vitro microbicidal ability of their 
whole blood was only about one-half of adult levels 
(Fig. I A). We did not detect a significant relation¬ 
ship between in vitro microbicidal ability of whole 
blood and nestling mass or right wing chord length. 
Similarly, Palacios et al. (2009) did not find a 
significant correlation between nestling body 
condition and complepient mediated cell lysis, 
their measure of innate immune function. The lack 
of relationships between MPK and Tree Swallow 
nestling mass and wing chord is consistent with 
Morrison el al.'s (2009) finding that in vitro 
microbicidal ability of the plasma of ND13 Tree 
Swallow nestlings was not correlated with nestling 
body condition, as estimated by a regression of 
body mass on head-bill length, and hematocrit. 
Our results, and those of Palacios et al. (2009), 
suggest the early stages of development of innate 
immunity in free Swallow nestlings may be 
relatively independent of body condition above a 
minimum threshold for healthy growth. However, 
a complete understanding of the relationship 
between development of innate immunity and 
nestling growth can best be evaluated by simul¬ 
taneously examining several components of the 
innate immune system (Norris and Evans 2000). 
