784 
THE WILSON JOURNAL OF ORNITHOLOGY . Vol. 123, No. 4. December 2011 
Our results add an additional component to the 
full assessment of immunity (cf Norris and Evans 
2000) in nestling Tree Swallows. It is likely that 
components of the immune system vary in their 
relative costs and benefits (Klasing and Lesh- 
chinsky 1999. Evans et al. 2000. Klasing 2004). 
and respond differently during different life 
history stages (Greenman et al. 2005) and phases 
of development (Martin 2005). 
The relative rates of development of different 
traits may provide clues about the possible causes of 
selection influencing their expression. For example, 
predation risk favors rapid growth and development 
of altricial nestlings (Skutch 1976. O'Connor 1984. 
Starck and Ricklefs 1998. Remes and Martin 2002). 
The data suggest that leaving the nest upon a 
predation attempt could be part of the normal 
behavioral repertoire of altricial nestlings (Redondo 
and Carranza 1989) and is associated with adapta¬ 
tions in nestling development (Bjorklund 1994). For 
example, patterns of growth in Meadow Pipit 
{Anthus pratensis) nestlings suggest directional 
selection for high growth rates; faster growing 
nestlings were better able to avoid predation while 
in the nest while slower than average growth rates 
placed pipit nestlings at a competitive disadvantage 
for food (Halpuka 1998). Post-fledging survival of 
slowly growing young Common Blackbirds (7Ur¬ 
dus merula) was relatively low compared to faster 
growing young suggesting mortality due to lack of 
food and by predation were synergistic, yet both 
acted independently Magrath (1991). Thus, nest 
predation is likely a strong factor selecting for rapid 
growth in many altricial bird species so they can 
leave the nest and fledge as quickly as possible, but 
with sufficient body mass to survive the transition 
period while learning to fly and And food 
independently. Selection pressures that favor mat¬ 
uration of innate immune responses are likely to be 
stronger post-fledging than pre-fledging when 
causes of selection act more strongly on growth 
and wing development. 
Our results are consistent with the view that 
predation risk could favor rapid growth and 
development at the expense of faster development 
of innate immunity. The in vitro microbicidal 
ability of whole blood in our study increased 
through each subsequent stage of development but 
adult immune potential was not reached until after 
fledging. The increase of in vitro microbicidal 
ability of whole blood to kill E, eoli had no 
apparent effect on nestling growth and supports 
the hypothesis of low developmental costs of 
innate immunity (Klasing 2004). This supports the 
hypothesis that causes of selection affecting 
nestling Tree Swallows ultimately favor rapid 
growth relative to the development of the in vitro 
ability of whole blood to kill E. coli. This is 
consistent with the prediction that a negative 
correlation should exist between growLh and 
development of nonspecific immune defense* 
(Lee 2006). For example. Mauck et al. (2005) 
found a negative correlation between growth and 
levels of immunoglobulin M (IgM), a component 
of the innate immune system, in nestling Leach s 
Storm-Petrels ( Oceanodroma leucorhoa). Exper¬ 
iments are required to directly test the hypothesis 
that an energetic trade-off exists between growth 
and development of innate immunity. 
Mid nest ling MPK-tn id parent MPK regression 
estimated high heritability (hr = 0.92) of innate 
immunity. However, this estimate is potentially 
biased by extra pair paternity (50-90% of nests) 
(Lifjeld et al. 1993, Dunn et al. 1994, Barbcrdal. 
1996. Laskemoen el al. 2010) which was not 
assessed in our study. Thus, we cannot estimate 
male parent genetic contribution to this trait. We 
also calculated a midnestling MPK-molher MPK 
regression to account for this potential bias that 
produced a reduced estimate of heritability Or = 
0.41). This result is (I) similar to the heritability of 
the cell-mediated immune response to phytohe- 
maggultinin (PHA) injection estimated from mid¬ 
nest ling-mother regressions of Tree Swallows in 
Tennessee (Ir = 0.42), but not in New York or 
Alaska (Ardia and Rice 2006), and (2) consistent 
with the strong effect that nest of origin had on the 
ability of ND13 nestlings to kill E. coli in cross- 
fostering experiments in Massachusetts (Morrison 
et al. 2009). Similarities in hcritabilities suggest 
similar mother genetic contributions to the cell- 
mediated immune response and microbicidal abil¬ 
ity. Nest of origin influences innate immune 
responses, including the microbicidal ability of 
nestlings (Foreman et al. 2008). Nest of ongin 
effects are likely due to both heritable and non- 
gcnetic maternal effects. For example, lysozyme, a 
protein that contributes to microbicidal ability, i* 
transmitted from mothers to their nestlings via egg 
albumin (Board and Fuller 1974). Evidence sug¬ 
gests the lysozyme activity of mothers and nestling* 
is highly correlated (e.g., Cucco et al. 2006). 
ACKNOWLEDGMENTS 
Dan Ardia, Jan-Ake Nilsson, and Maria Palacios 
provided helpful comments on previous versions of the 
