Biology Reference
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the expense of resources available for the immune system, so increased reproductive
effort reduces the breeder's physiological condition (Sheldon & Verhulst, 1996; Norris &
Evans, 2000).
Varying care in relation
to costs and benefits
Do parents vary their care in relation to these costs and benefits? Studies have
investigated this question both by comparing the behaviour of related species which
face different selection pressures, and by experiments in which costs and benefits have
been manipulated for particular species.
Comparing North and South American passerine birds
Cameron Ghalambor and Thomas Martin (2001) compared parental risk-taking in
response to predators by passerine birds breeding in North and South America. Species
in temperate North America tend to have large clutch sizes (typically four to six eggs)
and low adult survival to the next breeding season (around 50% or less). By contrast,
species in tropical South America tend to have small clutches (typically two to three
eggs) and high adult survival (around 75% or more). The selective forces leading to
these different life histories are likely to be complex, including a greater flush of food
during northern temperate breeding seasons (which permits larger clutches) but
harsher climatic conditions in the non-breeding seasons (which reduces adult survival).
Ghalambor and Martin predicted that these life history differences should lead to
different parental risk-taking responses. Their previous work had shown that increased
nest visits by parents led to increased risks of predation to both parents and young, by
attracting the attention of predators. Therefore, they measured parental responses in
terms of how much parents reduced their nest visits to a brood in the presence of
predators of the adult parents versus predators of the nestlings.
Five pairs of species were tested, matched for phylogeny and ecology, including a
North and South American species of each of the following families: flycatcher, thrush,
wren, bunting and warbler. Ghalambor and Martin predicted that South American
species should respond more strongly than their North American counterparts to a
predator of adult birds (a hawk) because South American parents have greater
expectations of future reproduction. Conversely, North American parents should
respond more strongly to a nest predator (a jay) because their current brood is more
valuable than is a brood for a South American parent. The data supported both these
predictions (Fig. 8.3). Therefore, parents modulate their risk-taking in response to both
the costs and benefit of parental investment.
Parent birds
modulate risk-
taking in relation
to value of
current versus
future broods
Flexible parental response to current brood demands
Prospects of future reproduction can also influence a parent's response to increased
demands from its current brood. Rose Thorogood and colleagues (2011) showed this in
a neat field experiment with a nectar-feeding passerine bird from New Zealand, the hihi