Bottlenecks, budgets and immunity: the costs and benefits of immune function over the annual cycle of red knots (Calidris canutus)
PhD thesis. 2008. Deborah M. Buehler, published by Drukkerij van Denderen, Groningen, NL
ISBN: 978-90-367-3594-0 (print), 978-90-367-3595-7 (electronic), PDF
This thesis is inspired by that question and aims to clarify our understanding of immune function over the annual cycle and in different environments.
Red knots, long distance migrating shorebirds, are used as a model species for experimental and field studies that provide several conclusions. First, migrant birds face bottlenecks or “tough times” during their annual cycle, especially during spring migration. Second, immune function varies significantly over the annual cycle, even in captive birds, and variation suggests that birds use different strategies during different annual cycle stages. Higher cost strategies are important during migration, but are down regulated during peak feather moult. Third, constitutive immunity persists under conditions that challenge energy balance, suggesting that a baseline level of immune function is compulsory and that birds save energy on more costly aspects of immunity when necessary. Fourth, in addition to available resources, pathogen pressure in the immediate environment likely shapes the strength and strategy of immune defence. Fifth, variation in melatonin does not correlate with immune function in knots. Thus, although melatonin may underlie the mechanism for annual variation in immune function in mammals, this is not likely the case in birds. Finally, in the wild immune function is affected by a myriad of factors including differences in available resources, energy expenditure and pathogen pressure in different environments.
Taken together my results establish that baseline immune function varies over the annual cycle, but that this variation is not driven by any of the factors I measured in captive birds. What then is driving this variation in immune function? An obvious answer is disease risk, an idea that requires testing in the future.