Abstract
Migratory flight is physiologically highly demanding and has been shown to negatively affect multiple parameters of constitutive immune function (CIF), an animal's first line of physiological defence against infections. In between migratory flights, most birds make stopovers, periods during which they accumulate fuel for the next flight(s). Stopovers are also commonly thought of as periods of rest and recovery, but what this encompasses is largely undefined. Here, we show that during stopover, northern wheatears Oenanthe oenanthe, a long-distance migratory bird, can rapidly increase constitutive innate immune function. We caught and temporarily caged birds under ad libitum food conditions at a stopover site in autumn. Within 2 days, most birds significantly increased complement activity and their ability to kill microbes. Changes in immune function were not related to the birds' food intake or extent of fuel accumulation. Our study suggests that stopovers may not only be important to refuel but also to restore immune function. Additionally, the increase in CIF could help migrating birds to deal with novel pathogens they may encounter at stopover sites.
Highlights
The immune system helps to keep an organism healthy, which is important for survival and for individual performance during the various annual-cycle stages
The rate of fuel expenditure in flight is higher than the rate of fuel accumulation at stopover [35]
Understanding stopover ecology is crucial to understanding bird migration
Summary
The immune system helps to keep an organism healthy, which is important for survival and for individual performance during the various annual-cycle stages. Mimicked bacterial infections prolong stopovers, the periods of fuel accumulation in between migratory flights, and change stopover behaviour in migrating birds [2]. This latter result suggests that getting infected during stopover could slow down migration, which may have negative carry-over effects into the breeding and wintering seasons [3,4]. While immune responses are important to ensure survival, immune responses are energetically costly [9,10], always include selfdamage, and often include ‘opportunity costs’, i.e. lost opportunities in life history, resulting in fitness costs [10,11,12,13]. A stronger CIF (baseline) should be favoured in order to avoid mounting an actual immune response
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