Ecophysiological assessment of blood haemosporidian infections in birds

Parasitism is one of the main selective forces underlying the evolution of species, as this will have direct consequences on the host reproduction or survival. However, the overall result of parasite-host interactions could be assessed through resistance or tolerance expressed by hosts and virulence exerted by parasites. In the case of avian malaria, these antagonistic interactions will drive the coevolution between birds and haemosporidian parasites. Hosts choose resistance strategies when trying to limit the infection intensity, whereas tolerance strategies will be selected when they need to reduce the damage caused by a specific parasitemia. In order to understand the ecological and evolutionary dynamics of these interactions, it would be necessary to recognize the effects of these parasites on those physiological or immunological traits that directly affect the host fitness at an individual level. Throughout this review, the aim is to address many of the variables used within the field of ecophysiology to understand how the avian organism reacts to haemosporidian infections, adopting resistance strategies (e.g. increases in immune responses) or tolerance (e.g. high levels of polychromatophilia, plasma corticosterone, haptoglobin) against parasites based on the hosts own biology or parasite strategies. The information collected in this review consolidates knowledge about the physiological sources of variation in resistance and tolerance strategies against haemosporidian parasites, and how this variation can affect avian fitness.