Figure 1. Examples of pre-contact avoidance measures as a first line of defence against different types of parasites and pathogens in birds. These are mainly behavioural measures to avoid contact with parasites and pathogens in the first place. Once a host has come into contact with parasites and pathogens, there is a second line of defence in the form of post-contact defensive measures which can be either behavioural, physiological or immunological. Here we focus on pre-contact avoidance measures. Figure based on Hart (1997). Copyright of David Thieltges.

Figure 2. Mechanisms of parasite and pathogen avoidance or defence are costly and thus reduce the fitness of resistant hosts compared to susceptible hosts in the absence of parasites. However, in the presence of parasites, the fitness of resistant hosts is higher than that of susceptible hosts with the latter suffering more from the costs of an infection. Pre-contact avoidance and post-contact defensive measures can only evolve if the cost of infection exceeds the cost of avoidance and defensive measures. Copyright of David Thieltges.

Figure 3. Selective prey selection can help to avoid acquiring parasites and pathogens. Oystercatchers take smaller cockle prey individuals with suboptimal energy intake. Smaller cockles harbour fewer infective stages of trematode parasites (which utilize the birds as final hosts) compared to larger cockles. By preferring smaller cockles the birds thus reduce their parasite intake and avoid acquiring high loads of parasites. Total avoidance of parasites is not possible in this case because all cockle sizes are infected to some extent. Schematic graph based on data taken from Norris (1992). Copyright of David Thieltges.

Figure 4. Forming groups can help to avoid parasites and pathogens. In birds and mammals, individual hosts suffer from reduced fly bites with increasing group size (dilution) as long as the larger group does not attract more flies due to a higher visibility (encounter). Positioning themselves at the centre of a large group can also help individual hosts to avoid fly bites and potential subsequent infections by the selfish–herd effect. Figure based on Hart (1997), by permission of Oxford University Press (www.oup.com).

Figure 5. Example of post-contact defensive measures against parasites and pathogens. Invertebrate immune systems can distinguish between self and nonself and eliminate parasites or pathogens for example by encapsulation and melanization. The picture shows an amphipod host in which invading trematode parasites (arrows) have been killed by encapsulation. Copyright of Robert Poulin.