Characteristics determining host suitability for a generalist parasite

Abstract

Host quality is critical for parasites. The common cuckoo Cuculus canorus is a generalist avian brood parasite, but individual females show strong preference for a specific host species. Here, we use three extensive datasets to investigate different host characteristics determining cuckoo host selection at the species level: (i) 1871 population-specific parasitism rates collected across Europe; (ii) 14 K cases of parasitism in the United Kingdom; and (iii) 16 K cases of parasitism in Germany, with data collected during the period 1735-2013. We find highly consistent effects of the different host species traits across our three datasets: the cuckoo prefers passerine host species of intermediate size that breed in grass- or shrubland and that feed their nestlings with insects, and avoids species that nest in cavities. Based on these results, we construct a novel host suitability index for all passerine species breeding in Europe, and show that host species known to have a corresponding cuckoo host race (gens) rank among the most suitable hosts in Europe. The distribution of our suitability index shows that host species cannot be classified as suitable or not but rather range within a continuum of suitability.

Figure 1

Predicted effects of ecological variables on population-specific parasitism rates (black) and number of parasitism events by cuckoos in UK (green) and Germany (blue) based on (a) nest structure (b) habitat of host (c) food type the host provide for its young and (d) the body size of the host. Note that values for number of parasitism cases in UK and Germany are scaled for easier comparison with parasitism rates and are not originally on the scale of the y-axis. Box blots show the mean predicted relative parasitism rate, outer box show the first and third quantile and the dotted lines show the 95% credible limits. See Materials and Methods for further explanation on how these predicted values were obtained.

Figure 2

Frequency plot of the host suitability index for passerine birds in Europe. Hatched bars = species with a corresponding cuckoo host race (gens) light grey bars include all passerine species in Europe.

Figure 3

Fit between our host suitability index based on European population-specific parasitism rates and number of parasitism events recorded in (a) UK (R² = 0.35) and (b) Germany (R² = 0.28). The grey dotted lines are predicted number of records for increasing suitability along the x-axis. This line is based on predictions from hurdle models (see main text), including only the suitability index as explanatory variable and recorded parasitism events as response variable in each of the two countries. Only species that are recorded in the specific country are included.