Limited consequences of infestation with a blood-feeding ectoparasite for the nestlings of two North Pacific seabirds

The seabird tick (Ixodes uriae) parasitizes over 60 host species in the circumpolar regions of both hemispheres, and acts as a vector for a number of potentially virulent pathogens. On Triangle Island, British Columbia, Canada, the nestlings of Cassin’s Auklet (Ptychoramphus aleuticus) and Rhinoceros Auklet (Cerorhinca monocerata) are often parasitized by seabird ticks, which may affect their growth and survival in the nest. We used a logistic growth model to interpolate between successive measures of mass (g) and wing chord (mm) for 558 Cassin’s Auklet and 344 Rhinoceros Auklet chicks over 11 years from 1996 to 2007. From the model, we estimated the asymptotic measure and the age at inflection point for each chick’s growth trajectory, and assessed the effect of tick load relative to other sources of annual and seasonal variation in growth. Most chicks (72.4% of Cassin’s Auklets, and 62.2% of Rhinoceros Auklets) hosted ≥1 ticks at least once while in the nest, and the median tick load w...