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 was two in both species. The probability of hosting a tick declined strongly with chick age, such that by day 40 after hatching less than 1% hosted ticks. We found evidence that tick load had a negative effect on asymptotic weights and wing lengths of both species, but the effect was minor relative to that of other sources of annual and seasonal variation. Only at very high loads – which were rare – did ticks have effects on growth that were likely to be biologically relevant. Tick load had little effect on survival to fledging in either species.We argue that these mild effects of ticks on their hosts are consistent with a co-evolutionary process that results in intermediate virulence when parasite transmission is linked to host recovery.