Ectoparasite effects on nestling European Storm-petrels

A colony of European Storm-petrels (Hydrobates pelagicus) on Benidorm Islan d was studied during 1996 breeding season. We experimentally tested whether ectoparasites affected growth and health of nestling storm-petrels. Four s pecies of ectoparasites were found on nestlings; the most common were a der manyssid mite and the flea Xenopsylla gratiosa. We manipulated ectoparasite loads on chicks by assigning them to one of three treatments: fumigated wi th insecticide, ectoparasite addition, or control. Nestlings in the fumigat ed group gained mass at a higher rate than those in the control group, and nestlings from the parasite addition treatment gained mass at the lowest ra te. Because slower mass gain may delay fledging or reduce survival, ectopar asites may be all important selective force on European Storm-petrels.