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.