Do egg spots influence levels of parental investment in the European Starling (Sturnus vulgaris)?

Abstract

Parasites can impose fitness consequences onto their hosts by reducing their reproductive success or offspring survival rates. Carnus hemapterus are external parasites that feed on the blood of European Starling (Sturnus vulgaris) nestlings and leave spots on starling eggs from their feces. Adult starlings might alter their offspring provisioning behaviours to compensate for fitness costs caused by Carnus hemapterus parasitism. I examined whether adult male European Starlings use egg spots as a cue to increase their offspring provisioning rates to offset the fitness costs of parasitism. To do so, 33 clutches were assigned to one of four groups: Control Spotted (n = 7), Control Unspotted (n = 6), Experimental Spotted (spots were added) (n = 10), and Experimental Unspotted (spots were washed off eggs) (n = 10). I predicted that nestlings hatching from clutches that were originally spotted before treatment would have lower condition. I also predicted that nestlings hatching from naturally and artificially spotted clutches would be paternally provisioned more than those from unspotted clutches. Nestling condition was determined using the residuals from regressing body mass vs. tarsus length on Day 11 of the nestling period. There was no significant difference in either nestling condition or paternal provisioning rates detected across all treatments. Carnus hemapterus did not impose any fitness costs in this population of European Starlings, and thus condition remained constant across all groups. Because of this high nestling condition, parents did not have to compensate for any harm caused by Carnus hemapterus parasitism, and as a result, fed at a similar rate as those in nests with no Carnus.