Lower begging efficiency can constrain host use in the specialist Screaming Cowbird

RIOVITTI B; GLOAG R; REBOREDA JC; DE MARSICO MARIA C.; FIORINI VD

Puerto Iguazu

Congreso; Ornithological Congress of the Americas; 2017

Aves Argentinas, Association of Field Ornithologists, Sociedade Brasileira de Ornitologia

A generalist strategy of use of hosts by avian brood parasites requires that parasitic chicks be able to survive in a broad range of hosts. Coevolution with any particular host may constrain host use if increased specialization results in a lower efficiency of parasitic chicks to successfully exploit other host species. To test this idea we compared the begging efficiency of the specialist screaming cowbird (Molothrus rufoaxillaris) and the generalist shiny cowbird (M. bonariensis) in an alternative host, the chalk-browed mockingbird (Mimus saturninus). We manipulated mockingbird nests during breeding seasons 2010- 2011 y 2011-2012 to create two-chick broods as follows: one mockingbird and one screaming cowbird, one mockingbird and one shiny cowbird and two mockingbirds. Experimental broods were filmed on days 4 (n = 12, 17 and 24 for each treatment, respectively) and 8 (n = 6, 13 and 19) post-hatching to record the feeding frequency and begging intensity and duration of all chicks. Overall nest provisioning rates (feeding/h) did not differ among treatments at any age. The proportion of feedings received by cowbird and host chicks did not deviate from the random expectation of 0.5. However, screaming cowbird chicks had to strive harder to obtain food as they begged for significantly longer periods than shiny cowbird and mockingbird chicks. Furthermore, screaming cowbird-broods, but not shiny cowbird-broods, were more likely to be depredated than unparasitized ones. Our findings suggest that the lower begging efficiency of screaming cowbird chicks, which results in increased predation costs, can constrain colonization of new host species.