| With the discovery that the ultraviolet (UV) spectrum is visible to birds, it has been suggested that mimicry in the UV range might explain why some Brown-headed Cowbird (Molothrus ater) hosts accept seemingly divergent parasitic eggs. The role UV light plays in egg recognition has not been well studied in cowbird hosts. The Northern Cardinal (Cardinalis cardinalis) is a common host that usually accepts cowbird eggs, but evidence suggests they occasionally reject immaculate or divergent eggs. The purpose of this study was to determine if a reduction in UV reflectance of cowbird eggs would elicit a rejection response from the cardinal. Four treatments were used: (1) cowbird eggs covered with UV-block; (2) cowbird eggs covered with creamy VaselineRTM used as a control; (3) uncoated cowbird eggs used as a control, and (4) cardinal eggs covered with UV-block. Cowbird and cardinal eggs with UV-block were not more likely to be rejected than eggs in either of the control treatments. However, 11.8% of UV-blocked cowbird eggs (n = 17) and 18.8% (n = 16) of UV-blocked cardinals eggs were rejected, which may indicate that UV light plays a role in egg recognition by the Northern Cardinal. The lack of a significant rejection response may be due to high intraclutch variation found within cardinal clutches and the overall similarity in appearance between cardinal and cowbird eggs. In addition, though cowbird eggs are typically smaller, they can overlap in size with cardinal eggs, which may be a constraint in the evolution of egg rejection. Furthermore, cowbird parasitism is not costly for cardinals, and therefore, selection pressures to evolve an ability to recognize cowbird eggs are likely minimal. |
