Perceived eggshell luminance does not predict rejection of experimental egg models in eastern bluebirds

Obligate avian brood parasites lay their eggs in other bird species' nests, leaving these hosts to rear the parasitic young. To eliminate or reduce the costs of parasitism, many hosts have evolved the ability to recognize and reject parasitic eggs, and most use eggshell color for recognition; however, color discrimination should be more challenging in the low-light conditions facing cavity-nesting hosts. Therefore, we hypothesized that instead of color, cavity-nesting birds could rely on perceived differences in brightness (i.e. luminance) and on light levels in their nests for egg recognition. Specifically, we expected that rejection rates would be higher when foreign eggs are lighter or darker than the hosts' own eggs or when nest light levels are higher. To investigate this hypothesis, we experimentally altered the luminance of 413 similarly colored (blue) experimental model eggs and added these experimental eggs to the nests of cavity-nesting eastern bluebirds Sialia sialis that typically lay immaculate blue eggs. Contrary to our expectations, neither the perceived luminance of the eggs nor the amount of light in the nest cavity predicted host responses. Therefore, it is unlikely that bluebirds base their antiparasitic decisions on perceived differences in eggshell luminance. Our work provides a foundation for future studies that can inform us about how cavity-nesting birds perceive salient visual stimuli in the dim light conditions of their nests.