American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Host responses to foreign eggs across the avian visual color space”

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Daniel Hanley, Karel Gern, Mark E. Hauber, and Tomáš Grim (July 2019)

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Scientists detect color biases in the rejection behavior of the blackbird

Vibrantly colored eggs provide insights into a bird’s mind

One of the authors (Karel Gern) hand painted eggs across the entire avian-visible spectrum following a color wheel (inset), measured them with a spectrometer, and deployed them in the field. (Credit: Daniel Hanley)
One of the authors (Karel Gern) hand painted eggs across the entire avian-visible spectrum following a color wheel (inset), measured them with a spectrometer, and deployed them in the field. (Credit: Daniel Hanley)

Painting eggs using vibrant colors may conjure pleasant springtime childhood memories for many people. A team of scientists led by Dr. Daniel Hanley of Long Island University–Post do this every season for science. Dr. Hanley and his lab are interested in natural colors: how they are used and how they evolve. Birds’ eggs range from blue-green to brown and can be variably speckled, and birds examine their own eggs for many reasons. One such reason is that some birds will lay their eggs in other birds’ nests. No bird wants to come back to their nest and find an additional mouth to feed, especially when that foster child may evict and kill its foster siblings. The appearance of an imposter egg is one way that birds evaluate whether an egg belongs to them or another bird. Traditional theory assumes that if the egg appears quite similar to their own a bird will likely accept it, but if it differs considerably the bird will reject it.

In a paper appearing in The American Naturalist, Dr. Hanley and his colleagues Dr. Mark Hauber (University of Illinois Urbana-Champaign), Tomáš Grim, and Karel Gern used an experimental approach to study how the common blackbird, an European species, responds to an unprecedented range of colors. Since birds can see a far greater range of colors than humans can, Dr. Hanley and co-author Karel Gern (shown above) made a bird-specific color wheel to paint eggs, manipulating the three main components of color: hue (e.g., blue, aquamarine, magenta, etc.), saturation (i.e. intensity), and brightness. They found that birds responded strongly to a far greater array of colors than had been previously tested. In addition, they found that the components of color interacted, such that birds may respond to one hue but only if it were an intense color. This group has also shown that hosts in North America, Europe, and South America all remove brown eggs at a much higher rate than blue-green eggs, despite how different they appear from their own eggs. These findings illustrate that we have a lot to learn about how wild birds use color information in their decision making; however, studies such as these are slowly closing that gap.


Despite extensive research on the sensory and cognitive processes of host rejection of avian brood parasites’ eggs, the underlying perceptual and cognitive mechanisms are not sufficiently understood. Historically, most studies of host egg discrimination assumed that hosts rejected a parasite’s egg from their nest based on the perceived color and pattern differences between the parasite’s egg and their own. A recent study used a continuous range of parasitic egg colors and discovered that hosts were more likely to reject browner foreign eggs than foreign eggs that were more blue-green, even when their absolute perceived color differences from the hosts’ own egg colors were similar. However, the extent of these color biases across the avian perceivable color space remains unclear. Therefore, we built upon this previous study by testing European blackbirds’ (Turdus merula) responses to model eggs spanning an unprecedented volume of the avian color space. We found that host decisions depended on avian perceived hue, saturation, and luminance of the parasite’s egg; hosts generally accepted eggs that were bluer or more blue-green, and more often rejected eggs that were less saturated or darker. We suggest that future studies investigate the underlying mechanisms of foreign egg discrimination in other host lineages to determine the prevalence and phylogenetic conservation of such perceptual biases among birds.