Hastad, O., Victorsson, J. and Odeen, A. (2005). Differences in color vision make passerines

less conspicuous in the eyes of their predators. Proceedings of the National Academy of

Sciences of the United States of America. 102(18) 6391-6394.

This particular research article is addressing the conflict that arises when animals are trying to signal to one another. In this case they are specifically referring to various bird species who display various types of coloration in order to attract mates, but at the same time these displays may make them more susceptible to predation. Thus there is a conflict, what is the optimum time that birds should display that will allow them to communicate with other birds but at the same time avoid predation? Or is there another way that birds can work around this issue?

As it turns out birds are able to work around this because birds have two types of color vision, both of which are sensitive to UV light. There is a UV sensitive type and Violet sensitive type. There is a difference in the wavelength of peak absorbance of the opsin in the UV/violet cone.

Prior research has shown that Passerida (thrush, sparrow, and warbler-like songbirds) have the UV type vision while the Violet type is present in raptors and Corvida (crows, ravens). Based on this research it was hypothesized that male songbirds could be able to communicate effectively with conspecifics without increasing their chances of being seen by predators. Thus the songbirds will be able to differentiate between the color displays of other songbirds and the surrounding environment and predators such as hawks will not be able to differentiate between the two.

One important thing to consider is that the forest is heterogeneous, it has many different types and colors of leaves that are exposed to various levels of light. In order to take this into account the researchers took reflectance measurements from breeding habitats. For the deciduous habitat they measured leaves from eight different tree species and for the coniferous habitat they measured needles and twigs of the Norway Spruce and Scots Pine. They measured plumage reflectance using skins from Swedish male birds at the Swedish Museum of Natural History. They decided to focus on measurements from the forehead and chest regions because these are two major areas of plumage. The researchers measured 18 species from nine Passerida families. An Ocean Optics S2000 diode-array spectrophotometer was used to take these measurements. The surface reflectances were measured using a TOP Sensor Systems DH-2000 combined with a deuterium-halogen light source connected to a fiber optic probe to the spectrophotometer.

The results from this experiment showed that male songbirds send a signal in such a form that only the intended receiver can see it. It was determined that the colors of the forehead and chest were significantly more conspicuous to other songbirds than to predators in both deciduous and coniferous forests. Using a signal that predators can’t detect reduces the perspective predation costs of signaling.

This article relates well to a previous lecture in which we discussed possible costs of signaling to the sender. If the songbirds were not able to send their signal in such a way that predators can’t detect it, they would be at high risk for predation. However the songbirds from this research study have eliminated this problem, thus they have a very low risk of predation while signaling.