Friday, 7 February 2014

The scary bright eyes of the Jackdaw

ResearchBlogging.orgJackdaws breed in loose colonies, and unusually for corvids, in cavities in trees, cliffs or buildings. There is often very strong competition for nest sites, and the pair will defend their nest fiercely against conspecifics. Jackdaws are also unusual for having very contrasting, almost white irides. In a recent paper, Gabrielle Davidson and her colleagues from Cambridge and Exeter Universities tested the hypothesis that the bright, contrasting eyes of the Jackdaw serve as a strong warning signal when adults are occupying a nest, warning intruders not to come in.
 They set up 80 identical nest boxes in woodland areas in Madingley (Cambridgeshire) during the pre-breeding season (February-April), when Jackdaws prospect potential breeding sites. The nest boxes had two perches, one at the base and the other just by the entrance. They fitted the inside of the entrances of the nest boxes with a circular printout, visible from the outside. Each nest box was randomly assigned one of four types of printout: a dark circle (control) a dark circle with white 'eyes'  of the same dimensions of a Jackdaw (eyes only), a printout of a face-on photo of a Jackdaw with eyes retouched to increase the brightness (bright eyes), and the same printout but with dark eyes (dark eyes). They used remote video cameras to record Jackdaw visits to the nests, and the time they spent sitting on the nest itself and the perches.
 The results were striking. Jackdaws alighted on the nest box significantly less when the bright eye print was in the nest entrance. In addition, they spent very little proportion of their visit time on the perch closest to the entrance both in the eye only and bright eye treatments. The visiting Jackdaws actually entered the nest box in two occasions, one in the control and another one in the dark eye treatment.

This experiment shows that eye colour can be important in communication with conspeficics. The pale eyes are a warning signal to prospecting Jackdaws that a nest is occupied, and they avoid approaching the nest entrance once they are on the nest box. This reaction to an occupied nest reduces intraspecific conflict, as it might save them from a fight and potential injury. Davidson et al speculate that the 'eye fear' response of Jackdaws might be more general, and they might react in the same way to sparrowhawk eyes, for example, as it is known that they are sensitive even to eye gaze in humans - they will delay their approach to food if a human is looking at them.

More information
Davidson GL, Clayton NS, & Thornton A (2014). Salient eyes deter conspecific nest intruders in wild jackdaws (Corvus monedula). Biology letters, 10 (2) PMID: 24501271


  1. What a very interesting post. Have added a link from my own blog. Do you think that Green Woodpeckers, which also have pale eyes, nest in holes, and are extremely shy when stared at by humans, share this behaviour?

  2. Thank you Ralph, I think you could be right, probably is a shared instinct of birds to stay shy of staring eyes and hole nesting birds have much to gain from keeping other birds away from their nests. I find it surprising that stock doves have black eyes though, they have been found to engage in fierce battles with jackdaws for nest boxes.

    1. I suppose the white eye signal prevents fights. Stock doves neither offer nor recognise the signal, so fights start between the species which would not have occurred in a jackdaw--jackdaw confrontation.

    2. Hi, It's really interesting you brought this up. In the experiment, I had many instances when stock doves landed at the nest boxes. I have yet to analyse the data, but when they were at the nest box, there was no indication that the stock doves responded to the images at all. -Gabrielle Davidson

    3. Thank you for commenting Gabrielle, I look forward to your analyses on stock doves.

  3. Don't they look amazingly intelligent though?

  4. They do look intelligent. I do wish my local jackdaws were tamer. The photo above I took in the park in Buxton, where they can almost be hand fed.