A predation risk experiment
The Black Sparrowhawk occurs in two colour morphs - previous research found that provisioning rates of prey to the nest differed between the morphs depending on light conditions, with dark morphs delivering more prey in dull light conditions (e.g. cloudy and rainy) whereas light morphs provided more prey in brighter conditions. This research suggested that this might be due to dark morphs having greater crypsis in duller light conditions because the prey did not spot them so easily.
We have now tested this idea experimentally as part of Carina Nebel’s PhD research. Carina and Adrien Pajot set up an indoor pulley system which enabled simulated predator attacks by taxidermy mounted Black Sparrowhawks on feral pigeons whilst controlling the light levels. During these simulations, the reaction time of feral pigeons towards the approaching predator were recorded.
In contrast to the hypothesis proposed by the original study, results from this experiment did not support the idea that dark morphs are more camouflaged in lower light conditions. However, the experiment did reveal that feral pigeons are generally much slower to react overall in darker conditions than in brighter conditions, and would thus be at a much higher risk of predation when it is darker. This may explain why feral pigeons avoid high-risk behaviours, i.e. foraging, in the early or late hours of the day.
We have now published the results from this experiment in the Royal Society journal Open Science. The paper is fully open access and can be found here: https://royalsocietypublishing.org/doi/10.1098/rsos.190677