The Relationship Between Relative Brain Size and Avoidance of Vehicular Collisions in Birds is Subtle

Møller and Erritzøe (2017) reported that birds killed by collisions with vehicles had on average relatively smaller brains than birds killed by other causes, which were not identified. Despite concerns about the method used to assess brain mass, we reanalyzed the published data of Møller and Erritzøe (2017) after excluding extraneous species and confirmed a subtle tendency for birds killed by vehicular collision to have somewhat small brains. Some groups of birds (owls, hawks, garden birds and migratory species) did not reflect the overall result. Surprisingly there was no effect of age or sex, whereas one would expect inexperienced immature birds and females in breeding condition to be vulnerable. Overall, plots of brain mass in birds killed by vehicular collisions and other causes greatly overlap, and in some species, individuals killed by collisions have relatively larger brains. That is, the tendency for birds hit by vehicles to be relatively smaller brained is not universal, nor in any species is there an absolute difference in brain size between the two categories. It is possible that in the short time birds have interacted with moving vehicles that selection has acted on brain size to avoid collisions, although we suggest that slightly larger brains might represent an innate tendency to avoid rapidly approaching objects. An interesting question from their study is what was the cause of mortality in the birds not hit by vehicles. Likely sources of mortality of birds post-nestling stage include depredation (natural and house cats) and collisions with windows and other human structures. In fact, relatively large-brained birds might be more susceptible to collisions with windows or being caught by cats (i.e., the other sources of mortality), for which having a relatively larger brain would not appear to mitigate these sources of mortality.