Thursday, 19 April 2018

Gulls mobbing Buzzard

From my office I hear several Herring Gulls start calling plaintively, kyeoh! kyeoh! a loud, far carrying call that I associate to buzzards overhead. I looked through the window. The local Carrion Crows had assembled atop a large poplar, watching nervously, calling and displaying wing shuffling and tail fanning.
A dozen gulls soared and called - all the local Herring gulls - slowly assembling and ascending on the thermals. Then I see a Buzzard, soaring higher. The gulls didn't reach the buzzard's height and it eventually drifted away and the gull calls died off.
 Gulls often alert me to raptors. A couple of years back it was a Honey Buzzard, migrating over town. I pay a lot of attention to the gulls alarm call since then!
 Later, at home, the scene appears to repeat itself. Alarm calling gulls. I grab the camera and go out into the garden. Two Lesser Black-backed gulls are mobbing a Buzzard.
 This gull alarm call is persistent and sounds distressed, it is hard to ignore. It seems to attract other gulls too, who soar and chase the potential predator. I wonder if it is how young gulls learn to identify the predators they are likely to come across in their lives. Crowd knowledge of the enemies, initiated by older gulls with previous experience, who in turn learned it in their youth. Immature offspring of the gulls initiating the mobbing are likely to be around. The adults could be 'teaching' young about potential predators and the young are likely to benefit from this at some point in the future. This is a hypothesis called 'cultural transmission of mobbing' which was explored in some experiments by E. Curio and collaborators in 1978. They showed that exposure of a novel object to a 'naive' Blackbird at the same time a 'teacher' blackbird is mobbing results in learning to mob the new object (in one of their experiments a colourful bottle). This teaching could be passed through six rounds of learning in which the learner becomes the teacher of a new bird, and so on. The fact that enemies often have to be recognised culturally was put to a sad test when captive reared endangered Hawaii crows (or Alalā) were released into their natural habitat. The last wild Alalā had died in 2002. Two of six newly released birds were predated by native hawks, to which the captive reared birds were completely naive in the first week after release. The conservationist recaptured the surviving birds and started an intensive 'predator training' program in which the captive crows were exposed to hawks calls, hawks flying overhead and simulated hawk attacks. Hopefully the savvy crows will do better when they are released next.
This time of the year there seem to be a passage of migrating or dispersing Buzzards over the city. The gulls are breeding, probably incubating now, and a buzzard could be a predator likely not for them, but for eggs left unattended or young chicks. It makes sense that the gulls see off the potential predator.
I got this short clip from the garden:

More information
Curio, E., Ernst, U. and Vieth, W. Cultural transmission of enemy recognition: one function of mobbing. Science 202, 899–901 (1978).

Different gull calls: here.

A blog post including some great photos of Herring gulls mobbing a Buzzard

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