Saturday, 20 March 2010

Uneasy neighbours: Carrion crows and Kestrels

A kestrel has taken residence in the neighbourhood, I have seen it flying almost every other day. The resident pair of crows does not seem happy about this. A couple of weeks ago I saw them mobbing the kestrel while regularly uttering their characteristic 'krrr' call high over the garden. The same happened yesterday. In both occasions, by the time I had taken the camera out they were already quite distant. As I wanted to write a post about mobbing, I decided to take out my Wacom Bamboo tablet and sketch the behaviour. Mobbing is found in many bird species and often is a communal affair by which potential prey call and harass a predator. Mobbing is a costly behaviour, it takes a lot energy to chase and call and dive bomb the predator. The crows can spot the kestrel in the distance and go out of its way to mob it. Once a kestrel was gliding quite high, and I watch a crow laboriously gaining hight in broad circles in order to mob it. During the actual mobbing, the crow flies slightly over the kestrel - a safe way to do it - and chases it, occasionally taking dives at it, while the kestrel avoids each dive, and occasionally turns to the crow. In rare occasions, the mobber can turn into a meal for the mobbed, but I am not sure this can happen in crows, which are quite robust and bigger than kestrels. Why do birds mob? what do they gain from this behaviour? There are many hypothesis and it is likely that a combination of some of them is the reason. One of them is that mobbing allows birds to transmit information on who the enemy is to the next generation (a form of cultural transmission). In some classic experiments with blackbirds, birds watching a harmless, novel bird being mobbed by another blackbird started to mob the new bird themselves. Mobbing can have also direct benefits for the mobbing individual, it could be showing off is good condition, a form of advertising that if wont be easy prey. Mobbing could confuse or distract a potential nest predator from their nest seeking behaviour. When the predator is chased and engaged it cannot pay attention to the local territory and gather information on the nest location, for example. In seagull colonies, mobbing on crows - yes, crows can be mobbers or mobbed themselves - decreased the chances of egg predation and colonially nesting gulls enjoy lower rates of nest predation than solitary nesting gulls. In addition, mobbing tends to intensify during the breeding season in solitary nesting birds.

This crow is gathering nest material, soft bark from the lime nearby. The photos were taken trough glass.
Crows are indeed nest building now. Yesterday and this morning we watched the pair gathering nest lining material, stripping pieces of lime bark dextrously using their feet and beaks. Their zealous harassing of the kestrel might well reflect the fact that they would rather not have him as neighbour when their vulnerable offspring hatches.

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