Wednesday, 13 December 2006

Who rules the roost

Now that the birds are beginning to return to my bird feeders, I am reminded of the hierarchy that exists within this avian community. As tits, finches and the occasional house sparrows jostle for position at the feeders, it soon becomes clear who are the bullies in this outdoor dining room. Size plays a role; great tits oust blue tits from favoured perches and, in turn, the blue tits see off the smaller coal tits. In fact, the coal tits seem to prefer to sneak in, grab a sunflower heart and then beat a hasty retreat to eat their meal elsewhere; a sensible strategy for a bird at the bottom of the pecking order. Not so the greenfinches who dominate the feeders, each one seemingly picking through the hearts to select the plumpest seed. These aggressive birds force their way onto the perches and then turn noisily towards any birds that try to usurp them.

Even within a species there exists a clear hierarchy. Top of the tree, so to speak, are the adult males and it is to these that the females and younger individuals defer. The hierarchy is maintained by a series of threats and displays and only rarely does a dispute turn physical. From a biological perspective it makes sense for two individuals to settle their differences in a non-physical manner. In the harsh reality of the natural world, any injury that results from conflict has the potential to be life threatening and, with such high stakes, it is better to resolve a dispute without bloodshed. Birds and other animals use display to warn a potential opponent of their strengths. As such, an inferior individual can assess if it has bitten off more than it can chew by facing up to an individual that is likely to win a physical contest.

One of the interesting results of such dominance hierarchies is that individuals from lower down the pecking order may be forced to feed in circumstances that are less than ideal. Work carried out a number of years ago illustrates this point rather well. Researchers put up a number of bird feeders in different locations within a garden, with some feeders positioned in cover provided by thick shrubs and others placed out in the open, where the risk from sparrowhawk predation was much higher. The researchers then watched to find out how the feeders were used by great tits of different ages and sexes. While the dominant adult males chose to use those feeders placed by cover, younger individuals were forced to use those positioned in the open. The same thing will be going on in your garden so spend some time watching and all will be revealed.

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