Archives for the month of: October, 2012

Two posts in one day is a bit much, but once my mind starts pondering, it’s best to get it written down!

I hail from Trowbridge, a leafy(ish) town in sunny Wiltshire, where a pigeon scandal is rife. A few years ago, ASBOs were handed out to pensioners for ‘excessively feeding pigeons’ and supposedly causing a massive over population. Granted, there are many pigeons in Trowbridge, possibly in excess of 1000 (I cannot find exact surveyed figures anywhere so this is an estimate).

Going by figures stated by Johnson and Janiga (1995), each pigeon must consume 35g of food on average per feed with generally two feeds per day. Therefore, 1000 pigeons, needing 70g of food per day, totals 70kg of food. If an average loaf of bread weighs 750g, then 93.3 loaves of bread would need to be fed to the pigeons daily. No mean feat for a pensioner!

Clearly, it’s inconceivable that such a large amount of food could be provided, even by ‘excessive feeders’, not to mention the costs spiralling (around £326 per week if feeding bread, far more than any pension would provide).

Studies have shown that feral pigeons consume far more ‘natural’ food than one would imagine, as clearly such large flocks could not live on bread alone.

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In human society we are often encouraged to be different, unique, to follow our own path or to stand out from the crowd. However, is the same true for pigeons?

I have been lucky enough to have been contacted by researchers interested in this topic, which got me thinking. They brought to my attention a study investigating goshawk (Accipiter gentilis) hunting habits (see the study here: www.cell.com/current-biology/abstract/S0960-9822(12)00315-6). It was found that rare or unusual phenotypes were chosen as prey items above and beyond the more common phenotypes in the flocks they were hunting.

You would imagine that such hunting pressure would select against the rare phenotype causing flocks to become uniform. However, negative assortative mating in pigeons means rare phenotypes are highly sought after as mates, perpetuating the ‘rare’ phenotypes. Is this the end of the matter? Is negative assortative mating sufficient to maintain plumage variation in face of hunting pressure?

Anthropogenic influences may also have an effect. For example, when counting pigeons earlier in the year, I encountered a man abandoning a fledgling captive bred white pigeon into a town flock. She was too young to survive on her own and so came home with me for hand rearing.

Given what is known about the oddity effect it would be unfair to release her and so she currently remains with me. But it does make you wonder how much of the gene pool consists of escaped/released individuals and how much is the result of negative assortative mating. Equally, it is known that melanic male pigeons have less gonadal regression and can breed for longer when they have access to sufficient food. Could white or red individuals have other unknown traits that help them prevail?

From what we know, being the odd one out for a feral pigeon is both a curse and a blessing. As an unusual phenotype an individual may find a mate more easily and (if male) may have more extra pair matings and therefore may have greater lifetime reproductive success. Given that unusual phenotypes are, just that, unusual, it is evident that selective pressure against such phenotypes must be strong.

It is clear there is still lots to be learned about these familiar birds. And as for the white pigeon, she is currently teaching me a lot about post juvenile moult by spreading her feathers all over my lounge. Here she is roosting on a clothes horse.

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