To Catch Crossbills

It has been a very dry April in our corner of the world. Cold too. Unseasonably cold and dry. There has been little sign of any April Showers and I don’t remember the last time I felt, saw or heard a deluge of rain drops.

The lack of rain may not only be causing consternation amongst farmers and gardeners, with our wild birds feeling the pinch too. As sources of open water dry up in the countryside and the earth becomes dried, hard, cracked and frozen, birds go in search of any remaining open water. Puddles, pools, ditches become focal points much like a feeding station full of nuts and seeds, even as the pools shrink and disappear before your eyes.

On farmland near Thetford a deep depression in the ground near the farmhouse, a pit from a bygone era, has become a tangle of trees, branches, logs and brambles. The sides are dotted with clusters of pale yellow primroses, a splash of colour amidst the browns and greys. The water that had covered the entire floor of this pit, and its labyrinth of debris, throughout the winter has shrunk to a bed of squelchy grey mud that sucks at your wellies. A slimy sheen of green algae coats the top of the mud and the decaying branches, logs and roots stuck there.

Now only a small puddle of water remains. And that is why this cluster of trees and bushes is alive with birds. Twittering flocks of Lesser Redpoll, Siskin and Goldfinch harmonise with the melodious song of Blackcap and Willow Warbler, and the onomatopoeic Chiffchaff. While there is certainly some scope for nesting here, most are here for the water.

As the shrinking pool becomes one of the only sources of water in the area, the birds gather in the trees, and then drop down to the waters edge to quench their thirst.

Situations like this present an opportunity for bird ringers. As when setting nets near feeders full of food, this is an opportunity to catch birds concentrated in a specific place. In the past it has been as rewarding, not only in terms of catching numbers, but also catching species that are usually more difficult to catch. Years ago we even caught Crossbills!

It has been over 10 years since I have seen one in the hand. Back then we had caught them coming to a garden pond during an extended cold snap. Since then conditions for catching this enigmatic species have not quite aligned.

This past autumn, winter and spring has been one of my best years for seeing Common Crossbill in Thetford Forest. Not since my spring and summer of fieldwork back in 2009 have I encountered so many of these ‘Upset Williams’ (yes that is one of my favourite ever blog titles!) whilst out in the Forest.

Stunning male Common Crossbill

Even on this farm, with its open fields surrounded by blocks of forest, they have been flying over, perching at the top of trees, their distinctive chatter filling the air.

And so with another cold, frosty start giving way to a warm, sunny morning with just a touch more cloud occasionally blocking out the sun, we set nets around that small pool at the bottom of that round pit with its mire of branches, trees and mud. The aim, once more to see if we can catch birds coming to the water to drink.

It did not disappoint, as finches converged on the pool and our nets. Not in huge numbers, but we were very pleasantly surprised by the variety. Siskin, Lesser Redpoll, Chaffinch, Linnet, Yellowhammer and Goldfinch. All coming down to the water. All caught in the net. And YES! Even a pair of those most wonderful finches, Common Crossbills! What an absolute thrill to once again hold this beautiful and unique species in my hand and see them up close. The wonderful colours, especially of the male with his feathers of gold, red, yellow and green. This was a young male, so not as many bright red feathers but stunning none the less, with an almost metallic sheen to each colour. The chunky body. Large head. And then that bill, uniquely crossed at the tips, designed for prising open conifer cones to extract the seeds within. Incredibly crossbills are either a lefty or a righty, depending which way the lower mandible or jaw crosses the upper one. Today, the male was a righty and the female a lefty. And so I found myself thinking why? What determines whether a crossbill is a righty or lefty?

Male and female Common Crossbill

Now for the science bit…

In most species of crossbill, including the Common Crossbill also known as Red Crossbill, the ratio of the two forms is approximately 1:1. As such it would appear the they way the bill crosses does not give any advantage or disadvantage. Crossbills predominantly feed on cones still attached to the tree. The theory as to how the 1:1 ratio is maintained is thought to be because birds which visit a cone already foraged on by a bird with the same bill direction would suffer from reduced food intake because the resource would already be depleted. A bird with a different direction of bill would be able to extract the remaining seeds in the cone. Therefore birds with same bill direction in those situations would experience lower survivorship as they take in less food, and the situation would favour birds with the other bill crossing direction. Ultimately this give and take results in a frequency dependent selection that is stable at a ratio of 1:1. All of this was summarised in a paper by Edelaar et al 2005.

This does rely on the assumption that the direction the bill crosses is heredity, and while this may be difficult to test in wild populations where ‘extra pair’ copulations means that the parents of all the chicks in a nest might not actually be those rearing them, it is not hard to imagine that bill crossing direction is at least in some way genetically determined.

Edelaar et al 2005 were unable to find any evidence of hereditability in bill crossing direction in captive crossbills, although their sample size was small and nearly every pair was made up of a lefty and a righty…

Of course given that the direction of bill does not seem to impart any advantage, it could all be just be a matter of random genetics…

Reference: Edelaar, P., E. Postma, P. Knops, and R. Phillips.  2005.  No support for genetic basis of mandible crossing
direction in crossbills (Loxia spp.). Auk 122:1123-1129.

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