Friday, 1 January 2016

The variable Redpoll

Redpolls are small finches with a tiny bill, a crimson spot on the forehead and a little black bib and mask giving them slanted-looking eyes. Their plumage is so dense that on their faces it can partially hide their bill, giving them a flat face. They are found from the Arctic (where they are resident), to subarctic and temperate open forest and scrub and specialise on small seeds, mainly from birch, but also alder, willow, spruce, larch (above) and pine cones. They will also feed on various wildflower seeds near the ground.
 Redpolls are very entertaining to watch as they feed, as they reach catkins and cones using acrobatic, agile movements, reminiscent of tits. In addition, they can use one foot to hold onto a stem, a bunch of seeds or catkin to reach the seeds more easily, like goldfinches. They can apparently store up to 2 g of seeds in a expandable pouch in their throat, to eat later in less exposed conditions. Redpolls have been seen 'bathing' and burrowing tunnels in the snow. These tunnels have unclear function, as they don't appear to use them to shelter from low temperatures, rather, they appear to fulfill a social role, or just be a form of play.
  Redpoll flocks have erratic, nomadic, movements in winter in search of seeds, often travelling in the company of Siskins. They also have 'irruption years', following failures on the seed crops on which they rely, or high population densities after a good crop year, often spurred by cold weather. One of the largest irruption years were in the winter of 1995-96, where Arctic and Common Redpolls arrived in the UK in good numbers.
 The taxonomy of Redpolls has been very fluid, with three species, Arctic, Common and Lesser -each with various described geographic subspecies- recognised, but at some point six were described. The browner, small and very streaked 'Lesser Redpoll' breeds in the UK. Diagnosis was based on the lightness and amount of streaking in the plumage and also bill shape and size and overall size. However identification is often difficult, as there is a lot of variation, and many individuals would fall in between the 'classic' species descriptions, resulting 'in much collective head-scratching' in the words of Riddington and colleagues.
  A recent study by Nicholas Mason and Scott Taylor from Cornell University, sampled 77 Redpolls from around their distribution range and looked at the diversity across their genomes, niche differentiation, morphological diversity and gene expression patterns. Surprisingly, all Redpolls were extremely similar genetically, with barely any genomic differentiation between 'species'. The different colour patterns and bill shape can be explained by different gene expression patterns (e.g in response to temperature), but the authors did not find a clear-cut morphological differentiation between species, as many individuals were intermediate. Although recent adaptation to particular local conditions, for example the dominant available seeds) might have happened, it is likely that the nomadic migratory movements of Redpolls result in a lot of dispersal and gene flow between populations and incipient regional varieties, preventing differentiation and subsequent speciation. Perhaps we should just enjoy Redpolls as a delightful bird on its own, and worry less about tidying individuals away into imaginary boxes.

More information
The Cornell Lab of Ornithology Cool Redpoll Facts.

Riddington, R., Votier, S. C., & Steele, J. (2000). The influx of redpolls into Western Europe, 1995/96. British Birds, 93: 59-67. Pdf here.

Gustave Axelson (2015) From Many, One: How Many Species Of Redpolls Are There? Commentary on Mason and Taylor paper on the The Cornell Lab of Ornithology blog.

Mason, N.A. and Taylor, S.A., 2015. Differentially expressed genes match bill morphology and plumage despite largely undifferentiated genomes in a Holarctic songbird. Molecular ecology, 24: 3009–3025. Here.

Collins, J. E., & Peterson, J. M. (2003). Snow burrowing by Common Redpolls (Carduelis flammea). The Kingbird, 53, 13-22. Here.

A male Redpoll feeding on Alder catkins
A Redpoll feeding on Rosebay Willowherb.

A male Redpoll on Alder
An individual on Alder, illustrating their ability to hang upside down from thin branches.


  1. Very interesting. Does the different expression of a similar genome account to some extent for the differences between Darwin's famous Galapagos finches?

  2. Hi Ralph, thank you for your interesting question. In fact, this year, Darwin's finches have had their genomes sequenced (this is a short article covering this that Darwin's finches have been evolving for around 1.5 million years, and the species status of most species was supported by genetic differences (which in the case of one gene, it matched bill shape), but the researchers also found a lot of evidence of hybridization, a common theme which is starting to dispel the myth of the Victorian 'pure' species. In the case of the Redpolls I wonder if growing at different temperatures determines the extent of whiteness in the plumage and I think that the researchers were planning to test this in future experiments. A bit like Siamese cats colour pattern is dependent of the temperature of the body part (in their case is colder areas like tail, face and ears are darker than the warmer areas of the body). I look forward to the results!

  3. Thanks a lot. Have read the article, with video and audio interview, and it's a very clear explanation. It was surprising to find that Darwin himself had not managed to collect enough data on the finches to be directly useful, and their full significance was only realised later.

    As for the influence of temperature on plumage colour, I wonder whether this to some extent explains the paleness of the desert-dwelling subspecies (Lilith) of the Little Owl. Equally, this could be wholly due to natural selection favouring better camouflage in sandy regions.