Thoughts on what constitutes a wild bird, a species and the impact on bird lists

   

Little Owl, a UK introduction almost universally accepted as tickable on a UK list

Many birders keep bird lists. These, for example, might record the number of birds seen in the UK, worldwide, or a particular country or area. Two issues arise when deciding what birds to include in such lists, what constitutes a species and how to identify a true wild bird as opposed to one which has escaped from an aviary. In this blog I will give my own personal thoughts on this somewhat controversial topic. My thinking on this issue will be guided by objectivity wherever possible driven by my scientific background.

 

The former is perhaps the easiest of these two issues to address. Modern affordable and rapid DNA sequencers provide the ability to study birds DNA rapidly and economically. It should be hence possible to define the level of DNA divergence between two birds  necessary for them to be recognised as separate species. Delving a bit further , there are a number of other aspects to consider. For example, is species differentiation defined by a total percentage variance in DNA or are there particular sectors of the genome that should be the focus for this decision? It would seem to be reasonable to say that separate species should not be able to interbreed to yield viable, i.e nonsterile, young. If this weren’t the case after a couple of generations any distinguishing features between the two would average out. So this begs the question as to whether areas of the genome that are associated with controlling this should be a particular focus for DNA divergence. 

 

We should also keep in mind that we are talking about very small divergences in DNA between separately identifiable species, after all we share 98.8% of our DNA with Chimpanzees. Each human cell contains roughly three billion base pairs, or bits of information. Just 1.2 percent of that equals about 35 million differences. Some of these have a big impact, others don't. And even two identical stretches of DNA can work differently in that they can be turned on or off in different amounts, in different places or at different times. 

 

Confused yet? 

 

Let’s try and put this into a little more context. Evolution is  generally a continuous process, and the propensity to divide life up into conspicuously distinct species and genera is less a reflection of fact than another human idiosyncrasy. I say generally continuous because evolution occasionally produces a sizable mutation providing a major evolutionary advantage.  For example, the Cuckoo presumably did not continuously evolve the trait of brood parasitism,  its totally discrete, i.e it either does or doesn’t lay its egg in someone else’s nest.

 

Let’s move onto the second and much more controversial issue of what does and doesn’t constitute  a true wild bird. Practicality often drives us down a path of a combinatorial approach of  objective facts and more subjective judgements. In this sense there are two readily identifiable sets of criteria. The first of which allows complete objectivity. For example, the presence of a collector’s ring or evidence of wing clipping are both necessary and sufficient criteria to conclude beyond reasonable doubt that a bird is of captive origin. There are also necessary and sufficient criteria to define a true wild bird.  Was it ringed in the nest in its normal  geography? Has isotopic analysis of a specimen shown that the bird has spent a long time in its normal geographic area? Unfortunately, most birds under consideration were not ringed in the nest nor is it possible to do Isotopic analysis so we are forced to consider other criteria.  The absence of a collectors ring or wing clipping is necessary but not sufficient to prove a bird is of wild origin as many collectors do not ring or wing clip their birds. 

 

Here we are into the second more subjective set of criteria. Does the bird appear overly tame? Has it been found with a know carrier species? Is it a bird that migrates and hence is susceptible to weather driven vagrancy? Could the bird have been ship assisted on its journey ( such events are known to have happened). Was it found at the migratory time of year ( although it may be found later if it has remained hidden?). 

 

It hence seems to me that is should be possible to write a comparatively simple process by which a candidate wild bird can be assessed. It may be that every box cannot be ticked due to lack of knowledge or contrary observations but it should be possible to define the number of more subjective criteria necessary to conclude that, on a balance of probabilities, a bird is wild. You could define this as 100% of subjective criteria ticked or, say 80%.

 

Let’s move to a specific example, the Paddyfield Pipit found in Cornwall in 2019. In truth, it is a rather dull bird which investigation indicates is not knowingly kept in european collections. It is a largely non-migratory tropical Asian species with no Western Palearctic records, for which there are only records of short-distance vagrancy as far as the Arabian Peninsula. It was not overtly tame but unusual feather damage was noted on the bird which might be suggestive of captive origin, but comparable wear has been documented in similar species of wild origin.  No other species with a similar distribution or life history has been observed as a vagrant to Britain. The British Ornithologists' Union Records Committee (BOURC) hence, by a majority vote, placed it in category D, a catch all category of birds that have not been proven as wild but future observations may result in it being reviewed and declared as wild. In this instance I would certainly say that the balance of probabilities is against a true wild origin although ship assisted must remain a real possibility.

  

Paddyfield Pipit condemned to category D by BOURC

Let’s look at a second specific example, the Lammergeier that roamed the peak district in the summer of 2020. Genetic tests on feathers discovered by a Yorkshire birder showed that it was a female which hatched the previous year in the French Alps to parents from a reintroductions scheme. The BOURC placed this bird in Category E of its British list. This category lists introductions, human-assisted transportees or escapees from captivity, as well as species whose breeding populations (if any) are not considered self-sustaining, and it was hence not added to the British list. Here we are in much more troubled waters. There are many examples of introductions that are on the UK list. As an example, consider the Little Owl which is not a native species, having been introduced to Britain in the 1870s. However, given the 150 years since its introduction, it is considered to be a self-sustaining population. Again , it seems to me to be possible scientifically to define self-sustaining to remove any subjectivity from the judgment, for example the population is constant in number  or growing over a number of generations. We all tick reintroduced Red Kites who were only reintroduced 30 years ago – at what point since the 1990’s did these become tickable I wonder? A definition of sustainability will have the somewhat strange consequence that, at some point, an offspring will become tickable but its parent is not.

     

Lammergeier condemned to category E by BOURC

The third and final example is the Least Bitten that was found on the south coast of mainland Shetland in October 2022. This bird had strong wild credentials and would represent the first UK record of this diminutive North American Bittern. It was very underweight and in a poor state and was taken into care where, unfortunately, it died. I was on the island at the time and like many other birders shot down to the south coast to see it. A scope had been set up for birders to see the bird although many people had not arrived before it was picked up. The question here is that is it still tickable when in hand? It would seem illogical to me to say it is no longer tickable after being handled, would this not exclude all wild birds that have been captured in mist nests and ringed?

Least Bittern in hand

In conclusion, it seems to me that it should be easily scientifically possible with current rapid DNA sequencers to define what constitutes a species. The question of wild or captive origin is much more complex but should be susceptible to a publicly published process which can be applied to all candidates.

 

I guess at the end of the day it comes down to an individual’s judgement as to what Uk list they keep and whether it is BOURC compliant.

 

Finally, I should say that everything in this blog constitutes my own opinion and I know others will have differing views so constructive comments are very welcome.

 

 Footnote – my blogs are posted with sometimes rather imaginative spelling and grammar due to my extreme dyslexia!