What motivates cross-breeding?

This post is the result of two earlier posts on a Sooty-headed Bulbul (Pycnonotus aurigaster) and a Yellow-vented Bulbul (P. goiavier) pairing up and starting a family – see here: 1 and 2. Another commenatory has been posted earlier.

“’John_singapore’ asks what motivates cross-breeding. This is a very complex issue which I am certainly not qualified to answer with any authority. However, I can offer a few well known and fairly simple examples of why it happens.

“The start line is acceptance of the basic fact that the innate desire in all wildlife to continue the gene line and to breed is a very powerful driving force. But the direction of this force can sometimes be mis-directed as would appear to be the case with these two bulbul species in question resulting in their cross-breeding and the production of one juvenile.

“1. The Y-v Bulbul which I have suggested may be the male in this liaison in a previous communication, may be deficient in some way so that it is less likely to attract a female of the same species. It may be that its song (mating call) is not up to scratch; its courting display behaviour is below standard; it may be just a young inexperienced bird that drew a blank even with equally young inexperienced females of the same species. Or, perhaps, there may be an inherent weakness problem with it that will always mean that it is relegated to the bottom of the pile when it comes to selecting, or being chosen by, a same-species female mate.

“2. The S-h Bulbul I have suggested may be the female in this partnership. This species shouldn’t even be in Singapore, but it is as a result of released birds for whatever reason which has resulted in an, at present, small population in a relatively small area of Singapore. It is unlikely that there can be an equal ratio of males to females, so some birds of either sex will be left “on the shelf” when it comes to mate selection for reasons not dissimilar to those mentioned above, or simply by an imbalance in the numbers.

“3. So therefore, there are likely to be a few unmated birds of both sexes of both species coming together where the two species exist in the same locality. The powerful driving force dictating the desire to breed will mean that in a few instances, those unfortunate birds “left on the shelf” – even of different though similar species – will come together in desperation and try to satisfy their innate breeding urge. And, in my opinion, this is probably the reason why these two birds have come together producing one hybrid youngster in the process.

“There are many bird species around the world where ranges overlap and hybrid birds (intergrades) are created. Higher infertility levels and lower breeding successes are generally much more apparent where these cross-breeding activities occur, and breeding success is even lower amongst the surviving hybrids themselves when, or if, they form a partnership which may be a back-cross with a pure species in its parentage, or attempt to continue the cross-breeding with another hybrid. All sorts of gene flow problems can arise in such liaisons leading to infertility, lower breeding successes and lower life expectation with the net result that such intergrade areas are, geographically, usually fairly small or narrow.

“I hope this may help ‘John_singapore’ a little, but this is a truly fascinating subject with lots of research going on, especially with gull species and intergrades stretching continuously right across northern Europe, across northern Asia and North America. No one can decide where one species ends the next one begins! Lots of blurred areas of intergrades!”

John Vickerman
United Kingdom
18th August 2009

John Vickerman is a regular contributor-commentator on bird behaviour to NaturePixels.org and the BESG.