When one species mates with another entirely different species, the result is a genetic blend of the two: a hybrid. Perhaps somewhat sadly we usually don’t get one half of one creature and the other half of the other in a perfect visual split, but there will be detectable differences in height, or length of tail, colouring, facial structure and so on. Mating between different species isn't all that unusual. However, the ability to then reproduce amongst hybrids is low; they are often born sterile, or any resultant offspring don’t tend to last too long after birth. Not so with birds, it seems.

Birders know to look among three types of birds for hybrids: ducks, large gulls, and hummingbirds.

Pick and mix

It turns out that around 10% of the known 10,000 or so bird species do cross-breed, and successfully, too. There are many instances of “forced” or captive hybridisation for various reasons, like falconers trying to create a super-hunter, or just among breeders of domestic canaries and finches (known as mules, as well), but wild hybrids are, by their nature, harder to track. Bird hybrids exist in plentiful amounts but they are not always easy to spot, and many birders regard seeing them as the equivalent to hitting the jackpot. One birder even earned the envy of his peers by identifying a triple hybrid…

Birders know to look among three types of birds for hybrids: ducks, large gulls, and hummingbirds. These make up a big chunk of the above-mentioned 10%. In fact, so many American black ducks have bred with mallards that there are now very few black ducks left.At first glance it would be hard to see any correlation between these types of birds that would account for so many hybrids, but the males of ducks and hummingbird families do in fact share common traits: they will aggressively pursue females if they are not willing enough to mate, they don’t form pair bonds (that is, once the deed is done the males will seek out other females), and they are also fairly poor at parenting, leaving the females to incubate and raise the young on their own. Quite why these traits lead to hybridisation is not yet clear, but do bear this in mind if you are struggling to identify something that for the most part looks like a particular species but somehow just isn’t.

There also seems to be a connection between high occurrence of hybrids in certain species and how pretty they are – plenty of evidence exists to suggest that the gaudier the males, the more likely you will find hybrids – there are 23 hybrid species of paradise flycatchers, for instance.

The gaudier the males, the more likely you will find hybrids.

If you keep those things in mind – high occurrence of resplendent males, notable single mothers – you may be able to locate some potential sites for some hybrid spotting depending on how well you know your own area. Not everyone is comfortable going to zoos, but if you happen to have one nearby then keep an eye out there too – captive species have very little choice.

Keep it in the family

Hybridisation will mostly happen between species that are already closely related, but it can occur in different genera (one above species); however, the animals must always come from the same family, the stage of classification that is two above species. For example, swans and geese come from the same family Anatidae, and can mate together, and when they do, you get the rather wonderfully named “swoose” (although I personally would have preferred a “goowan”, said in a west coast Irish accent). Pleasingly, in this instance, the resultant bird does indeed look like half swan, half goose.

‍So, why does hybridisation actually occur? Some have put it down to “accidents”, but the more accepted theory is the probable scarcity of males in the common species. That and the other chaps are just too darn attractive.

‍One family of birds where hybrids occur regularly are the warblers, and a notable union is between the golden-winged and blue-winged warbler. Depending on the genetic make up that shows in the plumage colouring, you can get either a Brewster’s warbler, or Lawrence’s warbler, identified in 1874, and both are found extensively throughout the eastern states of the US.

Lucky spot

Has there ever been an instance of three species in one bird? Well, I’m glad you asked: in May 2017 one very lucky birder was able to demonstrate that this does indeed happen, and perhaps surprisingly it was the very first time this has ever been recorded.

Lowell Burketknows his patch – he may not know many birds, but he knows those that frequent his neighbourhood, and he loves to photograph them. One sunny Pennsylvania morning he had his equipment set up around the local watering hole that the birds frequent, and he noticed a warbler that was displaying some peculiar markings on its chest, a smudge of brown. At first putting it down to mud or blood, he then noticed that the markings were identical on each side of the chest. But they were completely out of place on this otherwise seemingly obvious Brewster’s warbler.

Baby birds, male and female, learn their songs from the father, if he is around, or from the other males of their species. Lowell knew this, and with that knowledge he managed to get a recording of that now familiar bird singing. What he heard suddenly made everything clear – this bird’s song was that of the chestnut-sided warbler, which meant that the mother had to be a Brewster’s warbler, which in turn is a hybrid of the golden-winged and blue-winged warbler.

He sent his recordings and photographs off to the Cornell Lab of Ornithology in New York, hoping they wouldn’t think him mad, and a week later researcher Dr David Toews drove 240 miles to set up a mist net to see for himself. Within four minutes the bird had been captured, whereupon it was measured, photographed, weighed, and a small sample of blood taken. Genome sequencing confirmed Lowell’s theory, and this new bird is now known as Burket’s warbler, an honour if ever there was one.

Baby birds, male and female, learn their songs from the father.

If you are interested in learning more about hybrids, you can do no better than visit the website of the Avian Hybrid Project, set up by Jente Ottenburghs during his PhD at Wageningen University; it is a commendable and mammoth resource, bringing as many if not all of the published scientific literature in this field in one place, with some beautiful images that may help you with identification.

And remember, those brown streak marks on the chest of a somehow different looking bird may be dirt, or you may be looking at a brand-new species, showing just for you, on your feeder.