The black swift of the family

It's that time of year in the UK.

Several months have passed since the swifts, swallows and martins returned to our shores from Africa, and for many of us, it's hard to remember a time when they weren't here. Because of their reliance on man-made structures for nesting, these birds are a familiar sight to most people- it's no wonder that we notice their absence in autumn when the screaming of the swifts and the dolphin-like chatter of the house martins fall silent. In areas where all three species can be found, it can often be difficult to tell them apart: they are so closely linked in lifestyle and appearance, and in our minds, too. Shouldn't it be safe to assume that they are also linked in a family tree?

Swift

House martins

Swallows

Well, not quite...

There's an odd one out here. House martins and swallows are very close relatives: so close, in fact, that they can sometimes produce chicks together. Personally, I've never seen a half-martin half-swallow perched mournfully on a wire puzzling out its existence, but compared to most naturally-occurring hybridisations between species, experts consider this one to be quite common. Both species fall into a massive great grouping of birds called the Passerines, or the perching birds. Think of any bird that visits your garden regularly, and it'll probably be a Passerine- unless you keep chickens or have a feral emu in your neighbourhood.

Swifts, on the other hand, are something else all together. Aside from other swift species, which are found all over the world, their closest relatives are... hummingbirds?!

Yes, hummingbirds. Those shiny, colourful little birds that get away with living on a diet of sugar and seem unable to accept that they weren't born as butterflies. Aside from their impressive flying abilities, you wouldn't think they had anything to do with the dull-coloured jet planes that whizz around our cities in summer. But they do!

Scientists get thrown off by similarities and differences too. When building family trees of species, or phylogenies, we use similarities between organisms to see how likely they are to be related. But sometimes, distant relatives- like the swifts and the swallows- end up looking similar due to a process called convergent evolution. Animals respond to the challenges of their environments by evolving solutions- in this case, the similar challenges faced by swifts and swallows has led to the evolution of similar solutions. Both groups have to fly long distances twice a year, and catch insects on the wing- hence both have become streamlined and sleek, with long wings and short, sharp beaks. Hummingbirds have very different challenges in their environment, so it's no wonder that they have ended up looking very different. Long, sickle shaped wings are great for gliding hundreds of miles across continents, but they're no good for hovering in front of a flower.

 To get round the problem of distantly-related animals that look the same by chance, scientists look at LOTS of different features when building a phylogeny: the more that two species share, the more likely they are to be related. As it turns out, hummingbirds and swifts do share a great deal of features, but they're much more subtle. A big clue is in the collective name for hummingbirds and swifts: the Apodiformes, meaning "no feet" in Greek. While that's a bit of exaggeration, both hummingbirds and swifts have the silliest little legs imaginable- they allow them to have a rest in between flights, but not much else. Swifts are such great fliers that taking a rest doesn't really bother them: it's thought that after leaving the nest, a young swift may remain in the sky for several years before making contact with a solid structure again. You'd think that the sudden inertia after all that time would make them feel dizzy. If it weren't for the very serious business of breeding, swifts wouldn't really need to land at all: they do everything in mid-air, from mating to sleeping. Until they achieve the evolution of flying eggs, however, it seems that they're destined to come down to earth for a few short weeks each year.

As walking makes up such a tiny part of a swift's life, it's not really surprising that they're a bit rubbish at it. Oddly, while Apodiformes don't lack feet, they DO lack a scaly covering on them- further testament to how little they are used for anything more strenuous than supporting the bird's weight.

Take a look at this video, and how awkward these swifts look when they're grounded in the nest:

 Harder-to-spot similarities between hummingbirds and swifts include features in the skull, and an unusually short, broad humerus (the bone that forms the upper arm in humans). This latter feature is perfect for creating a strong wingstroke in hummingbirds, allowing them to hover while they feed. By considering all these features together, biologists have known about the relationship between hummingbirds and swifts for quite some time now.

Building phylogenies is still a tricky business, and of course, there are plenty of differing opinions on how many parts of the tree of life should look. Things have become better in recent years now that we can look into the similarities in DNA sequence between species: DNA tends to tell fewer fibs and be less subjective than morphology, as long as the data is handled correctly! Still, the false family of swifts, swallows and martins is an important lesson in looking beyond what our eyes first tell us, and avoiding the traps that convergent evolution throws up throughout the animal kingdom.

Related or not, though: they're all pretty damn cool.

Photo credits:
Swift: http://www.flickr.com/photos/billyboysfotocolection/4754795436
House martins: http://www.flickr.com/photos/tonymorris/6016706663/
Swallows: http://www.sxc.hu/profile/krzysiuc