Super pecs and 1,200 heartbeats per minute: The hidden biology of hummingbird flight

Hummingbirds can beat their wings so fast that their flight is audible to us as a sonorous hum. But how did these incredible birds evolve into aerial masters?

In 2004, Gerald Mayr, a palaeontologist at the Senckenberg Research Institute in Germany, found himself confronted by two fossilised birds found in rocks in Fraunweiler, a village in southern Germany.

Any fossilised bird is a thing of wonder, a window into a long-lost past when flight, leaving the bounds of the Earth, was still a relatively new innovation in the vertebrate world. But these birds on Mayr’s workbench were something more remarkable still…

The birds were small – just four centimetres long, with bills two and half times the length of their skulls, and wing bones that looked just like the short, stocky humerus typical of modern-day hummingbirds. Mayr recognised them as just that, early hummingbirds, albeit ones found an ocean away from the family’s present global range in the Americas.

South American hummingbird fossils found to date are relative youngsters, dating to just one million years ago – these fossils, and subsequent examples unearthed in Germany, Poland and France, are those ancient hummingbirds’ ancestors. Mayr duly named the Fraunweiler hummingbirds Eurotrochilus inexpectatus, or ‘unexpected European hummingbird’.

Anyone whose path has crossed that of a hummingbird cannot help but be struck by their hovering flight – their very name, hummingbird, is defined by it. Beating their wings between 50 and 200 times per second, their flight through the air is so fast it becomes audible to us as a sonorous hum.

No other family of birds come even close to this frequency of wingbeats. Some other birds can hover, with varying degrees of success, but none have the aerial mastery of the hummingbirds, capable even of backwards flight.

They are a pinnacle of evolution, their flight powered by pectoral or breast muscles that account for almost a third of their body weight – this is twice the pectoral muscle mass of most other birds – and hearts that beat up to 1,200 times per minute. To put that in to context: our human hearts average a mere 80 beats per minute.

Hummingbirds’ flight as a whole, and their wing rotation specifically, is made possible by skeletal differences that mark them out from almost all other birds.

Their sternum, or breast bone, is considerably larger than those of other birds, providing anchorage for their large pectoral muscles. Moreover, their wings are attached to the sternum with a tiny ball and socket type joint – unique to them and their distant cousins, the swifts – which allows their wings to achieve an extraordinarily efficient rotational movement. Hummingbirds, by rotating their wings, gain 25 per cent of their lift from the upbeat in addition to the balance of lift generated by the conventional downbeat.

To keep up with their bodies’ demands, hummingbirds fuel themselves with a high-octane, calorific diet that comprises mostly nectar they drink from flowers – though they do eat some small insects for protein and roughage. Were we to have the same fast metabolism as a hummingbird, we would need to consume around 155,000 calories a day.

But to understand how their distinctive hovering flight came to be, we need to follow the path of Gerald Mayr’s fossilised hummingbirds further millions of years back into the Cretaceous period, a warm period of rampant biological diversification some 66-145 million years ago.

At some point in the early Cretaceous period, flowers started to evolve scents and colours to signal the presence of pollen to passing insects. These insects, visiting many flowers, would inadvertently pollinate them. In time, flowers added nectar to the mix, giving insects an additional inducement to visit them – and simultaneously, these insects evolved the necessary equipment necessary to first find the flowers and then to extract pollen and nectar alike.

These coevolutionary interactions inadvertently provided the fuel for a much larger organism, those early hummingbird ancestors, to evolve in their own right to exploit this rich food source.

Source: BBC