Hover Craft

Many years ago I was searching for the elusive Grey Honeyeater Conopophila whitei in mulga woodland near Alice Springs (not the first or last time I've tried - and failed - to find this notoriously elusive and inconspicuous little bird). However the heart definitely beat faster on finding a relatively nondescript bird repeatedly hovering at the foliage; it took a little while to give myself a good mental shake and accept that it was in fact a not-really-very-similar and very much commoner young Western Gerygone Gerygone fusca. What it was doing however was very worthy of our attention and admiration.

Immature Western Gerygone hovering at Mulga Acacia aneura foliage.
The blunt bill alone should have told me immediately that my preliminary id was based on hope rather than science!
(This is an old photo from the pre-digital past.)

Hovering is a remarkable trick, which seems to defy commonsense. In its purest form it involves - to put it simplistically - pushing forward at the same speed as the bird is pushing backwards, while of course also cancelling out gravity, in order to hang in the air. It is often claimed that only the American hummingbirds can truly hover, but I suspect a small amount of Northern Hemisphere bias there; this character was making a pretty fair attempt at it. Elsewhere the two species of Australian spinebill Acanthorhynchus spp. (also honeyeaters) and the African sunbirds are likewise pretty adept, at least to my eyes.

However, there is no doubt that the hummingbirds are the masters; to see them park motionless in front of a flower or a feeder, then suddenly apparently vanish, to rematerialise some metres away, can be quite disconcerting. Not to mention absolutely spell-binding.

Green Thorntail Discosura conversii, Umbrellabird Lodge, Ecuador. (This is a female, hence the lack of thorny tail!)
Despite being taken at 1/100 of a second, the wings are a mere blur. Not surprising, given that they may be beating at up to 80 times a second.

It is that last observation that is the key to it all. The wings are moving in a shallow figure-8, at speeds that make my shoulders ache just thinking about it, basically flying forwards and backwards almost simultaneously. The energy costs are horrendous; their flight muscles comprise 30% of its body weight, far more than for any other bird group, and their heart weighs relatively five times what ours does. At rest a hummer breathes 300 times a minute, again five times what we require. To power such a profligate lifestyle they must fill the tank constantly, drinking up to 150% of their body weight of nectar daily - for me that would mean drinking 120 litres a day!

Bearded Mountaineer Oreonympha nobilis (above) and
Green Violetear Colibri thalassinus (below), Peruvian Andes near Cusco,
both feeding on wild tobacco, Nicotiana sp.
The mountaineer is limited to this area, while the violetear is much more widespread.
These are bigger birds than the thorntail above, and the shots were both taken at 1/500th of a second,
but even so the wings are still blurred.

Other birds - terns and some small birds of prey for instance - also hover, but using a somewhat different principle, using the wind to provide the backward thrust to balance against. It is still a very sophisticated trick, constantly adjusting for the vagaries of the wind by fanning and part-closing wings and tail, while altering power and speed of the wing strokes. 

Nankeen Kestrel Falco cenchroides, near Canberra.
This bird was operating in a relatively light breeze, as the fully fanned tail and erect posture suggest,
both maximising the body's resistance to it.

Hovering in these situations provides a sort of platform for the bird to scan the ground for prey. Remarkably, but necessarily, the head remains almost motionless while the body is making its continual adjustments - indeed it has been calculated that a kestrel's head 'wobbles' by no more than 5mm while hovering.

In a stronger breeze the bird can afford to hover in more of a horizontal position, using 'scooping' wing beats to oppose the wind.

Black-shouldered Kite Elanus notatus (above, near Forbes, New South Wales)
and Nankeen Kestrel (below, Sturt National Park, New South Wales),

I am awed by the precision and power required for such a misleadingly effortless-seeming behaviour; wouldn't you love to hover go?

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