Thursday, 3 May 2018

Gannets and Boobies; formidable fishers

Gannets and boobies form a Family of 10 distinctive large seabirds, now known to be most closely related to the cormorants and darters. They are strikingly handsome birds, and dramatic to watch, as they hurtle headlong into the water from high above it (mostly from between 10 and 30 metres up, but in the case of Masked Boobies this may be as much as 100 metres). They close their wings as they hit it (at speeds of up to 100kph, according to Bryan Nelson's classic The Gannet) and continue to chase the fish or squid beneath it. This chase can take them as far as 25 metres beneath the surface; fish are usually swallowed before they resurface. This latter strategy is useful for avoiding the thieves - frigatebirds, gulls, skuas - which are lurking above with intent to steal the rewards of their skill and labour.
Blue-footed Boobies Sula nebouxii fishing, Puerto Vilamil, Isla Isabela, Galápagos, above and below.
You can see the streamlined shape they adopt just before hitting the water.
This was a spectacular sight, right in the harbour.

'Gannet' is based on the same Old English word root as 'gander', which at that stage simply meant a goose, with no implication of gender; for reasons uncertain it came to be applied to the Northern Gannet, which in northern Scotland is still referred to as 'Solway Goose'. The general usage of the word didn't occur until the 19th century. 'Booby' was used in the sense of a 'foolish fellow' - presumably for sitting trustingly in colonies in the face of marauding sailors. We really are shameless! The gannet genus Morus means the same thing, from the same Greek word which gave rise to 'moron'. If I were a gannet (or booby) I'd be suing...

It was long believed that they were closely related to pelicans, but that is no longer the case; pelicans are closer to herons and ibis. The gannets and boobies share the Order Sulidae with frigatebirds, as well as cormorants and darters. (From Sulidae comes the world sulid, referring to a member of the family; I will use it here, not to be smart but because it's easier than saying 'gannets and boobies' each time!) One characteristic that they share with pelicans, which contributed to the misunderstanding, is having all four toes connected by webbing for better swimming power. It seems however that they and the pelicans evolved this adaptation independently.
Blue-footed Booby feet (as if you couldn't have guessed!), Puerto Ayora, Galápagos.
The hind toe has been dragged around to the side to enable the more extensive area of webbing.
In ducks and gulls, for instance, only the front three toes are joined.
The plunging lifestyle inevitably requires other adaptations too.
Nazca Booby pair Sula granti, Española, Galápagos.
The eyes are set right alongside the bill, enabling binocular vision, which is most unusual in birds.
They seem to choose the prey from high up and adjust their dive as come down to follow it, which of course
requires very accurate depth perception. Moreover in a vigorous headlong dive, external nostrils would
be a serious hazard, so they have been dispensed with them; instead they open inside the bill.
A less obvious adaptation is in the breast, which is cushioned to protect internal organs by a remarkably developed set of air-sacs (bronchial extensions which spread throughout the body of a bird to enable their unique respiratory system), like bubble wrap. Their neck muscles are unusually strong, they have membranes to protect the eyes, and a layer of spongy bone at the base of the bill to absorb the impact.

The sulid bill has small serrations to assist in handling fish; on the other hand it lacks the hooked tip of many other large fishing birds.
Red-footed Booby Sula sula, Genovesa, Galápagos.
You can see the serrations here if you click on the photo to enlarge it.
Within the family many (but not all) authorities recognise three genera. The oldest seems to be the Abbott's Booby Papasula abbotti of Christmas Island and surrounding oceans, which apparently split from the ancestors of the other species some 22 million years ago. The ancestral gannets and other boobies parted company 17 million years ago. Despite this the differences between the two groups are not particularly striking. The six Sula boobies are essentially tropical and subtropical, while the larger three Morus gannets are mostly birds of temperate seas. The boobies are generally darker (though as with the Nazca Boobies above there are exceptions). Gannets are white (except for wings and tail) with yellowish heads.
Australasian Gannet Morus serrator, south coast New South Wales.
These long narrow pointed wings are typical of the family.
Here are a couple more examples of these very elegant wings.
Masked Booby Sula dactylatra, Muttonbird Point, Lord Howe Island.

Nazca Boobies, Galápagos.

Peruvian Boobies Sula variegata, Islas Balasteros, Peru.
A further difference lies in the sex size ratios; female boobies are notably bigger than their males, but this is not the case with gannets.

Most sulids are completely white beneath, presumably to provide camouflage from predators coming from beneath them in the ocean. However there are two exceptions; the Brown Booby Sula leucogaster, found throughout most of the world's tropical oceans, and some Red-footed Boobies... This strange latter statement is based on the fact that Red-footed Boobies come in two basic morphs, a brown (shown above) and a white - plus variations on those themes!
Brown Booby, Lady Elliot Island, Queensland.
Even here the belly is white.
All sulids nest in colonies, though this is presumably mostly due to necessity - suitable island sites are not easy to find. The size of the colony and the breeding area will also determine how densely packed the colony is.
Nazca Booby colony at sunset, San Cristóbal, Galápagos.

Masked Booby colony, Lord Howe Island.
Cape Gannet colony Morus capensis, Lambert Bay, South Africa.
This is an extraordinary experience; the huge colony comes right up to a viewing hide.
All these species nest on the ground - in most situations there is no choice, and the gannets in particular are large birds. The nest is no more than a scrape on the ground.
Nazca Booby nest scrape.

This nesting Masked Booby on Lord Howe Island seems to have prepared even a scrape among the grasses.
 However Red-footed Boobies, the smallest sulids, nest in shrubs and trees.
Red-footed Booby colony, Genovesa, Galápagos.
Most Galápagos Red-foots are brown morphs...

... but not all! This is a white morph Red-footed Booby, with an egg.
At first sight, this looks like the brooding strategy of most birds, but sulids lack brood pouches (featherless areas of the belly which have blood vessels close to the surface for warming eggs and chicks) - perhaps they would cause too much heat loss in the water. This parent - they both take turns at brooding - is covering the egg with its feet under the body. These feet have many blood vessels, which warm the egg. On the other hand the bird is basically standing on the egg, so it must have a particularly thick shell!
Nazca Booby and egg, Genovesa.
Most sulids lay only one egg, though Blue-footed and Peruvian Boobies breeding in the great anchoveta grounds of the Humboldt Current lay two or three and aim to raise them all. In other cases, where two eggs are laid (ie Brown and Masked Boobies) the second is simply insurance against loss of the first, and the second chick always dies.
The two eggs of this Nazca Booby on Genovesa represent a most unusual situation for this species,
but they would certainly not have resulted in two fledged chicks.
For the first few days of the chick's life it is brooded on the warming feet, beneath the parents' bodies.
Nazca Booby with young chick on feet, Genovesa.
Here are some more chicks at increasing ages; ages cited are of course approximate. Except where specified, all photos were taken at different colonies on Genovesa.
Nazca Booby with one week old chick, just growing down feathers.
Nazca Booby and 2-3 week old chick.
Red-footed Booby chicks, 2-3 weeks.
Nazca Booby feeding 3-4 week old chick, growing but still quite helpless.
Blue-footed Booby and 3-4 week old chick, North Seymour, Galápagos.
By now the parents may start to leave the chick on its own, as it is now able to adequately manage its body temperature, while both parents forage. However fledging in most species takes around 15 weeks - and this is after some six weeks of egg incubation!
Red-footed Booby chick losing the last of its baby down.
Red-footed Booby chick, now fledged, practising flapping its wings in preparation for
its first flight lesson.
But even after fledging the youngster will depend on its parents for food for another few weeks; it presumably takes this long to learn the complexities of the plunge-diving feeding system.

This relaxed approach to breeding means less stress on the parents, a strategy which is made possible by their longevity, which may be 20 or more years. They may not start breeding until six years old.

Displays, both for reinforcing pair bonds, and for defending territories in crowded colonies, are highly ritualised. Famously, Blue-footed Boobies flaunt their feet, presenting them overtly when landing, and performing slow dances featuring the feet, lifted for inspection. As in most animal blue bits, the effect is a trick, utilising bubbles in cells stacked in precise ways to reflect blue light; more on that here. However Blue-footed Boobies' feet aren't just blue - they cunningly brighten them up by incorporating yellow carotenoids to form the most sought-after shades, which are bright aquamarine. (Mind you, when I started to look up this colour on line I found many shades all called aquamarine, so I'm no wiser as to which is the optimal Blue-footed Booby foot.) Birds with feet artificially dulled by experimenters lost their attraction to their mates. And there was a real correlation between foot colour and fitness. Here are a couple of very different shades I've seen in Blue-footed feet.
Puerto Ayora, Galápagos.

Pucusana, Peru.
And that is probably enough for today, though I've greatly enjoyed preparing this tribute to the wonderful sulids; I hope you've enjoyed it too. Here are just a couple more images...

Peruvian Boobies, Islas Balasteros.

Masked Booby preening, Lord Howe Island.
(And remember that you can get a reminder when the next post appears by putting your email address in the Follow by Email box in the top right of this screen.
And I'd love to receive your comments - it's easy and you don't need to sign in!)

1 comment:

Flabmeister said...

I seem to have been seeing more Australian Gannets at Mallacoota this year than in the past. I'm not sure whether that is because there are more Gannets in the area or if I am mor observant this year!