When looking at the planet’s skies and oceans, it is no surprise that Earth has been given the moniker ‘blue planet’. Yet in nature, the colour blue is rare.
For example, fewer than 10 per cent of all plants possess blue flowers, while the colour is even rarer among animals – which is why the blue-footed booby’s feet stick out like the proverbial sore thumb.
Confined to the eastern Pacific coast, from California in the north to Peru in the south, the blue-footed booby (Sula nebouxii) is surely the most celebrated of the seven species in its genus.
These birds, together with their temperate relatives – the gannets – form the Sulidae family. (One other species, Abbott’s booby, is distinct enough to have its own genus.)
Like gannets, boobies are masters of their maritime environment. Their name emanates from the Spanish word bobo, meaning foolish or clown, which doubtless refers to their clumsy movements on land.
Adults have brown upper wings and a dusky streaked head, contrasting with their bright white underparts. But few take notice of the booby’s plumage as it’s the startlingly blue feet that grab the attention.
As ever in the natural world, striking colours and appendages exist for a specific reason, and in the case of the blue-footed booby, the blueness of the feet plays an important role in both attracting and maintaining a partner.
The colour itself is obtained from the bird’s fresh, fishy prey, in the form of carotenoid pigments that, in addition to boosting the immune system, can also be concentrated in the feet to dazzling effect.
How do blue-footed boobies court?
The advantage becomes glaringly apparent when observing the boobies courting. Here, the male often marches around his prospective mate with a high-stepping motion to show off his fabulous feet in turn.
This foot-flirting is frequently accompanied with both the ‘gifting’ of basic nesting materials and the pointing of his beak, wings and tail to the sky in a blatant and brazen attempt to seal the deal.
Research has shown not only that females tend to prefer partners with more intensely coloured feet, but also that the blueness of a male’s feet is a reliable indication of his health and reproductive fitness.
Any males, for example, that are old, ill or weak may struggle to catch as much food, and this directly impacts the amount of pigment they are able to sequester, ultimately resulting in duller feet.
Males with the brightest feet are effectively saying, “I’m so fit and strong that I can ‘afford’ to use the pigment in my feet, without immune function being compromised”.
How do blue-footed boobies mate?
Boobies often mate for life, with the female laying two to three eggs each breeding cycle, in a very simple nest – little more than a shallow scrape in the ground – demarcated by a territorial ring of guano. With no brood patch (a bare, heat-transferring area of skin), the adults use their feet to keep the eggs warm, and the young finally hatch after around 45 days.
While the chicks are young, the male undertakes most of the fishing expeditions, with studies showing that chicks raised by brighter-footed males tend to receive more food (in the form of regurgitated fish).
As a result, they’re able to grow faster than those cared for by dull-footed dads.
In the 1960s, almost half the world’s blue-footed boobies bred in the Galápagos, with a population of around 20,000, but by 2012, numbers had plummeted to around 6,400.
Research indicates the decline has primarily been driven by a slump in sardine stocks, which boobies appear to need in large quantities if they are to breed.
With plastic pollution also believed to play a part in the downturn, the mantle of ‘feeling blue’ could be passed directly from the boobies to us humans.
