A blue-footed booby's feet are a health report its mate reads daily
Those famous turquoise feet are not just for show. Their colour depends on how well a booby has eaten in the past day or two, and potential partners watch closely. When researchers dulled a male's feet with makeup after his mate laid her first egg, she responded by laying a smaller second one.
The blue comes from two sources working together. Collagen fibres in the skin are lined up in a way that makes it look a flat purplish blue. Pigments called carotenoids, taken from a diet of fresh fish, then shift that shade toward bright aquamarine. The same pigments support the immune system, so bright feet signal a well-fed, healthy bird. The signal is fast and hard to fake: after 48 hours without food, a booby's feet visibly fade.
Courtship puts the feet front and centre. A male struts before a female, lifting each foot in turn, offers nesting material, then shows his feet once more, pointing his bill skyward with wings and tail raised. Brightness fades with age, and females tend to pick younger, brighter-footed males, who make better fathers. In a fostering experiment, chicks raised by brighter-footed foster fathers grew faster. Males judge too: paired with a bright-footed female, they would tend small eggs as carefully as large ones, whereas partners of dull-footed females incubated only the larger eggs.
The feet also serve practical ends. Boobies have no bare brood patch, so they warm their eggs by standing on them. Hunting is dramatic. Parties of about a dozen fly over schools of sardines or anchovies until a leader spots fish and signals, and they all plunge together like arrows, sometimes from 100 metres up. They hit the water at around 97 kilometres an hour and can reach 25 metres deep. Their nostrils are permanently sealed for diving, so they breathe through the corners of their mouths, and air sacs in the skull cushion the brain against the impact.
Spanish sailors called them bobo, meaning clown, for their clumsiness on land and apparent lack of fear. Roughly one in two breeding pairs raise their young on the Galápagos Islands.
Source: Blue-footed booby
