Seabirds that can’t get wet: the bizarre lifestyle of frigatebirds

Seabirds that can’t get wet: the bizarre lifestyle of frigatebirds

There are a lot of weird birds in the world. A couple that spring to mind are White Storks and Turkey Vultures, which defecate on their own legs to keep cool. Australia’s own Bassian Thrush, which uses flatulence to disturb worms and make them easier to find, is also pretty weird. But topping this list for me must surely be the frigatebirds. You may have seen these extraordinary birds in Attenborough documentaries, or if you are very fortunate and have spent time in the coastal tropics, you may have seen one for yourself.

Frigatebirds are seabirds. By definition, this means they are dependent on food from our oceans and seas. In my experience the ocean is a pretty wet place, so it seems logical that any animal that spends most of its days at sea should be comfortable with getting a little bit wet from time to time. Not so the frigatebird, a seabird whose plumage is not waterproof. If a frigatebird lands on the ocean surface, its feathers quickly become waterlogged, making it too heavy to take off again, and the bird will eventually drown.

frigatebird
If this male Lesser Frigatebird was to land on the sea, it would be unable to take off again and eventually drown. Image: Rowan Mott

How is it that a seabird that can’t get wet is able to catch enough fish and squid to survive? Fortunately for frigatebirds, their primary prey also have wings. Flying fish and flying squid (yes, surprisingly these do exist!) make up the bulk of the frigatebird’s diet. However, flying fish and flying squid rarely waste their energy on getting airborne, so frigatebirds need a little help to catch a meal. The wings of flying fish and flying squid offer them the chance of escape from predators below the surface, such as tuna, mackerel, and dolphins. When threatened by these sub-surface predators, flying fish and flying squid take to the air to evade their water-bound pursuers. However, they may be leaping out of the metaphorical frying pan and into the fire if a frigatebird has spotted the chaos taking place. Frigatebirds are incredibly agile in flight. Their thin, pointed wings and specialised tail bestow an amazing ability for mid-air turning. This manoeuvrability means they are able to snatch flying fish and flying squid right out of the air without coming into contact with the sea surface at all.

Flying fish take to the air where they are safe from predatory fish and dolphins. However, above the water, they can be snatched by an agile frigatebird. Image: Rowan Mott

The specialised body plan of a frigatebird has evolved in part to maximise their ability to spot feeding frenzies of tunas and dolphins. In tropical oceans feeding opportunities are few and far between, so frigatebirds have become uniquely adapted to be able to cover vast distances at minimal energy costs while searching for prey being pushed to the surface. They have a wingspan of around two metres or more. This is only a little bit smaller than a Wedge-tailed Eagle. But, whereas a Wedge-tailed Eagle can tip the scales at more than 5 kilograms, frigatebirds usually weigh about one kilogram. Their bones are packed with airspaces (known as pneumatised bones), more so than any other species of bird. Similarly, they have evolved additional weight minimisation measures including reducing the size of their preen gland (a gland at the base of the tail that produces oil to help keep feathers waterproof) to such an extent that it is now ineffective at stopping their feathers becoming waterlogged.

Together, these adaptations mean that they have the largest wing surface area for their body weight of any bird and are consequently able to glide and soar with ease. And soar they do! Most seabirds fly low over the water surface. Some, such as gannets, boobies and terns, will occasionally fly several tens of metres above the water to plunge-dive down on prey. However, frigatebirds have been recorded flying more than four kilometres up. From this vantage, they can spot a school of tuna or pod of dolphins from miles away and quickly descend to catch any flying fish or flying squid that may be flushed into the air. The gliding and soaring ability of frigatebirds is so efficient that they have been recorded staying airborne for more than two months at a time. During this time they can even sleep on the wing.

If ultra-efficient flight isn’t enough to score them a meal, some individuals also steal food from other seabirds. This is one of the most spectacular sights to behold – a jet-black wraith tearing through the sky as a terrified booby, tropicbird, noddy, or tern dodges and weaves to avoid being caught. In its panic, the pursued bird will often disgorge its last meal, at which point the frigatebird breaks off the chase to grab the cheap snack. This behaviour, called kleptoparasitism, has made frigatebirds famous and has even earned them the colloquial name “man-o’-war bird”. However, the spectacular nature of kleptoparasitism probably means it gets more attention than it really deserves. It seems that this behaviour may be practised by only a small minority of individuals and possibly by only immature birds and non-breeding adults.

kleptoparasitism practised by frigatebird
Frigatebirds are perhaps best known for chasing other seabirds to steal their food. Here, an unfortunate Brown Booby is the victim. This behaviour (kleptoparasitism) is spectacular, but probably overrepresented in terms of how important it is for their overall diet. Image: Rowan Mott

There’s plenty more that’s extraordinary about frigatebirds. For instance, the males have a bright red gular pouch (a sac of skin on the throat) that they inflate to the size of an Aussie Rules football to attract a mate. Also, frigatebirds have the longest period of parental care of any bird in the world. Reportedly it takes 14 months for a pair to complete a breeding cycle, meaning they (or at least the female) cannot breed every year.

Male Great Frigatebird in nest
This male Great Frigatebird is trying to impress potential mates by puffing up his gular sac into a huge red orb. A Red-footed Booby isn’t distracted by the spectacle and continues to add to its nest. Image: Rowan Mott
frigatebird chick
A juvenile Lesser Frigatebird is dependent on food from its parents. The slow growth rate of these birds means they have the longest period of parental care of any bird in the world. Image: Rowan Mott

It would be fantastic if frigatebirds were more accessible to us than they are. Yet, these fascinating birds go about their daily lives largely beyond the realm of our landlocked consciousness. This leaves them vulnerable to the ‘out of sight, out of mind’ conservation problem that plagues species we rarely encounter. Three of the world’s five species of frigatebirds live in Australia and one, the Christmas Frigatebird, is critically endangered. I hope this article has made a small connection between you and our frigatebirds, and that this in turn might improve the outlook for the frigatebird future – it would be a far more boring world without this bizarre bird.


Banner image courtesy of Rowan Mott

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