Sunday, August 20, 2017

A-hole of the Skies

If, in reading my previous blog posts, you’ve gotten the impression that birds are inherently noble or good, let me disabuse you of that notion by telling you about the Great Frigatebird (Fregata minor).

An adult female Great Frigatebird soars above Sand Island on Midway Atoll.

We generally admire predators, understanding that that they need to kill other animals in order to survive. After all, everyone’s got to eat and most of us humans sacrifice the lives of other creatures for our daily sustenance. And it’s certainly true that most animal species, including our own, sometimes find themselves in conflict with others of their kind, so when fights break out between individuals over territory or mates, we tend to shrug our shoulders and attribute such violence to Darwin’s theory. Albatross, for example, are generally peaceful creatures but at the start of the mating season, things can get pretty ugly.

What is it about the frigatebird then that illicits such negative reactions from people? Why do I often find the word “asshole” forming on my lips as I watch them fly by?

In terms of public image, frigatebirds face some real challenges. First, they are very large and kind of scary looking. Cloaked in black feathers and having a wingspan exceeding 2 m (80 inches) and a long hooked beak, they are imposing looking as they circle above Midway in vulture-like fashion. Unlike vultures, they are not scavengers, which becomes apparent when they suddenly burst into rapid flight chasing down a seabird as it returns from a foraging trip to sea and harasses it mercilessly until it drops whatever food it's carrying. Then, before the regurgitated fish or squid hits the water, the frigatebird swoops down and catches the morsel in mid-air. A pretty clever trick, really, and it sure beats having fly out into the open ocean to catch your own food. In biological jingo this is known as kleptoparasitism, a portmanteau of klepto (Greek for stealing) and parasitism (to harm without killing). The Hawaiian word for this species, in fact, is ‘Iwa, meaning thief.

Watching frigatebirds engage in their thieving ways must surely rank as one of the most amazing wildlife spectacles, and on Midway this behavior is very easy to observe, especially this time of year, when frigatebirds are nesting and must feed not only themselves but also their hungry chicks. My favorite place to watch frigatebirds is at the Cargo Pier where you can sit as the sun sets, drink a beer or three, and get a really good view of the action. Just look for a Red-Footed Booby or Red-Tailed Tropicbird heading towards land and there’s a decent chance you’ll get to see a chase. (Male frigatebirds pursue sooty terns and noddies as well, but for whatever reason this seems more difficult to observe.) For the victim there’s a lot at stake – losing the food they are carrying might diminish the bird's chance of successfully fledging a chick – so they don’t give in easily. The result is often a truly spectacular aerial chase in which the booby or tropicbird attempts every flying maneuver it has in its repertoire. The bird in pursuit flies as fast as it possibly can, then suddenly twists and turns to evade its pursuer. Unfortunately for boobies and tropicbirds, the frigatebird possesses exceptional flying skills, a consequence of its large wing area to body mass ratio as well as a very long forked tail which makes it incredibly agile for its size. Frigatebirds also often attack in small “gangs”, making it even less likely the bird being pursued will succedd in escaping.

Frigatebirds soar high among White Terns scanning the skies for their next “victim” from which to steal a meal

Reasons to despise the great frigatebird don’t end there. These birds have an unsavory habit of eating eggs or small chicks (especially terns and noddies) when a nest is left unguarded.  I’ve seen them flying with Sooty Tern chicks in their bills on Eastern Island. They apparently will engage in cannibalism too, stealing eggs or chicks from other frigate nests. Frigatebirds may also attack vulnerable fledglings when they are trying to learn to fly. I once watched two frigates torment a young Bonin Petrel that was floating in the water off West Beach; they dove at the petrel repeatedly for several minutes, sometimes grabbing the fledgling, flying up a few feet, and then tossing it violently. Eventually the frigates tired of the game and left the petrel there in the water, looking much the worse for wear (I’m guessing it died).

Frigatebirds thieving ways are not limited to stealing food. Both male and female Great Frigatebirds work together to construct their nests; the male gathering the nest material and bringing it to the female who does the bulk of the building. Sometimes though, a male will find it easier to steal nest material from other birds rather than collecting it himself. Red-footed boobies, which often nest in very close proximity to frigatebirds, are a common victim. The situation is even worse if you’re a Black Noddy though, as the frigatebird is known to steal their entire nest! Then there is the issue of the nest itself which, because the frigatebird chicks lack the ability to eject their feces away, becomes covered in shit and quite stinky as time goes on.

Great Frigatebirds prefer to nest in shrubs or trees. On Eastern Island at Midway Atoll they find suitable habitat atop tree heliotrope (Tournefortia argentea).

So is the frigatebird really an avian A-hole? When I the idea of writing a blog post about the Great Frigatebird was hatched in my mind (sorry!), I was pretty convinced that this was so. But as I was researching this species I discovered that the story was much more complicated.

Frigatebirds, it turns out, share some of the same admirable traits possessed by albatrosses. Parents are monogamous (at least during a given breeding season), engage in interesting, if not completely endearing, courtship behavior, which include various postures including the male’s display of its gular sac. Both male and female equitably share in the incubation, brooding, and feeding of their one and only chick. This is no small feat as it can take over six months to raise one. Great Frigatebirds might even hold the record for avian parental dedication. Unlike albatrosses and many other seabirds, whose care of their chick ends at the time of fledging, adult frigatebirds continue to feed their slow-growing offspring for up to a year after they leave the nest.

A male Great Frigatebird displays its gular sac in an attempt to attract a female

OK, maybe you’re saying "so sure they are good parents but what about the stealing bullshit?". Having grown up in a neighborhood with a lot of mobsters I know what you're saying. Some of them were good parents too when they weren't out stealing and killing!  Well it turns out that kleptoparasitism accounts for just a small part, by some accounts just 5%, of the Great Frigatebirds diet, meaning that they obtain the bulk of their food – squid and flying fish, primarily – by (anthropocentrically-speaking) honest means. 

Frigatebirds certainly should be admired for their incredible flying abilities. Here on Midway you can often see them hovering very high in the sky, like large black kites, riding the air currents and rarely flapping their wings. Like albatrosses, they engage in a flight technique known as dynamic soaring, a sophisticated means of using differences in air speed to fly quickly and efficiently; frigatebirds cruise nearly effortlessly at speeds of up to 50 km / hr (30 mph). The frigatebird’s aerial prowess has enabled them to colonize a large swatch of the tropical and subtropical Pacific Ocean, from the South China Sea, to the Great Barrier Reef, and to Mexico and Christmas Island in the South Pacific. Some populations are even migratory. Midway lies just about at the northern extent of their range. This hyper-adaptation to a flying lifestyle does not come without a cost. The Great Frigatebird’s small legs and feet create difficulties for them when trying to take off; they don’t land in the water because if they did they might get stuck there.

Living around frigatebirds is definitely a unique opportunity and I feel a little embarrassed at not having taken the effort to look beyond the stereotypes and appreciate their other behaviors until now, over a year since beginning my assignment here. It makes me wonder how much stereotypes and lack of information, limit my appreciation of other species (or even my fellow humans!). An important reminder, I think, to question your assumptions and spend more time questioning and learning and less time judging.

Additional reading for the exceptionally curious:

Gilardi, J. D. 1994. Great Frigatebird kleptoparasitism: sex-specific host choice and age-related proficiency. Condor 96:987-993.