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.
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.
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.
Wow! Might be your best post yet! You definitely had me thinking "total a-hole" in the first part, but then when I heard about their devoted parenting, their flight skills, and lastly, their goofy little legs & feet, I think they are...sweet. :-)
ReplyDeleteAnother fantastic post! Frigatebirds definitely get a bad reputation sometimes and their use of kleptoparasitism sometimes overshadows some of their truly amazing characteristics, like their use of dynamic soaring and exploiting thermal fronts.
ReplyDeleteInteresting. Thanks! Funny, but I never thought of Frigatebirds as assholes. My memories of them are from a summer of adventuring in Baja many years ago, watching them catching fish in the Sea of Cortez. Now, baby Coots also are kleptoparasites, but nobody thinks of them as being assholes. I guess if you're that cute you can get away with anything.
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