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Great & lesser Frigatebirds (frigate birds) - lifespan, habitat.

The magnificent Flying Machine

The frigatebird is one of the most dramatic birds of the air and Seychelles is one of the best places to appreciate it.  Aldabra has possibly the world as second largest breeding site, but visitors do not need to journey to this remote site to see them.  A few may be seen off the coasts of any of the islands but for real drama, take a trip to Aride Island Nature Reserve.  The climax of the Aride nature trail is breathtaking, with a spectacular vista over the northern cliffs, the last outcrop of granite before India and the sight of up to five thousand frigatebirds soaring over the crashing waves.

Just what makes them so special?  Frigate birds are seabirds yet they cannot even land on water.  They have tiny feet, incapable of giving the propulsion needed for so large a beast to become airbourne from the surface of the sea.  Unlike most seabirds they do not even have webbed toes to help them paddle through water.  In addition, their feathers are not waterproof so should one be unfortunate enough to misjudge its dive to the surface to snatch fish it would be helpless.  It would soon be swallowed by the ocean that is its home.

In fact, so tiny are their feet that they cannot even walk.  When they land at their nesting sites in the mangroves their approach must be timed to perfection because once they have settled they are virtually immobilized. The truth is that the need to build a nest and raise young is the only reason why they ever land at all. If it was possible to build a nest in the sky, no doubt they would do so, but gravity dictates otherwise.

Ah, these must be very primitive birds, people often say. Certainly they look at it, not just because of their physical appearance, reminiscent of pterosaurs with long V-shaped wings and a long narrow body.  A seabird that can't swim also seems a bit of a design fault.  However, frigatebirds are highly evolved and purpose built.  For a start they display extreme sexual dimorphism. In other words the male and the female look very different.  Males have the most bizarre, inflatable sacs on their throats which they inflate to try and impress the ladies.  Strong differences between the sexes are a very recent evolutionary trait found in all five species of frigatebird but not in a single one of the other 250-plus species of their group.  On the whole, seabirds are a rather drab looking lot, with little or no difference obvious between male and female, let alone something as weird and wonderful as a red party balloon.  It is not unusual for land birds to show big differences between the sexes, especially where evolution has had time to work its wonders.  Evidently there is something strange going on here.

These birds are maestros of the air and while most birds can fly, they have developed flight to a new art.  They have the lowest wing factor of any bird on the planet  (that is total weight in relation to wing area). They are huge birds with a wingspan of about two metres (6 feet), yet they weigh little more than a medium sized duck at around 1.6 kilograms or less.  They get away with this partly because of their narrow bodies but also because of their incredibly light, flexible bones.  The skeleton makes up just five percent of the total weight, a lower percentage than any other bird.

They are adapted to warm tropical waters such as those of Seychelles.  There are times of the year when there can be little or no wind in the tropics, yet they will hang in the air, seemingly with no effort at all, on even the calmest days as it rides the warm thermal updrafts from the ocean.

Another problem for tropical seabirds is that the warm ocean can sometimes make the waters equivalent to a desert.        A thick layer of warm surface water restricts the flow of nutrients which makes prey for seabirds more scattered and difficult to find.  Therefore, tropical seabirds need to adopt a strategy of energy-efficient flight to keep them going between meals when food is short.

Unlike any other group of birds the pectoral girdle bones of frigate birds are fused together like the rudder of a plane.  This gives them tremendous agility in the air, helping them to twist and turn quickly despite their enormous size.  They are quite capable of chasing and catching flying fish, but will sometimes take the easier option of piracy.  The hapless booby is the main victim at Aldabra.  The booby has design features which enable it to dive deep into the water to catch fish that a frigatebird would never hope to reach.  However, once it returns to the sky with its catch, the booby is at a disadvantage compared to the highly manoeuvrable opponent.  Sometimes several birds will mercilessly chase a booby to try and force it to regurgitate its catch.  So successful is this approach the Seychelles Creole name for the Masked Booby is Fou Zenero, or generous booby, a reference to the fact that birds will usually quickly relinquish their prey rather than face the onslaught of a frigatebird. Frigatebirds will also make piratical attacks even on each other to steal nesting material.  At the nesting sites in Aldabra, care has to be exercised by the nature reserve staff of Seychelles Island Foundation to ensure birds are not disturbed from their nests.  Should birds be forced to fly, others will soon swoop in to try to steal nesting material.  Despite their rivalry at sea, they breed alongside boobies in the same colonies, where they are apparently good neighbors. Maybe the boobies take comfort from having such fearsome birds to patrol the nesting sites!

At Aride Island Nature Reserve where there are no nesting boobies, frigatebirds may pick on tropicbirds or terns nesting on the island.  Attacks on Aride rarely seem to produce much reward, but perhaps that is not the point.  They may not be taking it all too seriously.  It may be that Aride is used as a practice ground for younger birds learning their skills and almost enjoying reminding the other birds of who is boss.

Being capable of traveling the most enormous distances, they can sleep on the wing, one of the few birds species known to have this ability . In the pacific, where they have been studied more intensively than the Indian Ocean it has been shown they can cross entire oceans.  Tagged birds have traveled nearly 8,000 kilometres (5,000 miles) from their nesting site to pay a visit to other popular roosts, yet it seems they return home to breed, and there is a very strong genetic identity between colonies.

In Seychelles, and indeed most of the Indian Ocean, there are many questions that still need to be answered.  The only large breeding colony in Seychelles is at Aldabra where 10,000 pairs of two species breed.  At Aldabra there are roughly 6,000 pairs of Lesser Frigatebird and 4,000 Great Frigatebird, yet at most of the major roosts elsewhere in the islands there are far more Great Frigatebirds.  At Aride Island, by far the largest roost in the granite islands, about 90 percent of the thousands of birds using the roost are Great Frigatebirds. There are other big roosts at D´┐ŻArros, St Joseph Atoll, Coetivy and Farquhar.  Add up the total number of Great Frigatebirds and it seems possible that there are too many to be accounted for by Aldabra.  The distance from Aldabra to Aride is nearly 1,200 kilometres (750 miles).  Yet many birds may be coming from further afield.  Staff on Aride Island Nature Reserve have in the past reported seeing birds with coloured wing tags which must have come from outside Seychelles as no one in the country has such a program, but where.

Local conservation organisations in Seychelles are looking at ways to answer some of the mysteries of the migrations.  It may be possible to collect fresh feathers for DNA analysis to try to link the information with studies being carried out elsewhere in the world.  Though a lot more expensive, it may also be possible to track frigatebirds via satellite.  One study at Europa Island in the Mozambique Channel showed that frigatebirds may go on foraging trips for anything up to 12 days when the partner is incubating eggs, or about three days when chicks need to be fed.  These foraging trips will take them on journeys of almost 4,000 kilometeres (2,500 miles) each time.  Data from the satellite tags has shown they will ride the warm air masses as high as 3,000 metres (10,000 feet), much higher than almost any other bird.  Then about six times per day, they go into attack mode and descend towards the surface to hunt. 

Along with those other giants, the albatrosses, they have the longest period to reach maturity of any bird species.     They do not breed until 10-12 years of age.  Even when they finally get round to breeding they take their time about it.  They have the longest breeding cycle of any bird, a full 12 months, leaving them no option but to take a year or more off between each successful breeding attempt.  They spend seven months with chicks after fledging, perhaps passing on their skills, a higher investment in parenting than that made by any others.  To compensate for this low productivity they are very long-lived birds surviving for 30 to 40 years or perhaps even longer.

As if all this was not strange enough, there is one final enigma to the family tree.  To look at this bird you could be forgiven for thinking it was related to other large seabirds.  Maybe albatrosses, you might think?  Linnaeus pondered the question and decided they be placed in the same genus as pelicans.  However, Linnaeus did not have the benefit of being able to sue DNA, and recent studies have shifted them substantially in the greater order of things.  They are now placed in the superfamily Procellarioidea or alongside birds including penguins.  So, this tropical non-swimming, master of flight with its light narrow body and unwebed feet ends up alongside the polar, master of swimming, dumpy, flightless penguins. Now that really is strange!
 

 




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