2015 Darwin Science Expedition - Day 11 - Underwater searches and seabirds on the islands


Spent the morning looking for Autonomous Reef Monitoring Systems off the island of Petit Coquillage. These systems were secured to the reef two years ago in a similar fashion to two other sets of the devices at other points around the archipelago. At the other sites we have found them in good condition but here they seem to have vanished. So although our dive did not achieve its purpose it did turn into a long duration underwater as we searched for the ARMS. This long swim allowed the perfect opportunity to view a large tract of this stunning reef.


The majority of the reef bed consisted of porites and soft corals like sinularia. Beds of these mounds and encrusting corals as far as the eye could see in each direction. And overhead a multitude of fish. Through the course of the dive we saw five turtle and four octopus whilst our boat cover crew saw several manta ray in deeper water as they hovered over the drop off waiting for us to surface.

So although we were unsuccessful in recovering the ARMS from this site it was still a beautiful morning. Fortunately the data from the other two sets of three devices that were also deployed two years ago is more than enough for this particular project to be successful.

This afternoon the terrestrial team travelled to Ile Yeye before we depart Peros Banhos and make our way across to Salomon Atoll further to the east. It is a while since we have heard from Pete on the terrestrial front so I've included an update from him. Read on...

Northern Peros Banhos

On completion of the Vache Marine work I conducted breeding seabird surveys of all of the islands of western Peros Banhos. All of these islands have been environmentally devastated by man. Introduced invasive rats are certainly present on all the islands that were farmed for coconuts and these islands have also had the vast majority of their original vegetation removed and replaced by monocultures of coconut. If environmental triage was undertaken on them, they would be placed in a corner to die. If ever they are to be environmentally restored, the only way to ecologically improve them would be to take them down to “ground zero,” exterminate the rats and then replant them with native trees; a task that would need decades to see fruition.

Coconuts are wonderful plants, hardy like few other trees, capable of being afloat in sea water for months and still being able to germinate when washed ashore; they are a cornerstone species for island building and the long-term stability of shorelines in the Chagos Archipelago. Naturally, coconut is found along shorelines and occasionally further inland where storm-surges have washed nuts (seeds) further inland. However, in western Peros Banhos man has removed over 90% of the original vegetation from all across the islands (i.e. inland as well as along shorelines) and replaced with coconuts as a crop. The landscape is now akin to the wheat and corn fields of Europe and America, stretching for miles with no other vegetation and very little wildlife. The first very long days’ surveying involved boating along the miles of shoreline, lagoon and ocean-side, staring at lifeless tracts of former plantations.

The potential of what could happen through environmental improvement of these islands is demonstrated by a large seabird called Red-footed Booby. This warm-weather member of the gannet family is a pan-tropical breeder, nests above the ground and generally prefers to nest on undisturbed islands. The Chagos Archipelago is likely to be unique for this species in that at least since 1996 (when the first full breeding seabird study was undertaken) it has been expanding its’ breeding range and overall numbers. It is thought this is the only Red-footed Booby population in the world that is increasing.



On the tips of some of the islands of western Peros Banhos are tinyvestiges of the former oceanic climax forest that once covered these islands; often literally two or three trees. In what appears to be a demonstration of the art of the possible, six of these relicts of the past now have Red-footed Booby breeding in them, four of which are new breeding islands that have been colonised since the last survey. Sadly these colonies are unlikely to increase in any significant numbers as the structural architecture of coconuts prevent boobies breeding in them (I have only ever encountered one Red-footed Booby nest in a coconut tree in the Chagos after nearly a decade of survey work).



After anchoring off the island of Ile Diamant a full day was spent surveying the islands technically and legally defined as eastern Peros Banhos. In the morning I was to look at Passe and Moresby and in the afternoon move further afield to visit Parasol and Longue. Passe is similar to a western Peros Banhos island, it is infested with rats and was farmed for coconuts. It is an environmental catastrophe that holds between 10-15 pairs of Red-footed Booby in the remaining coastal trees that are not coconut. Moresby also has rats and was once farmed as a plantation but this near-unique island in the Chagos holds a secret; it has a mangrove swamp and Red-footed Booby thrive in these, despite the presence of rats.

After surveying the lagoon-side (former coconut plantation) of the island where, again, a few Red-footed Booby nest in relict hardwoods, I entered the mangrove swamp from the north-eastern end. Small-leaved Mangrove (Lumnitzera racemosa Willd) only grows on two islands in the archipelago, Moresby and Eagle Island. Both of these mangrove stands require management if they are to survive in to the future. Both are being encroached upon and dried out by the former coconut crops and in the case of Moresby where seawater occasionally overflows in to the swamp, it is inundated with what man so readily discards in to the seas; piles of rubbish such as plastic bottles, polystyrene, fishing buoys and flip-flops now line the edge. Fortunately the boobies do not recognise or mind the poison below them and are still nesting in healthy numbers.



The afternoon took us further along the northern boundary of Peros Banhos atoll and on to the first of the IUCN listed Important Bird Areas (IBAs) of Parasol and Longue. These two islands exemplify the problems with how IBAs are classified in the Chagos. At present it is based upon individual islands, and there are ten designated and two proposed IBAs. However, not all of these islands meet all of the criteria every year, for reasons that became fully apparent on last year’s scientific expedition and were first suspected in 2009. In 2009 I visited Longue as part of the British military working out of Diego Garcia. I noticed many deserted Sooty Tern nests and several dying and dead chicks in amongst what had been a few weeks earlier a thriving Sooty Tern. I examined some of the dying chicks and they all had avian ticks attached. Last year the same phenomena was found on Parasol where some 32,000 pairs of birds had deserted the island and the vast majority of the remaining chicks were again infested with ticks, some with a load of over 20 of these parasites. This was evidence enough that periodic desertions of these islands was occurring and was caused by the tick-load of the island tipping the balance in favour of the ticks, not the survival of chicks. Knowing Longue was abandoned for at least two years after the 2009 infestation, if the breeding cycle of Sooty Tern coincided with our visit (they do not breed on a 12 month cycle in the Chagos), I did not expect Sooty Terns to breeding on Parasol. They were not, neither were they breeding on Longue. I have recorded island desertions by Sooty Terns on the Bois Mangues and Coquillages too but did not investigate whether ticks were the cause but suspect so).

So my point on IBA classification in the Chagos Archipelago is it should be amended from individual islands (in most cases) to groups of islands. Had an ornithologist been surveying these islands for the first time with no knowledge of the breeding phenology and distribution of Sooty Terns, both Longue and Parasol would not have been recognised, yet, at a period as yet undetermined, likely to be about three years, these islands provide rat-free havens for thousands of the highly communal breeders. For Peros Banhos I have proposed elsewhere that this IBA be called the Eastern Peros Banhos Island Group and constitute all of the islands that fall under the Strict Nature Reserve ordnance under BIOT Law (forbidding anyone without permission to land on them) and runs from the recently rat-cleared Vache Marine east encompassing the rat-free Coquillages, Bois Mangues, Parasol and Longue and, significantly, the rat-infested islands of Yēyē, Manōel, Moresby and Passe. The latter four ornithologically poor islands being included in the IBA to signify their importance as islands requiring ecological improvement (by removing rats and managing the former coconut plantations).



Longue did provide one pleasant, if expected surprise and that was an increase in the Brown Booby breeding population. Throughout the Indian Ocean, if not the world, this species is in decline, primarily through human interference of some sort. Not so in the Chagos where again, since the first comprehensive counts in 1996, this species has been increasing in numbers and expanding its breeding range to new islands. In 2014 it was recorded as breeding for the first time on Ile Longue with a single pair. This year there are two pairs. Brown Booby in the Chagos does appear to have any set breeding period and on Longue, one nest contained two eggs and the other a 10 week old chick. Unlike Red-footed Booby, Brown always breeds on the ground and, as such, rat-free islands are a prerequisite for colonisation.


The following day saw a long boat transit across the top of the atoll to the paired islands of Petite and Grande Bois Mangues (loosely translated as woods of the mangos). Both islands are rat-free and both qualify as IBAs. Similar to Longue and Parasol, these islands periodically host Sooty Tern breeding colonies, though none were present at the time of our visit. The Bois Mangues speciality are their Pisonia grandis glades and associated with them, massive numbers of breeding Lesser Noddies. At the time of our visit Petite Ile Bios Mangue held about 1100 breeding pairs and Grande about 11,000. The Pisonia stands on Grande Ile Bios mangue have been described as “Lesser Noddy churches.” This is due to the way the branches of the veteran trees intermingle to form domed-likestructures and, all along the branches are nests and roosting Lesser Noddy. The noise produced by chattering birds; the acrid smell of centuries of accumulated guano underneath the domes; the incredible site of thousands of birds in a confined area, some inquisitive enough to hover very close by like huge dragonflies as they inspect you, all taking place in amongst storm-smashed and broken trees is one of the unforgettable experiences of the Chagos Archipelago.

Pisonia has evolved with seabirds and is reliant upon them for seed dispersal. It does this by having sticky seeds that when a breeding or roosting seabird brushes against them the seed sticks to the feathers of the bird and eventually the stickiness wears out and the seed drops of in a new location to germinate. On this visit I counted 53 Lesser Noddy under the nesting glades totally covered in seeds and adjoining twigs. These were doomed and it was only a matter of time before the land and hermit crabs moved in to devour them. Whilst I had heard and read of bird mortalities associated with sticky Pisonia seeds, I had never witnessed it and the numbers involved in this event appeared high (actually ˃ 0.1% of the island breeding population). It is a natural phenomenon and the seemingly high numbers are likely due to large numbers of breeding birds coinciding with a bumper crop of seeds.



The penultimate day in Peros Banhos was spent surveying the rat-free and near natural Grande and Petite Coqillages. Both of these islands are IBAs on account of breeding Sooty Terns but as elsewhere in this atoll, the terns were not having a breeding episode during this visit. It was still an absolute pleasure to circumnavigate the island counting the breeding seabirds; the speciality on Grande Coquillage being breeding Great Frigatebird. To see the males displaying on their flimsy twig nests is another Chagos treat; they raise their huge wings out to their sides and fan-wave them, inflate their crimson gular (throat) sacs to balloon like proportions and then give out a wonderful “Red Indian war cry” to try and attract a female.
The final day in Peros Banhos was spent on rat-infested Yēyē. The total lack of breeding Red-footed Booby on this island is something of a mystery to me. From previous research in the Chagos it has been demonstrated that Red-foots preferentially breed on islands without rats, though, on islands that have stands of mature climax forest trees on the coast, particularly headlands, they will nest. The surveys of the previous few days where birds were found colonising rat-infested islands in western Peros Banhos being examples of this. Yēyē is a quite large for a Chagos island (c. 60 ha.) and its shoreline holds several stands of mature Takamaka Calophyllum inophyllum and Guettarda speciosa, both readily nested in by these arboreal booby. It has windy headlands and sheltered bays, again, both favoured breeding haunts if the right trees are present. So, why, on this island nestled in amongst six IBAs that all hold breeding populations of boobies, is there not a single pair? There are several possible solutions. A super-abundance of rats? This is thought unlikely and, the breeding stronghold of Diego Garcia has a rat population assessed to be as high as on any island in the world. An undiscovered relict population of cats left over from then this island held a very small human settlement? Again unlikely, Diego Garcia still has a tiny population of cats and these do not appear to impact the breeding boobies to any great extent. The most likely theory is human disturbance. Research towards my Masters showed that islands with little or no disturbance such as Nelson’s and Danger Island are preferred for breeding sites over islands that have regular human disturbance around the breeding colonies. Yēyē has had poaching camps sited on it over the last decade and it may be that boobies were killed for food by poachers and this is the reason they have not colonised this island.

As we left Yēyē heading back to the mother-ship we stopped to take a brief visit to a tiny unnamed island some 150m of Yēyē’s shores. I first landed on this island in 2008 and it had certainly grown over the ensuing seven years. There was evidence that as in 2008, Black-naped Terns breed on the coral rubble that forms the island and the spit running northwest out from it. This species had been joined by Crested Tern as a breeding bird. Three Madagascar Fody, an introduced species that has colonised the entire archipelago, were foraging on the Scaveola that was emerging out of the substrate. But, it was the amount of washed up plastic that has given me my lasting memory of this island. The inner ring enclosed by the coral rubble resembled a commercial rubbish tip. This was a sad reminder that even these truly remote oceanic islands cannot escape the nightmares man is inflicting upon the planet. After an uneventful sail from Peros Banos in calm waters and very light winds we anchored in the circular lagoon of the Salomon Islands as the sun was spectacularly setting.


2015 Darwin Science Expedition