Seawatch SW: Project Shearwater – spring 2011 (part 2)

In Part 1 of this report I described the first week of our 2011 field expedition to tag and track the Critically Endangered Balearic Shearwater at its breeding grounds on Mallorca. One of the biggest surprises was the discovery of a breeding Yelkouan-type Shearwater in the large Balearic Shearwater cave colony at Sa Cella. Part 2 covers our second week of fieldwork, when some of the team continue working at Sa Cella to recover GPS devices deployed the previous week, while I head over to Menorca with one of my PhD students, Lavinia Suberg, to search for Yelkouan-type 'Menorcan' Shearwaters at the cliff colony of La Mola.

29th March 2011

Lavinia and I get up early for our flight over to Menorca, and by mid-afternoon we arrive at the Jaume Ferrer biological field station at La Mola that will be our base for the next few days. The aim is to scout out accessible nests in the colony in readiness for Tim to come over and deploy the geolocator devices. However, the high levels of predation by cats and rats at this colony means that most nests are deep inside vertical shafts and well beyond our grasp. Last year we only managed to get hold of two birds during a full day of work, while a scouting mission in November 2010 was hampered by heavy rainfall and unstable cliff conditions, with few additional nests identified. We're hoping to have better luck this time, as we're very keen to find out where these pale 'Yelkouan-type' birds go outside the breeding season!

After dumping our stuff and meeting the friendly staff at the field station we head straight into the shearwater colony, accompanied by two Booted Eagles soaring overhead. On reaching the cliff we are stunned to see that, since our visit in November, a large rockfall has occurred that has breached the outer wall of the fort. This makes the traverse along the cliff base even more treacherous than usual and we pick our way carefully across, testing every boulder as we go.

Part of the rockfall along the outer wall of the fort of La Mola; the displaced boulders might eventually become new nesting habitat for Menorcan Shearwaters! (photo: Russell Wynn).

The first task is to check the 'tunnel cave' where we deployed a single geolocator on a Menorcan Shearwater last year. The tunnel cave basically consists of a horizontal tube that extends into the cliff for a few tens of metres, and is only just large enough for a person to crawl through. It is normally Ana who works in this cave, as she is the smallest and most adept in tight spaces. However, this time it is my responsibility, which is somewhat daunting as I'm over six feet tall and not particularly bendy! The first few metres are reasonably comfortable, and I manage to slide along on my belly fairly quickly, brushing aside cobwebs as I go. Things get more interesting when I reach the inner cave, which is quite a squeeze but does contain four birds, two of which are on eggs. Unfortunately, neither bird has a geolocator or a metal ring, so I return them to their nests and retreat.

Russ exiting the 'tunnel cave' at La Mola (photo: Lavinia Suberg).

After dusting myself down we head over to the 'wire burrow', which was the other nest where we deployed a geolocator last year. A shearwater can be seen inside sitting on an egg, and after some careful manipulation I manage to get a good view of the bird's legs, which are unringed. I therefore retreat without handling the bird and move across to the 'fig tree cave', which consists of a series of burrows underneath a cluster of huge limestone boulders that have detached from the cliff. A further three accessible birds are identified, together with another in 'pigeon poo cave', so called because of the impressive tower of guano underneath a pigeon roost! As evening approaches we return to the field station for dinner, having located about ten birds, up to seven of which are accessible. A quick call to the guys on Mallorca brings good news: two of the birds carrying GPS devices have already returned to Sa Cella and the data have provided good tracks of their foraging trips. We also hear that the 'Yelkouan-type Shearwater' that we found on 25 March is paired with a pale Balearic Shearwater!

At dusk Lavinia and I return to the colony in order to check out a good-sized cave that we discovered in November. The aim is to be in place before the incoming birds arrive, so we can assess which burrows inside the cave are occupied. As darkness falls we sit at the entrance enjoying the first calls of Scopoli's Shearwaters, but only when it is fully dark do we hear the first Menorcan Shearwaters gathering offshore. This is our cue and we swiftly take up our positions tucked away at the back of the cave. Within a few minutes the first bird comes crashing through the cave entrance, and in seconds it scuttles into a burrow and is noisily greeted by its mate. Another two presumed breeding birds follow the same procedure, in contrast to a presumed non-breeder that wanders aimlessly around the cave for several minutes.

Lavinia awaits the first returning shearwaters at La Mola as dusk approaches (photo: Russell Wynn).

After an hour or so we quietly exit the cave and check the rest of the colony for calling birds. By midnight we find 17 apparently occupied burrows and see several presumed non-breeders wandering around at the surface in the dark; the latter are worryingly approachable, and allow you to touch them before they scuttle away! This is bad news when there are cats in the vicinity and, in addition to the abundant rat droppings, we see plenty of fresh cat scats and one or two piles of shearwater feathers that are probably cat kills.

30th March 2011

In the morning we return to La Mola to continue our scouting mission. A reconnaissance camera deployed at the entrance to the 'tunnel cave' indicates that at least two rats entered the cave overnight, but only one Menorcan Shearwater is seen and we suspect that they are flying directly into the cave entrance above the laser trigger.

Infrared images from inside the tunnel cave at La Mola. The upper image shows the sinister form of a rat entering the cave, while the lower images shows a Menorcan Shearwater leaving the cave just 45 minutes later. Note the clean white underparts of the shearwater (photo: Russell Wynn).

As well as rats and cats, the shearwaters in the tunnel cave also regularly fall victim to the resident Peregrine, and as we move along the cliff base we see and hear the male screeching overhead. Further bad news comes when we check the 'wire burrow' nest; it is unoccupied and I fear that the bird may have abandoned. It certainly appeared skittish the previous day, and it is possible that it was an inexperienced bird or had previously been the victim of an attempted attacked by cats or rats. I keep my fingers crossed that it will return tonight.

In the afternoon we check another large cave in the east of the colony, but there are no easily accessible nests. However, the grounds of the fort at La Mola are alive with birds, and as well as the resident Booted Eagles we see Audouin's Gull, Stone Curlew, Hoopoe, Zitting Cisticola and Subalpine Warbler.

Shearwater caves at La Mola are often quite wet and muddy! (photo: Lavinia Suberg).

Tim arrives in the evening and we return to the colony for further nocturnal monitoring. I see eight birds enter the tunnel cave in the first hour of darkness, and Tim sees a remarkable ten birds enter the small cave we had occupied the previous night. We also find several new apparently occupied burrows, and overall we think we can map just over 50 territories in the colony. Our walk back in the dark is punctuated by the haunting calls of Stone Curlews and Scops Owls, which seem appropriate in the eerie atmosphere of the deserted fortress.

31st March 2011

We arrive at the colony mid-morning, pumped up and with a set of geolocators ready to go. Our first mission is to try to safely extract the two birds on eggs that I had previously located in the 'tunnel cave'. This is not going to be a straightforward operation, and Tim and I spend some time discussing the best way to undertake this task. Tim eventually crawls into the cave with me following close behind, while Lavinia waits at the entrance. Once we have wriggled our way into the vicinity of the first occupied nest, we turn our headlamps off and Tim edges forward in the darkness. He manages to get hold of the bird and safely takes it off the egg, but then has to get the kicking and pecking bundle safely secured inside a cloth bag while lying on his back. Once done, Tim hands the bird to me and I wait quietly while he moves on to the second bird at the back of the cave. After a few minutes I see his red light go off and all goes quiet. Lying in the darkness I can hear the shearwater clutched to my chest breathing steadily. Suddenly there is a minor commotion at the far end of the cave followed by the sound of Tim retreating towards me. Good news — he has the second bird secured! We both squeeze our way out of the cave grunting and swearing; it's much harder to wriggle forwards when one arm is carefully cradling a Critically Endangered seabird!

Russ and Tim exit the 'tunnel cave' at La Mola, each carefully carrying a bagged Menorcan Shearwater. Tim is a highly experienced expedition caver, but the added tension of handling endangered birds in such a tight space is etched onto his face (photo: Lavinia Suberg).

Once we're out of the cave we work quickly in order to minimise disturbance to the birds. The geolocators are attached and the birds are weighed and photographed. Tim and I then return them to their nests as quickly as we can, with the whole process completed in about 30 minutes. We also attach new metal rings to two presumed non-breeders that we extract from the middle of the cave but which are not sitting on eggs.

Menorcan Shearwater at La Mola, successfully tagged with a geolocator. Let's hope this one makes it back next year (photo: Russell Wynn).

We next move on to the 'fig tree cave' and are disappointed to only extract one bird for tagging, when we found three accessible birds on our scouting trip (the other two were presumably visiting non-breeders). Things get more complex when I check the vacated 'wire burrow' and find there is now a bird sitting on the egg again! Although I'm relieved that the burrow has been reoccupied, I'm aware that the bird inside may be carrying a geolocator from last year and we will therefore need to get hold of it. After much deliberation, I decide to first look inside and try and manipulate the bird in order to see its legs, without actually extracting it. This procedure is partially successful, as I can see a metal ring on one leg but can't see the other leg due to the bird's position. The bird seen a couple of days ago was not ringed so this is clearly the partner, and we make the decision to extract it. After carefully easing the bird away from the egg I'm able to remove it and am disappointed to see it is not carrying a geolocator. However, we record the metal ring number, add a new geolocator and return it to the nest.

The final accessible nest is in 'pigeon poo cave', although this one is a real stretch! I have to strip down and squeeze my arm in as far as it can go to reach this bird. After a fair bit of effort I manage to ease it away from the egg and am able to remove it safely with one hand. Tim adds a geolocator and we return it to its burrow within 14 minutes of extraction. So overall we have added five new geolocators to the two we deployed in this colony in 2010. With seven birds now carrying devices I'm more confident that we will recapture one or two next spring, even with the high predation levels. In addition to finding out where these pale Menorcan Shearwaters go, I have also used the opportunity of handling the birds to take photos; these will contribute to a long-planned paper on 'Yelkouan-type Shearwaters' that will probably now be delayed until next spring when we (hopefully) will have recovered some vital geolocator data.

On the trek back to the field station we stumble across a tortoise in the grounds of the fort, and stop to take some photos of this fantastic creature. In the evening we catch a return flight back to Mallorca and get prepared for our final couple of days working at Sa Cella.

Tortoise at La Mola: a nice surprise! (photo: Russell Wynn).

1st–3rd April 2011

The final two days of fieldwork see us continue our work in Sa Cella, deploying and recovering GPS devices. Maite Louzao, who is representing our colleagues in the EU Future of Atlantic Marine Environment project, has joined us for these two days. The FAME project team will be working on Eivissa for the first two weeks of April with the aim of deploying up to 40 geolocators on Balearic Shearwaters, so it is important to discuss methods and exchange best practice. It will certainly be interesting to see whether their results indicate that birds breeding on different islands have different migration routes and strategies.

Removing a GPS device from a bird at Sa Cella that has just returned from a foraging trip (photo: Russell Wynn).

Careful removal of the device ensures limited impact on the bird, with just a few small feathers on the back being removed or cut (photo: Russell Wynn).

On 3rd April we fly back home to the UK, well pleased with our initial results but still worrying about the birds with GPS devices that have not yet returned. Over our two spring expeditions we have deployed about 50 geolocators on Mallorca and Menorca, with at least 20 individual GPS deployments. We are already looking forward to spring 2012, where we plan to recover another year of geolocator data that will hopefully include some of those enigmatic Menorcan Shearwaters!

The project team in the field at Sa Dragonera (photo: Russell Wynn).

Postscript

Miguel and Ana continued to visit the cave regularly after our departure, and recovered all but two of the deployed GPS devices. The two missing devices were attached to birds that abandoned their nesting attempts after their eggs were damaged, probably due to scuffles breaking out in that part of the colony — Miguel has commented that each year a small number of eggs get broken in the colony through natural causes. The GPS tape attachment is designed to degrade after a few weeks allowing the device to fall off, so hopefully the birds will not suffer any long-term effects.

 

Some of the geolocator data that we have recovered this year will contribute to a scientific paper that we plan to submit in autumn 2011. The results will also be presented at the Seabird Group annual conference in Plymouth in September and the MEDMARAVIS conference in Sardinia in October. Detailed analysis of all of our tracking data will then be undertaken by a recently recruited PhD student, Rhiannon Meier, who has won a lucrative three-year NERC studentship (sponsored by Total Foundation) based at the National Oceanography Centre in collaboration with Tim at Oxford, Steve Votier at the University of Plymouth and Clive Trueman at the University of Southampton.

A final surprise came on 14th May, when a group of feeding Balearic Shearwaters off Cap Salou, Catalonia, was seen to include a bird carrying a geolocator. The observer, Marta Mas, managed to get some decent photos, confirming that it was one of our birds from Sa Cella! Several of our GPS-tagged birds had been feeding in this area during the incubation period, but nevertheless it was a surprise to see one photographed in the field!

Balearic Shearwater plunge-diving off Cap Salou, Catalonia; note the orange darvic ring and geolocator! (photo: Marta Mas).

Acknowledgements

We are grateful to Joan Mayol Serra and the Conservation Department of the Balearic Islands Government for providing the necessary permits for our fieldwork; we fully support them in their efforts to eradicate rats from Sa Dragonera. We thank Joan Moranta Mesquida and the staff at Jaume Ferrer biological field station at La Mola for providing accommodation on Menorca, and Jordi Mayol and the staff at Sa Dragonera Natural Park for again providing accommodation on Mallorca. British Antarctic Survey supplied the geolocators that are so essential to the project. Microsoft Research is financially supporting some aspects of Tim's seabird tracking work, while SeaWatch SW is financially supported by Total Foundation, RSPB, BTO, SAHFOS, RNBWS, Seabird Group, BirdGuides, and Marine Information Ltd.