Seawatch SW: Project Shearwater – spring 2011 (part 1)

In spring 2010, I was part of a small team of British and Mallorcan researchers that spent ten days working in the largest known cave colony of the Balearic Shearwater, at Sa Cella on northwest Mallorca. The objective was to deploy miniature geolocator devices on this Critically Endangered species for the first time, in the hope that the data would provide important clues as to the birds' distribution and behaviour throughout the whole year. Conservation of the Balearic Shearwater is certainly a pan-European issue; for example, over 5,000 birds (up to 25% of the estimated world population) were present off northwest Brittany in July 2010, and about 1% of the world population was seen in a single day passing the SeaWatch SW watchpoint at Gwennap Head (Cornwall) in September 2010. Our new tracking data would hopefully provide more information about the birds' favoured foraging areas and flyways along the west European coast, and therefore contribute to future designation of Important Bird Areas and other conservation plans. Accounts of the successful 2010 expedition can be found here and here.

Balearic Shearwater at Sa Cella tagged with a geolocator device in spring 2010 (photo: Russell Wynn).

This article outlines the results of our follow-up expedition in spring 2011, when we returned to the Balearic Islands to (a) retrieve the geolocator devices deployed at Sa Cella in spring 2010, (b) deploy GPS tags at Sa Cella to determine favoured flyways and foraging areas during the incubation phase; and (c) deploy new geolocators on 'Yelkouan-type' Shearwaters at La Mola on Menorca, to investigate whether they disperse into northeast Atlantic waters after the breeding season. The team members were Russell Wynn, Alice Jones, Lavinia Suberg and Phil Collins (National Oceanography Centre), Tim Guilford (University of Oxford), Lou Maurice (British Geological Survey) and Miguel McMinn and Ana Rodriguez (Skua SLP, Mallorca).

Initial visits to the cave colony at Sa Cella by Miguel and Ana in late autumn 2010 produced some encouraging results, with deployment of an infrared reconnaissance camera yielding great images of several returning shearwaters carrying our geolocator devices. So on 20th March 2011 we flew out to Palma full of optimism, but also very aware that we needed to ensure our work was not impacting this vulnerable species. It was therefore an excited and anxious group of researchers that arrived on Mallorca that evening, and we made sure we got a good night's sleep in preparation for the adventures ahead!

Infrared image taken inside the cave at Sa Cella on 29th October 2010; a geolocator is clearly visible on the rear bird, while the bird in the foreground has a metal ring (photo: Miguel McMinn).

21st March 2011

After buying food supplies we transfer to the island of Sa Dragonera (our base for the expedition) and get all our gear organised before heading to Sa Cella mid-afternoon. On the crossing we immediately see one of the local Ospreys together with a few Scopoli's and Balearic Shearwaters. Our first boat-to-shore transfer goes smoothly despite the swell, although Miguel has to stay in the RIB and is nearly hit by a rock dislodged from the cliff by a goat!

In the cave we start work on the accessible nests closest to the entrance, and within four hours have recovered four geolocators. A great start! All the birds are in good condition with no obvious wear on the legs or the geolocators, and all four devices provide good data; this procedure involves direct connection of a laptop to the geolocator and takes about 15 minutes, during which time Alice and Lou have to keep the bird entirely still. The birds are then weighed and returned to the nest, with the geolocators still attached and reset in readiness to gather another year of valuable data. The whole process taking about 30 minutes, and once returned to the nest the birds invariably settle down and resume incubating immediately. Everything goes well until one of the birds unloads all over Alice, Lou and the nearby laptop, which leads to a frantic few seconds locating tissues and preventing the sloppy mess penetrating into the keyboard!

Lou and Alice hold on to a tagged Balearic Shearwater as the geolocator data are downloaded, while Tim frantically deals with the aftermath of an unloading event! (photo: Russell Wynn).

At dusk we return to Sa Dragonera for a hearty meal of spaghetti bolognese washed down by red wine, before eagerly gathering around the laptop for the first data analysis. The geolocators provide a whole year of coarse-scale location data (accurate to within about 150–200 km) as well as saltwater immersion data that provide indications of the birds' behaviour every ten minutes. Initial results from the first bird indicate that it left the Balearic Islands in late May 2010 before summering in the northeast Atlantic and returning to the cave sporadically from mid-October onwards (further details will be provided once the data have been fully processed and published). So, for the first time, we had successfully tracked a Balearic Shearwater throughout the whole year — a good excuse for a celebratory nightcap!

The project team discussing initial results of the first ever year-round tracking of a Balearic Shearwater (photo: Russell Wynn).

22nd March 2011

We wake up to a bright, sunny morning, with two Ospreys fishing offshore and Serins, Sardinian Warblers and Blackcaps hopping around the bushes. After breakfast we head over to Sa Cella in an increasing wind and, after transferring us to the cave, Miguel has to retreat with the RIB to more sheltered waters. We soon get to work in the cave and recover another three birds with geolocators. However, the sea state continues to worsen, prompting a discussion about strategy. We all agree to stay overnight in the cave for one or two nights so we can continue recovering birds. Miguel therefore returns with food supplies before he and Ana wave goodbye and return to Sa Dragonera; we are now alone in the cave with no way out until they return!

In the afternoon we recover a further seven birds with geolocators, all of which are in good condition and carrying intact devices that download good data. So we have already recovered geolocators from 14 nests out of a total of 17; we just have to get birds from three more nests and then we have to wait for changeovers so we can recover devices from the partner birds. Most birds are on the same nests as last year, but a couple have moved a metre or so to an adjacent nest; the reasons for this will only become clear in the coming days.

Tim emerges from the cave carefully carrying a tagged Balearic Shearwater (photo: Russell Wynn).

At the end of the day we have the remains of the spaghetti bolognese and settle down for our first night in the cave. We set up camp in an elevated area adjacent to the colony but away from any nests and shearwater 'highways'. At dusk we sit at the cave entrance watching Balearic and Scopoli's Shearwaters flitting by and phosphorescence glistening in the surf. Once it is fully dark, we retreat to our sleeping bags accompanied by a cacophony of calling shearwaters, amplified by the sound of waves crashing against the shore. It's great to hear so many different calls, including a sporadic deep 'guk guk guk guk' nearby that continues for long periods and is often very loud. Miguel later informs me that this call is made by copulating Balearic Shearwaters, so I was obviously sleeping next to a busy couple!

Tim and Lou prepare for a night in the cave (photo: Russell Wynn).

23rd March 2011

We have breakfast of tea and muffins at the cave mouth while watching Audouin's Gulls, Crag Martins and a pair of Ospreys overhead. I managed to get some sleep overnight, despite being paranoid about rolling over a roaming shearwater. I was certainly more fortunate than Lou, who late in the night was rudely awakened by a bird that scuttled over her head and another one that tried to get into her sleeping bag!

After checking all the nests in the cave we find that three contain birds that have changed over in the night, meaning that the partner birds are now incubating. We successfully recover the geolocator data from all three, before preparing a series of back-mounted GPS tags for deployment. These battery-powered devices are set to obtain an accurate positional fix from satellites once every hour, and are designed to stay on the bird during a single foraging trip (which equates to a few days). The results will provide insights into favoured foraging areas and flyways during the incubation period, and will build upon previous studies that have indicated that large numbers of Balearic Shearwaters forage off the Catalonian coast during the breeding season. A total of four GPS devices are deployed on previously untagged birds, accompanied by geolocators so that saltwater immersion data can also be collected during the foraging trips. The total weight of the two devices and attachments is about 19 g, approximately 4% of body weight.

GPS device mounted on the back of a Balearic Shearwater at Sa Cella; this will provide fine-scale location data during a single foraging trip (photo: Russell Wynn).

Miniature geolocator device attached to the leg of a GPS-tagged Balearic Shearwater. The geolocators have a saltwater immersion sensor, which, when combined with the GPS locations, will provide information on the bird's activity in different areas (photo: Russell Wynn).

In the evening we hear an approaching RIB and see Miguel coming to check we are all OK. It is still too rough for us to board the RIB, so we opt to stay in the cave for a second night. At dusk we hungrily devour the last of the spaghetti bolognese, while an Osprey calls from its perch above us.

24th March 2011

The previous night seemed relatively quiet in the cave compared to the one before, and this is borne out when we discover that none of the nests containing geolocator-tagged birds have seen any changeovers. However, one of the new GPS-tagged birds has left, while the other three are still incubating on their nests. We therefore decide to deploy a further six GPS devices on new birds. While exploring the cave entrance I stumble across an incubating Balearic Shearwater in an unusually exposed location, within a few metres of the cave opening. This is presumably an inexperienced bird, as in strong northerly winds the waves can wash over this area. I take a photo, wish it good weather for the coming weeks, and leave it in peace. By mid-afternoon the weather has improved sufficiently for Miguel and Ana to rejoin us in the cave, before we all return to Sa Dragonera at dusk for pizza and beer.

Incubating Balearic Shearwater at Sa Cella; this bird could be photographed without use of a flash as it was unusually close to the cave entrance (photo: Russell Wynn).

25th March 2011

A very nice morning sees us heading over to Sa Cella early on, where we deploy four new GPS devices and recover a total of seven birds carrying geolocators. The improvement in the weather has evidently triggered a lot of changeovers in the cave! In total we have now recovered 24 geolocators, all of which yielded good data with no obvious impacts to the bird. One additional bird that was tagged with a geolocator in 2010 had returned with no device attached, but there was no sign of damage to the leg and the bird was in good condition so it may have just been a faulty attachment.

The project team in action in the cave at Sa Cella. Lou and Tim are holding a shearwater as geolocator data are being downloaded, while Tim prepares to install the reconnaissance camera (photo: Russell Wynn).

The biggest surprise of the day comes when Miguel emerges from the inner cave clutching a very pale shearwater. My first impression when it is taken out of its bag is that it is a Yelkouan Shearwater, and it certainly looks very clean white underneath and is markedly small in the hand. It is the first time that Miguel has seen a bird looking like this in Sa Cella, and its small size is supported by its weight of 390 g, about 50 g smaller than any of the Balearic Shearwaters we have handled. The small size of the bird means we are reluctant to deploy a back-mounted GPS tag, so we decide to return it to the nest and come back tomorrow in order to take some photos and attach a geolocator. A further surprise comes when one of the birds sheds a small parasite, probably a louse, which is retained for further analysis!

This nasty-looking beast crawled off a tagged Balearic Shearwater! (photo: Russell Wynn).

26th March 2011

In the morning we accompany Miguel and Ana to the western end of Sa Dragonera to assist with monitoring of rats. During the previous winter they have been working with the Balearic Islands government on a rat-eradication programme on the island, and are now involved in monitoring its success. We had certainly not seen any rats during our first few days on the island, in marked contrast to the previous spring when we were amazed by the sheer numbers that appeared once it got dark. However, to ensure that the rats are really gone, a series of transects with baited ink traps are set in different parts of the island. The ink traps record the footprints of any animal walking over them, and happily only showed evidence for lots of lizards on the transect we checked.

Ana checking an ink trap baited with peanut butter. The only footprints were those of lizards, with no evidence of rats (photo: Russell Wynn).

After lunch we head back to Sa Cella and have another look at the 'Yelkouan-type' Shearwater that was discovered the previous day. The metal ring it is carrying indicates that it was ringed as an adult Balearic Shearwater at Sa Cella in 1993, so it is at least 20 years old! Morphometrics and blood samples are taken to aid with racial identification, and I obtain a series of photos once we have attached a geolocator.

'Yelkouan-type' Shearwater at Sa Cella. Note the pure white underparts that extend back to the undertail coverts. The poor quality and red tone of the photo is because it was taken in near darkness at 6400 ISO with red light illumination; use of a camera flash and white light is avoided when handling birds in Sa Cella to prevent unnecessary disturbance (photo: Russell Wynn).

We also recovered the images from our reconnaissance camera, deployed the previous evening near the main shearwater 'highway' just inside the cave entrance. The camera takes a burst of five shots every time a movement is detected, and it was evidently a busy night with over 3700 images to sift through! The incoming birds started arriving at 19:00 and the last bird left the cave at 05:00. Most of the breeding birds (including some carrying our geolocators) rapidly move in and out of the cave, but small numbers of presumed non-breeding birds, and a few Scopoli's Shearwaters, are seen lingering near the cave entrance and indulging in mutual preening and occasional squabbling!

Infrared camera image from inside the cave at Sa Cella, showing a pair of preening Scopoli's Shearwaters in the foreground and an inquisitive Balearic Shearwater behind (photo: Russell Wynn).

27th March 2011

After a morning session at Sa Cella where we recover another two geolocators (one of which had malfunctioned) we return to Sa Dragonera for lunch. While munching on sandwiches I'm casually scanning the skies above when I see a flock of 18 Griffon Vultures soaring overhead! These birds had wandered over from the Mallorcan mainland, and Miguel thinks they might be a group that was recently blown across from Iberia and are now essentially 'trapped' on the archipelago. They certainly seem to be looking out to sea to the northwest, but after a while they drift back over the water to the main island. In the evening we see two Bottlenose Dolphins offshore, but rather more surprising is a Great Skua moving west, harassing a rafting group of Scopoli's Shearwaters as it passes.

Griffon Vultures over Sa Dragonera, part of a flock of 18 (photo: Russell Wynn).

28th March 2011

Strong winds overnight meant that there was a lack of changeover activity in the cave at Sa Cella, with our reconnaissance camera only generating about 470 images. The trip across to the cave is most memorable for the boat transfer. Getting into the cave isn't too bad, but an increasing northerly swell means that when we come to leave a couple of hours later things are looking rather more interesting! In the end we all manage to scramble or leap into the RIB safely, but not before some us get absolutely soaked by large waves crashing over the cave entrance!

Russ gets ready for one of the more interesting RIB transfers! (photo: Tim Guilford).

Back on Sa Dragonera, we get another visit by the vultures, but this time there are 37 Griffon Vultures and they are accompanied by five Black Vultures, at least 25 Booted Eagles, 18 Ravens, two Peregrines and a Red Kite! A brief respite in the schedule then allows us some time for paperwork and a much-needed siesta. In the evening we do a quick search of the cove adjacent to our lodgings, which produces one incubating Balearic Shearwater and indications of at least one other occupied burrow. This find is highly encouraging as it is apparently the first incubating shearwater on this part of the island for several years, and may be an indication that the birds are responding positively to the rat eradication programme.

An incubating Balearic Shearwater in its burrow on Sa Dragonera. This bird is in a relatively exposed location, and it is the first occupied nest found in this area since the rat eradication programme was carried out (photo: Russell Wynn).

At the end of our first week we have accounted for 29 of the 34 birds tagged with geolocators at Sa Cella in spring 2010. Of these, 27 have yielded good data, with one geolocator malfunctioning and one having fallen off. Overall our bird-return rate is 88%, which is actually higher than the estimated adult survival rate. Given that the five birds not accounted for may have simply skipped breeding or moved to another unchecked nest, the actual survival rate may well be higher. In addition, Miguel and Ana compared the productivity of nests containing tagged and untagged birds during the 2010 breeding season, and found no significant difference in productivity. So overall, we are extremely happy and relieved that our geolocator work is not impacting the birds to any significant degree.

The second part of this report will focus on our subsequent week of fieldwork, including more results from Sa Cella and a trip to Menorca to tag Yelkouan-type 'Menorcan' Shearwaters.

Miguel, Ana and Alice in action on Sa Dragonera (photo: Russell Wynn).