Seawatch SW: 2010 field expedition - Part 1


The project team working in a Balearic Shearwater cave colony on Mallorca (photo: Russell Wynn)


The SeaWatch SW project (www.seawatch-sw.org) has collected a huge amount of data on the Critically Endangered Balearic Shearwater in UK/Irish waters since 2007. Almost 1000 hours of effort-based observations have been carried out every autumn at the prime watchpoint of Gwennap Head in southwest Cornwall, supplemented by:

  • intensive observations from several sister sites
  • boat-based observations collected by our partners at Marinelife
  • over 1000 records per year from the wider sea-watching community

These data are providing important information on the fine-scale distribution of this vulnerable species in our waters, including the occurrence of internationally important aggregations (>100 birds).

Recent studies have indicated that low at-sea survival rates are one of the major threats to the survival of the Balearic Shearwater, with increasing evidence to suggest that fisheries bycatch is a contributing factor (particularly in longlines). However, a lack of information on the birds' favoured foraging areas throughout the northeast Atlantic has hindered attempts to tackle this problem. Therefore, in 2009 I approached Prof Tim Guilford of Oxford University to set up a project to tag Balearic Shearwaters with the aim of tracking their movements throughout the year. Tim has recently been tracking Manx Shearwaters at Skomer Island in Pembrokeshire, and his methods have been proven to be reliable and highly successful (see BBC News article).

At the same time I approached Miguel McMinn and Ana Rodriguez on Mallorca, who run a small consultancy that undertakes conservation work for the Balearic Islands government (www.skuasl.com). Miguel and Ana have worked on Balearic Shearwaters for many years and have a wealth of knowledge on the species' breeding sites and habits. With an experienced team taking shape, it was time to focus on the methods we would use in the field.

Tracking technology and methods

Recent technological advances have ensured that a wide variety of ultra-lightweight devices are now available for use in bird tracking studies. Our first aim was to attach lightweight geolocators (<1% of="" body="" weight)="" to="" the="" legs="" of="" balearic="" shearwaters="" during="" the="" breeding="" season.="" tim="" ordered="" a="" total="" of="" 40="" devices="" from="" the="" suppliers="" at="" british="" antarctic="" survey,="" the="" majority="" of="" which="" would="" be="" deployed="" on="" balearic="" shearwaters="" in="" a="" large="" cave="" in="" northwest="" mallorca="" (sa="" cella).="" a="" small="" number="" were="" also="" allocated="" for="" deployment="" on="" birds="" at="" a="" colony="" on="" menorca="" (la="" mola="" de="" mao);="" these="" 'menorcan'="" shearwaters="" are="" the="" subject="" of="" much="" taxonomic="" debate,="" genetically="" falling="" between="" balearic="" and="">Yelkouan Shearwater but having plumage, structure, vocalisation and breeding phenology more akin to Yelkouan Shearwater (I am personally very interested in where these Menorcan birds go outside the breeding season, given the recent upsurge in records of 'Yelkouan-type' shearwaters off the southwest UK).

Geolocators are attached to the bird's leg and have enough battery power to operate for several years. They collect information on the bird's location every day throughout the whole year, using day–night length ratios to derive an approximate geographical location. This method is accurate to within 150–200 km (although a sea-surface temperature sensor collects additional data that can be cross-referenced with satellite-based ocean temperature data, potentially enabling the spatial resolution to be improved). Detailed at-sea behavioural data, e.g. foraging, roosting and migrating, can also be extracted for every ten-minute period through analysis of salt-water immersion records. Geolocator results should provide invaluable information on foraging areas and migration patterns in Mediterranean and northeast Atlantic waters, aiding targeted conservation measures in the future.

The second aim was to attach a small number of trial GPS devices to the backs of Balearic Shearwaters at Sa Cella, so we could assess the birds' reaction to carrying devices in this manner. If the trials were successful, this would lead to deployment of a series of GPS devices in spring 2011, in order to obtain fine-scale movement patterns and foraging areas in the western Mediterranean during the breeding season. GPS devices have already been used successfully on British Manx Shearwaters at Skomer and Lundy Islands: www.ox.ac.uk/media/news_stories/2008/080222b.html.

Logistics and project team

The first field expedition was scheduled for early April 2010, so that we could deploy devices on birds during the incubation phase. Miguel advised that this would facilitate capture and handling of the birds, and would minimise disruption in the tightly packed cave colony at Sa Cella. This cave was the focus of our study, as prior monitoring undertaken by Miguel and Ana had established a series of marked nests, providing a robust monitoring framework for our tagging work. This was particularly important given our aim of catching a series of male–female pairs, to see if there were sexual differences in foraging areas and migration strategy.

With Miguel's assistance, we obtained a permit from the Conservation Department of the Balearic Islands Government allowing us to attach geolocators and trial GPS devices to Balearic Shearwaters on Mallorca and Menorca. We also began corresponding with colleagues in Spain and France, who are also planning to deploy geolocators on Balearic Shearwaters and other seabirds as part of an EU-funded project (FAME) to determine marine Important Bird Areas (IBAs) in northeast Atlantic waters. We hope to be able to invite their Balearic Shearwater researcher to join us on future expeditions, so that we can effectively share resources and results.

The spring 2010 expedition team comprised Russell Wynn and Alice Jones (SeaWatch SW and National Oceanography Centre, Southampton), Tim Guilford (Oxford University) and his partner Lou Maurice (British Geological Survey), and Miguel McMinn and Ana Rodriguez (Skua, Mallorca). Miguel and Ana kindly arranged our accommodation during the expedition, and also supplied their own Rigid Inflatable Boat (RIB) (essential for getting to the cave at Sa Cella). It should be noted that although the team receives funding for various aspects of seabird work, this expedition was essentially self-funded and was as much about voluntary conservation work as scientific research.

Day 1: 1st April 2010

The team arrives at Palma airport, and Miguel whisks us away to the small port of Sant Elm in the northeast corner of Mallorca. Ana soon arrives on their faithful 10-year old RIB and we quickly transfer to the spectacular island nature reserve of Sa Dragonera (Dragon Island), which would be our base for the next few days.

Sa Dragonera as viewed from the east (photo: Russell Wynn).

On the short journey across we get great views of a restless raft of about 100 Scopoli's Shearwaters, which are mostly breeding birds from a small island colony immediately offshore of Sant Elm. We also see several stunning Audouin's Gulls, and get our first views of Balearic Shearwaters on their home territory.

Scopoli's Shearwater, offshore of the Sant Elm colony. Note the metal ring on the right leg (photo: Russell Wynn).

Our lodgings on the island overlook a small, picturesque cove, whose crystal-clear waters are teeming with fish and other marine life. A short walk before dinner produces Blue Rock Thrush and numerous Sardinian Warblers, as well as more familiar species such as Song Thrush, Robin and Willow Warbler.

Male Blue Rock Thrush on Sa Dragonera (photo: Russell Wynn).

The dominant species is Yellow-legged Gull, with many thousands of raucous birds dotted amongst the rocks and scrub on breeding territories. These birds are in the initial stages of their breeding cycle, and it's rare to have a moment on the island (day or night) when you can't hear them noisily going about their business!

Adult Yellow-legged Gull on Sa Dragonera (photo: Russell Wynn).

As dusk falls we have a glass of wine on the terrace and are thrilled to hear our first calling Balearic Shearwaters, as well as a few distant Scopoli's Shearwaters. However, the idyllic scene is rather spoilt by the profusion of rats that appear soon after, including one jumping at the bars on the window catching moths attracted to the lights inside! Before I retire to bed I take a walk alone, just drinking in the haunting sounds of calling shearwaters in the darkness.

Day 2: 2nd April 2010

We wake soon after dawn to a clear blue sky and a light breeze. After walking out onto the terrace half-asleep I'm soon hurrying back inside to grab my binoculars; an Osprey is fishing in the cove just 100 metres away! Everyone is eager to get to work, so after breakfast we load all the gear into the RIB and head off to the cave at Sa Cella, passing a colony of raucous Audouin's Gulls on the way. We also see several Shags of the Mediterranean race desmarestii, including a few strikingly pale juvenile birds.

The project team in the RIB and ready to catch some shearwaters! (Photo: Russell Wynn)

Tim, Lou and Alice are dropped off at the cave mouth while Miguel, Ana and I recover the geolocators from a site a few hundred metres further east along the coast. The devices were placed in this location a few days earlier to enable them to calibrate with the local day–night cycle. A sound recording unit is also recovered to see if the calibration site has received any nocturnal visits from breeding shearwaters.

The cliff at Sa Cella, with the cave just visible at the base. The inaccessible location means that the only way in and out is by boat (photo: Russell Wynn).

Entering the cave at Sa Cella. This expedition would not have been suitable for anyone prone to seasickness, claustrophobia or vertigo! (Photo: Russell Wynn)

Once inside the cave we are confronted by the bizarre sight of stalactites and stalagmites, covered in Balearic Shearwater guano! The birds themselves are actually nesting in shallow scrapes and burrows right inside the cave, often in complete darkness several tens of metres from the entrance. We decide to set up camp in the outer cave area and bring birds out individually for processing to minimise disturbance.

Inside the cave at Sa Cella. Note small patches of Balearic Shearwater guano on the cave floor (photo: Russell Wynn).

Miguel and Ana soon recover the first incubating bird from a marked nest site and bring it to the outer cave area. Red light is used to minimise disturbance, and the bird is placed in a restraining sock. The first task is to record the metal ring number and obtain morphological data (tarsus length/width, bill depth, and bill, head, tail and wing length). The underbody colour phase is noted, on a scale of 0 (pure white, like Manx Shearwater) to 4 (sooty-brown, like Sooty Shearwater). Almost all birds at Ca Sella score 2 or 3. The bird is then weighed and blood samples are taken from the leg joint to enable confirmation of sexing.

Measuring wing length of a Balearic Shearwater at Sa Cella. Males average larger than females (photo: Russell Wynn).

Taking a blood sample from a Balearic Shearwater at Sa Cella, in order to confirm the sex of the bird (photo: Russell Wynn).

The next phase is attachment of the geolocator, which involves a red plastic darvic ring being attached to the left leg below the knee joint. The geolocator is then fixed to this ring using two cable ties and a drop of glue. Once the glue is dry a small dab of white paint is put on the bird's nape to indicate it is the first bird of a pair to be caught (this would aid the process of checking birds in the cave in subsequent days to see if they had changed over incubation duties). Throughout the whole process the bird is surprisingly docile, and is visibly relaxed by Miguel's sporadic preening of the head and neck!

Miguel 'preening' a Balearic Shearwater at Sa Cella (photo: Russell Wynn).

Once the paint is dry Miguel and Ana return the bird to the nest and set it down gently in close proximity to the egg; it rapidly returns to incubation once released. The whole procedure takes about 15 minutes (although the fact that incubating birds can leave the egg exposed for several hours in the relative warmth and shelter of the cave means that speed is not absolutely crucial). So our first bird is successfully tagged, and we collectively breathe a huge sigh of relief that everything has gone so smoothly!

Our first tagged Balearic Shearwater! Note the geolocator on the left leg, and Miguel's hand gently preening the bird's head! (Photo: Russell Wynn)

Close up view of the geolocator device, which weighs less than 1% of the bird's body weight and yet is able to collect detailed information for several years (photo: Russell Wynn).

A total of 15 birds are subsequently caught and tagged over a period of several hours, and the team soon gets into a good rhythm working within the peaceful confines of the cave. The presence of incubating birds nearby is occasionally advertised by a bout of calling, but otherwise the only sound is the noise of waves washing against the cave entrance. Once the work is completed we are treated to a spectacular trip back to the island, with a massive feeding frenzy of Yellow-legged Gulls, Audouin's Gulls and Scopoli's Shearwaters providing some great photo opportunities.

Yellow-legged and Audouin's Gulls feeding on baitfish off Sa Dragonera. (Photos: Russell Wynn)

An evening walk on the island produces a pale-phase Booted Eagle, Osprey, Serin and several Balearic Warblers (the local race of Marmora's Warbler). Before dinner we quickly check the sound recording unit, which reveals a few Balearic Shearwater calls.

Day 3: 3rd April 2010

A relaxed morning is spent setting up a series of four trial GPS devices. The previous day we had attached an inactive device to the back feathers of an incubating bird to see how it would respond. If everything appeared OK this would give us the go-ahead to install the four trial devices and see how the birds respond to the weight load during a complete foraging trip (which typically last several days before they change over incubation duties). In the warm sunshine we also take a few moments to admire the resident lizards, which are busily feeding on nectar and petals of rosemary plants!

The real dragons of Dragon Island! (Photo: Russell Wynn)

After lunch we return to the cave at Sa Cella and enjoy good views of a Crag Martin flying around the entrance. The first task is to check the bird with the dummy GPS device; fortunately it is happily incubating and the tag remains attached and intact.

Removal of the inactive GPS device. If done carefully the impact on the bird is minimal, with just the occasional feather being lost. (Photos: Russell Wynn)

After carefully removing the device we weigh the bird and find it has lost about 10 g overnight, which gives an indication of how much weight the birds need to put on each day during their foraging trips to remain in condition.

Miguel and Ana weighing a Balearic Shearwater in the cave (photo: Russell Wynn).

We then check the 15 birds with geolocators (marked with correction fluid on the nape) to see if any have been replaced by their partners overnight. In total we find four newly arrived birds and these are caught and geolocators attached (including one that was not metal-ringed and is therefore probably a new breeder in this colony). We add white marks to the backs of these 'new' birds to help identify them and provide more information on changeover rates.

Metal ring being attached to Balearic Shearwater at Sa Cella (photo: Russell Wynn).

Balearic Shearwater with correction fluid applied to its back; this bird had just arrived in the cave and had taken over incubating duties from its tagged mate (photo: Russell Wynn).

The next task is to see how incubating birds respond to the flashing blue light on the trial GPS tags; in the darkness of the cave these lights looked like a small army of approaching police cars! Fortunately none of the incubating birds show the slightest bit of interest in the lights, so we catch two birds and attach live GPS devices to the back using TESA marine cloth tape.

Tim and Lou attaching a trial GPS device to the back of a Balearic Shearwater at Sa Cella (photo: Russell Wynn).

Balearic Shearwater with trial GPS device attached (photo: Russell Wynn).

The first two devices are successfully attached, so we decide to collect the other two devices from Sa Dragonera and deploy those as well. Once this done we finally return to the island at dusk, and while eating dinner are surprised to hear a Barn Owl screech in the darkness.

Day 4: 4th April 2010

A strong north wind and rain showers means we are grounded for the day as the swell is too big to enter the cave; I therefore take the opportunity to do some birding on Sa Dragonera. A group of 100 Balearic Shearwaters are lingering off the sheltered south side of the island mid-morning. However, the most interesting sighting is a Scopoli's Shearwater spiralling up in the thermals with Yellow-legged Gulls at a height of at least 50 m! Migrants on the island include several Hoopoes and Subalpine Warblers, together with a good variety of commoner species. Both dark- and pale-phase Booted Eagles are recorded, with one of the latter being mobbed by a Peregrine. A couple of sparring Ospreys are also seen and photographed.

Ospreys sparring off Sa Dragonera (photo: Russell Wynn).

After lunch I go snorkelling in the cold waters of the cove and see a fantastic variety of fish and what appears to be a half-buried cannonball! In the evening Miguel and I go out on the RIB to look for Balearic Shearwaters. We again see the raft of Scopoli's Shearwaters off Sant Elm but only a small number of our target species.

Scopoli's Shearwater off Sa Dragonera (photo: Russell Wynn).

Scopoli's Shearwater off Sa Dragonera (photo: Russell Wynn).

Balearic Shearwater in flight, with two Scopoli's Shearwaters in the background (photo: Russell Wynn).

The trip ends with me getting a soaking in the cove, thanks to a misunderstanding with Miguel who was reversing the RIB while I was still hanging onto a mooring rope anchored to a rocky cliff!

Day 5: 5th April 2010

The weather has improved and it's a lovely sunny day with light winds. The Osprey is seen fishing successfully in the cove as we ease the RIB out of the harbour and head back to Sa Cella. The remnants of a big swell from the previous day means the transfer from RIB to rocky shore is entertaining!

Tim demonstrating perfect timing during a 'shore-to-ship' transfer at Sa Cella. Getting the timing wrong inevitably resulted in wet feet! (Photo: Russell Wynn)

Miguel and Ana collect a further ten calibrated geolocators from along the coast and we enter the cave to start work. A total of five birds have changed over and four of these are fitted with geolocators. The fifth bird is quite skittish so we leave it alone. We then select another two nests for geolocator tagging, giving us a target of 17 male–female pairs.

The team at work in the cave at Sa Cella (photo: Russell Wynn).

The four birds carrying GPS tags are checked last. One bird has departed and has been replaced by its partner, but the other three are still incubating and flashing away! It is noticeable that 'incoming' birds are quite heavy, presumably as they are full of fish to keep them sustained during incubation duties. The weights of the tagged birds are actually quite variable, ranging from 440g to 610g. The four unused recovered geolocators are returned to the calibration site prior to our return to Sa Dragonera.

Miguel weighing a Balearic Shearwater in the cave at Sa Cella (photo: Russell Wynn).

In the evening we embark on a short boat trip in search of Balearic Shearwater nests along the south coast of the island. Despite all of us clambering about amongst rocks and boulders we only find a single occupied burrow, so on our return to the lodge four of us decide to go on a nocturnal excursion searching for more nests in the relatively unexplored centre of the island. After hearing a few birds calling we split up and I strike out along a rugged karst ridge that climbs towards the spine of the island. Scrambling in the dark with just a headlamp illuminating the way gets the adrenalin flowing, but that's nothing compared to the experience of having the call of a Balearic Shearwater resonating from the rock beneath my feet!

I soon discover that a small number of birds are holed up inside small clefts in the limestone; how they get in and out of vertical shafts no more than a foot or so across is a mystery, but it's great sticking your head in a burrow while the bird is calling from a few inches away! In total I find five birds in three holes, and hear at least three other stationary birds in adjacent valleys. The other guys find another two calling birds in burrows near to the path, so in total we have discovered at least ten 'new' territories that haven't been recorded for at least a decade. It's easy to see why estimates of the known global breeding population (2000–2500 pairs) don't match the numbers being recorded at sea in the Western Mediterranean (20,000–30,000 individuals); working in such challenging terrain is difficult and time-consuming, and there could be many more pairs waiting to be discovered at inaccessible locations all around the Balearic Islands.

Day 6: 6th April 2010

After breakfast I return to the karst crag to photograph the nest sites located the previous evening, while Tim and Lou go climbing on the mainland. On the walk back I hear a Tree Pipit overhead and see three Crossbills.

The rocky ridge that I scrambled up the previous night when looking for Balearic Shearwater nest sites (photo: Russell Wynn).

Balearic Shearwater nest hole in karst limestone. Note the small size of the hole when compared to the camera lens cap alongside (photo: Russell Wynn).

After lunch I explore the northeast coast of the island and see three Booted Eagles, two Ospreys, Redstart, Black Redstart and good numbers of Willow Warbler. The Audouin's Gull colony provides some nice photo opportunities, and then it's back to the lodge for a nap before an evening trip out to the cave.

Audouin's Gulls at the colony on Sa Dragonera. (Photos: Russell Wynn)

As we arrive back at Sa Cella we pick up the remaining geolocators from the calibration site and enter the cave. The skittish bird from the previous day is now more relaxed and so we duly attach a geolocator, as well as a further four birds that have changed over. We have now attached geolocators to 13 male–female pairs, and have just four birds remaining to catch when they come in and change over.

Lou and Alice counting the rapidly dwindling stock of geolocators. Happy days! (Photo: Russell Wynn)

As dusk approaches we remain in the cave, and after an hour or so the first birds start to arrive at the entrance and calls from inside begin to increase in frequency. Near the cave entrance there is enough light to see birds as they land on the approach ramp and quickly flutter up into the cave. Some of the birds clamber over and around us on their route to their burrows, while a small number of presumed non-breeders loiter around the entrance. Despite the increasing cacophony of calls the most memorable noise is actually the metallic tinkling of metal rings making contact with the rocky cave floor; it's clear that Miguel and Ana have been busy in this cave!

Balearic Shearwater calls: Get the Flash Player to see this player.

As the Balearic Shearwater calls peak in intensity a single Scopoli's Shearwater joins the show, and we take this as our cue to leave the birds to it and retreat to the cave entrance. However, as I get up from my cramped sitting position and switch on my red headlight I'm surprised to find a Balearic Shearwater sitting quietly under my crooked knee! Miguel then arrives with the RIB and plucks us off the cliff in total darkness, which goes surprisingly smoothly until Lou dives in head first and ends up in a heap in the bottom of the boat (luckily she was wearing a caving helmet). The night ends with Miguel taking us around the Scopoli's Shearwater colony at Sant Elm islet to take in the sounds and sights, before we head back to the lodge for a well-earned nightcap.

Day 7: 7th April 2010

We're woken up early by the sound of an Osprey calling outside and are heading back to Sa Cella by mid-morning. In the cave we catch one 'changeover' bird making it 14 complete male–female pairs. Three of the GPS-tagged birds are still at the nest, with one now being accompanied by its partner.

Tim and Lou attach the 28th geolocator to a Balearic Shearwater at Sa Cella (photo: Russell Wynn).

After leaving the cave we tour the spectacular north coast of Sa Dragonera looking for potential Balearic Shearwater nest sites. Having Tim and Lou with us proves to be extremely useful as they are experienced climbers and well capable of scaling apparently inaccessible cliffs. They search two previously unexplored caves but find no Balearic Shearwaters, although they discover an old European Storm-petrel egg, which is apparently a very unusual record for the island.

Tim (marked with white arrow) goes searching for Balearic Shearwater nest sites on the spectacular north cliffs of Sa Dragonera. Don't try this at home! (Photo: Russell Wynn)

Tim watches on as Lou gets stuck into the search for Balearic Shearwater nest sites! (Photo: Russell Wynn)

We get great views of a perched Osprey on the cliff and a brief glimpse of a probable Eleonora's Falcon high overhead. We then return to the lodge and get all our gear packed for a transfer to Palma, where we will be bunking in Miguel and Ana's office for the remainder of the trip.

Osprey on the north cliffs of Sa Dragonera (photo: Russell Wynn).

Part 1 summary

The first part of the trip has gone extremely well, with 14 complete pairs of Balearic Shearwaters tagged with geolocator devices. However, in Part 2, the tension rises as we monitor the response of the four birds carrying trial GPS devices, and take a day trip to try and tag some elusive 'Menorcan' Shearwaters at a threatened colony at La Mola.

Written by: Russell Wynn, SeaWatch SW co-ordinator