Formentera is the smallest inhabited island of Spain’s Balearic Archipelago in the western Mediterranean, with a total surface of 83.2 km² and a highest point of 192 m.  Formentera lies seven kilometres off the southern tip of Ibiza (Eivissa in Catalan), separated by a shallow strait containing several small islands.  During the last Ice Age the strait was a land bridge connecting Formentera, Ibiza and the small islands, forming a single landmass.

La Mola de Formentera with Punta des Garrovet and Punta de la Xindria

The horizontal stratification of the limestone sea-cliff outcrops with hundreds of caves and crevices makes a habitat for the cave-breeding Balearic Shearwater

A Balearic Shearwater at its breeding site, photograph by Daniel Oro

The resident human population of Formentera is just over 10 000, although Ibiza and Formentera together received over 2.7 million tourists in 2014.  Formentera is a popular one-day destination for tourists that stay in Ibiza.  It also has its own tourist network of hotels and holiday resorts, some of them very near to seabird breeding sites.

The population of the ACAP-listed and Critically Endangered Balearic Shearwater Puffinus mauretanicus on Formentera and islands in the strait was estimated in 2003 as 692 pairs.  There are three colonies on the island of Formentera itself: la Mola (460 pairs), Cap Barbaria (110 pairs) and Punta Prima (50 pairs).  The shearwaters also breed on the nearby small islands of Espalmador (32 pairs) and Espardell (40 pairs). Scopoli’s Shearwaters Calonectris diomedea, Mediterranean Cormorants Phalacrocorax aristotelis desmarestii and Yellow-legged Gulls Larus michahellis also breed on the sea cliffs of Formentera and on the neighbouring islands.

Coves de l’Arena at la Mola

A quaternary sand dune covers the floor of the caves providing soft material for breeding

The Balearic Shearwater is the most abundant bird species found in archaeological sites on Formentara.  Subfossil shearwater remains can be found in nearly every cave or crevice along the island’s limestone coastline, and there is archaeological evidence from several sites that they were an important food source for the first human settlers, with exploitation continuing right up until the 1990s.  Early shearwater sites were easily accessible and likely both young and adult birds were taken, but as populations dwindled and birds became largely restricted to the most inaccessible ledges of the massive sea cliffs of la Mola and Cap Barbaria, only adult birds were captured as they entered the small caves and crevices along the ledges.

Large caves near cliff tops are easily accessible. A project to build a “disco-restaurant” in this cave was abandoned, leaving behind the bulldozer.  Balearic Shearwater bones litter the floor

Shearwater bones and Black Rat and feral cat footprints are common in all the large accessible caves in the sea cliffs of Formentera  

 Collecting Balearic Shearwaters was a dangerous activity, dangling down cliffs with woven ropes made of grass, and was limited to a few experienced families.  Bird carcasses were often exchanged for food and other household foods: the lighthouse keeper of la Mola used to exchange paraffin petroleum for shearwaters.

Stone walls were built to trap adult shearwaters by blocking up most of the entrance of inaccessible caves and crevices

Nooses were then used to snare birds

 Site of Balearic Shearwater colony on Espalmador, Natural Park of Ses Salines, off the north coast of Formentera

La Mola Lighthouse, a perfect place to listen to Balearic Shearwaters at night

 A “virotador “ (virot is the local Catalan name for a shearwater) helped discover the last breeding sites of the Balearic Shearwater on Formentera, but field work was discontinued and there is little current information on the island’s breeding population.

Exploitation of seabirds and the introduction of invasive predators such as Domestic Cats Felis catus (which turned feral) and Black Rats Rattus rattus have caused a great impact on the seabirds of Formentera.  Habitat destruction is an issue in some of the breeding areas, and a problem not yet evaluated is artificial pollution from new tourist facilities.

ACAP Latest News has previously covered the shearwaters of Sa Cella and the Cabrera Archipelago Maritime-Terrestrial National Park on Mallorca and in Menorca in the Balearic Islands.

Selected Literature:

Alcover, J.A. 1989.  Les Aus Marines Fóssils de les Pitiüses en el Context de la Mediterrània. In: López-Jurado, C. (Ed.). Palma de Mallorca, Grup Balear d'Ornitologia i Defensa de la Naturalesa. Aves Marinas. Actas de la IV Reunión dek Grupo Ibérico de Aves Marinas, Sant Francesc Xavier de Formentera 29 de Octubre al 1 de Noviembre de 1988.  pp. 33-43.

Alcover, J.A., Florit, F., Mourer-Chauviré, C. & Weesie, P.D.M. 1992.  The avifaunas of the isolated Mediterranean islands during the Middle and Late Pleistocene.  Papers in Avian Paleontology 36: 273-283.

Alcover, J.A., McMinn, M. & Altaba, C.R. 1994.  Eivissa: a Pleistocene oceanic-like island in the Mediterranean. National Geographic Research & Exploration 10: 236-238.

Arcos, J.M. 2011.  International Species Action Plan for the Balearic Shearwater Puffinus mauretanicus.  Madrid: SEO/BirdLIfe & BirdLife International.  51 pp.

García, D. & Arbona, P. 2001.  Nueva localidad de nidificación de la Pardela Balear Puffinus mauretanicus en el islote de s'Espalmador. Anuari Ornitològic de les Balears 16: 69-70.

Mayol, J. 1986.  Human impact on seabirds in the Balearic Islands.  In: Monbailliu, X.G. (Ed.).  Mediterranean Marine Avifauna.  Population Studies and Conservation.  Berlin Heidelberg: Springer-Verlag.  pp. 379-408.

Ramis, D., López-Gari, J.M., McMinn, M., Martínez, J.A. & Quintana, J. 2011.  Els mamífers i les aus del jaciment arqueològic de la cova des Riuets (Formentera).  Eivissa: Consell Insular d'Eivissa.  17 pp.

Ruiz, A. & Martí, R. 2004.  La Pardela Balear.  Madrid:  SEO BirdLife & Conselleria de Medi Ambient del Govern de les Illes Balears.  [no.] pp.

Miguel McMinn, Skua Gabinet d'Estudis Ambientals SLP, Mallorca, Spain & John Cooper, ACAP Information Officer, 14 April 2015

Rachel Buxton (Department of Zoology, University of Otago, Dunedin, New Zealand) and colleagues have published in the journal Biological Invasions on what happens with breeding burrowing petrels when Pacific Rats Rattus exulans are eradicated from their breeding islands.  The species considered are Flesh-footed Shearwater Puffinus carneipes, Fluttering Shearwater P. gavia, Little Shearwater P. assimilis, Sooty Shearwater P. griseus, Grey-faced Petrel Pterodroma macroptera gouldi, Pycroft’s Petrel P. pycrofti and Common Diving Petrel Pelecanoides urinatrix.

The paper’s abstract follows:

“Introduced mammals have been eradicated from many offshore islands around the world, removing predation pressure from burrow-nesting seabirds and other affected wildlife.  Nest-site selection in procellariiform seabirds is mediated by nesting habitat characteristics and social information, although it is unclear if, or how, nest-site selection will affect post-eradication colony growth.  Using a Bayesian hierarchical modeling approach we assessed how nest-site selection differs among burrow-nesting seabird colonies at different stages of recovery after Pacific rat (Rattus exulans) eradication.  We compared nest-site selection in a community of seven procellariiform species among six offshore islands in northeastern New Zealand: four designated rat-free over a continuum within the last 26 years, an island which never had rats, and an island with rats present.  We hypothesized that, immediately after eradication, birds would be constrained to nesting habitat where they were less vulnerable to predation, and as time since eradication increased birds would eventually spread to new habitat.  We found a positive relationship between mean burrow density and time since rat eradication.  Soil depth was the most important predictor of burrow presence, abundance, and occupancy in plots among islands, with more burrows found in deeper soil.  We found that the relationships between habitat covariates and nest-site selection decreased with increasing time since eradication.  The probability of a covariate having a significant effect on nest-site selection decreased with increasing time since eradication and decreasing variability in the covariate across an island.  Our results suggest that the eradication of rodents reduced constraints on petrel nesting distribution and that nest-site selection in burrow-nesting petrels may be influenced by burrow density, where selection of particular nesting habitat characteristics may be relatively more important in small recovering populations.  We conclude that colony expansion immediately after predator removal is complex, influenced by numerous interacting factors, but may be partly limited by the availability of suitable nesting habitat.”

Pycroft's Petrel

With thanks to Barry Baker and Rachel Buxton for information.

Reference:

Buxton, R.T., Anderson, D., Moller, H., Jones, C.J. & Lyver, P.O'B. 2014.  Release of constraints on nest-site selection in burrow-nesting petrels following invasive rat eradication.  Biological Invasions 17: 1453-1470.

John Cooper, ACAP Information Officer, 13 April 2015

Susan Waugh (Museum of New Zealand Te Papa Tongarewa, Wellington, New Zealand) and colleagues have published in the journal The Condor Ornithological Applications on aspects of the ecology of the ACAP-listed and New Zealand endemic Westland Petrel Procellaria westlandica.

The paper’s abstract follows:

“The use of long-term ecological datasets to explore the importance of the effects of environmental variability on higher predator populations has been focused mainly on high-latitude areas.  We modeled the population dynamics of the Westland Petrel (Procellaria westlandica), which spends its time mostly in subtropical waters during both breeding and the interbreeding migration across the Pacific Ocean.  We found that the population has slowly increased since the early 1970s, a result of high adult survival, high fecundity (0.6 of all eggs laid survived to fledge) and moderate mean age at first return to the colony (7.7 yr; a recruitment age typical for this genus), strong recruitment rate of juveniles, and negligible emigration.  The modeled population trends were supported by similar rates of increase in nest occupancy since 2001 and nest density since 2007.  Annual adult survival for breeders was the same for both sexes (0.954, 95% CI: 0.918–0.975) and constant across years.  However, nonbreeders had lower survival rates than breeders, and, among nonbreeders, males tended to survive better (0.926, 95% CI: 0.917–0.934) than females (0.917, 95% CI: 0.900–0.931).  Breeders transitioned to the nonbreeding state at a rate of 0.232 and nonbreeders to the breeding state at a rate of 0.295.  Sea-surface temperature anomalies had a negative effect on adult survival during the breeding period and a positive effect on survival outside the breeding season.  Local marine productivity as measured by fishery catches was strongly correlated with adult survival: Years with a greater fish catch were also years of higher adult survival.  Despite many threats operating throughout the breeding and foraging range of Westland Petrels, it appears that marine environmental change is a strongly influential factor for the species, with uncertainty in population growth due to predicted increases in sea-surface temperature in the future.”

Westland Petrel at its breeding site, photograph by Sue Waugh

With thanks to Barry Baker for information.

Reference:

Waugh, S.M., Barbraud, C., Adams, L., Freeman, A.N.D., Wilson, K.-J., Wood, G., Landers, T.J. & Baker, G.B. 2015.  Modeling the demography and population dynamics of a subtropical seabird, and the influence of environmental factors.  The Condor Ornithological Applications 117: 147-164.

John Cooper, ACAP Information Officer, 12 April 2015

Ruth Brown (Ecosystems Programme, British Antarctic Survey, Cambridge, United Kingdom) and colleagues discuss hybridization in giant petrels Macronectes spp. in the open-access online journal PLoS ONE.

The paper’s abstract follows:

“Hybridization in natural populations provides an opportunity to study the evolutionary processes that shape divergence and genetic isolation of species. The emergence of pre-mating barriers is often the precursor to complete reproductive isolation.  However, in recently diverged species, pre-mating barriers may be incomplete, leading to hybridization between seemingly distinct taxa.  Here we report results of a long-term study at Bird Island, South Georgia, of the extent of hybridization, mate fidelity, timing of breeding and breeding success in mixed and conspecific pairs of the sibling species,Macronectes halli (northern giant petrel) and M.giganteus (southern giant petrel).  The proportion of mixed-species pairs varied annually from 0.4–2.4% (mean of 1.5%), and showed no linear trend with time.  Mean laying date in mixed-species pairs tended to be later than in northern giant petrel, and always earlier than in southern giant petrel pairs, and their breeding success (15.6%) was lower than that of conspecific pairs.  By comparison, mixed-species pairs at both Marion and Macquarie islands always failed before hatching.  Histories of birds in mixed-species pairs at Bird Island were variable; some bred previously or subsequently with a conspecific partner, others subsequently with a different allospecific partner, and some mixed-species pairs remained together for multiple seasons.  We also report the first verified back-crossing of a hybrid giant petrel with a female northern giant petrel.  We discuss the potential causes and evolutionary consequences of hybridization and back-crossing in giant petrels and summarize the incidence of back-crossing in other seabird species.”

Northern Giant Petrel on Marion Island, photograph by Marienne de Villiers

With thanks to Richard Phillips for information.

Reference:

Brown, R.M., Techow, N.M.S.M., Wood, A.G. & Phillips, R.A. 2015.  Hybridization and back-crossing in giant petrels (Macronectes giganteus and M. halli) at Bird Island, South Georgia, and a summary of hybridization in seabirds.  PLoS ONE 10(3): e0121688. doi:10.1371/journal.pone.0121688.

John Cooper, ACAP Information Officer, 11 April 2015