Azores glory: Europe's most threatened songbird rebounds
Think invasive species and images of chick-munching cats and rats immediately spring to mind. But many local extinctions have their roots in alien invaders of another kind: plants.
The introduction of alien plants, many of which are adapted to survive in far less welcoming climes, can be devastating to finely balanced ecosystems. Rugged species such as the Giant Hogweed Heracleum mantegazzianum, for example, have evolved the ability to develop thousands of seeds to compensate for the poor soil quality in its natural range in the Caucasus Mountains. Its introduction into gardens throughout Europe and North America, and subsequent spread into the wild, has had terrible consequences for the local fauna and flora. Evolution has given this hulking species the means to spread aggressively, and it outcompetes native plants by shading them from the sun. This has a knock-on effect for the area’s native wildlife, which have adapted their needs around these now vanquished plants.
As is to be expected, the effects of invasive plants are often most acutely felt on remote islands, where species are forced to specialise their diet and behaviour around limited resources. Such is the plight that nearly starved the Azores Bullfinch Pyrrhula murina to extinction. A small, plump bird similar in looks to the female Eurasian Bullfinch Pyrrhula pyrrhula (however in the Azorean species, there is no colour difference between the two sexes), the Azores Bullfinch is endemic to São Miguel, an island that forms part of the Azores archipelago; an autonomous region of Portugal located some 1,360 km from the mainland. By all accounts, it was once a common sight on the island and was considered a pest of fruit orchards. Yet today, the species finds itself confined to a few square kilometres of fragmented laurel forest in the island's mountainous east.
The Azores Bullfinch is known affectionately to locals as the 'priolo'. (© Carlos Ribeiro)
Both hunting and forest clearance have taken their toll on the species, but the main driver for its dramatic population collapse throughout the twentieth century has been the spread of exotic plants. “Plants were introduced into the Azores archipelago for various purposes - mainly ornamental and agricultural”, says Azucena de la Cruz, LIFE Terras do Priolo Project Assistant at SPEA (BirdLife in Portugal). “Some of those species became invasive and presently cover large areas of the island. Today, more than 60% of the vascular flora of the Azores is considered exotic.”
This rude encroachment is a problem for the Azores Bullfinch in particular because, in order to combat year-round food shortages, the species has developed very specialised feeding habits that change throughout the seasons to adapt to what food is available. In spring, it feeds on flower buds; in summer, herb seeds; in autumn, it turns its attention to fruits; and, during winter, it is reliant on fern spores. The only thing that doesn’t change, is that the Azores Bullfinch’s various food sources consist of plant species endemic to the archipelago; these include the Azorean holly Ilex azorica, Hawkbit Leontodon rigens and the nearly extinct Azorean plum Prunus azorica.
Thus in order to survive, the Azores Bullfinch is reliant on Azorean laurel forest habitat which can boast a rich tapestry of these endemic species. However, in the last 100 years, these habitats have come under attack from exotic invaders that have escaped from gardens and naturalised. One such species, the Kahili ginger Hedychian gardneranum of India and Nepal, is recognised by the IUCN Invasive Species Specialist Group as one of the world’s worst 100 invasives. It is able to quickly colonise large areas of land by forming dense blankets of foliage that crowd out native seedlings. Today, only around two percent of the island’s native laurel forest remains. This loss of local biodiversity nearly spelled the end for the Azores Bullfinch; before SPEA began work to protect and restore the species’ habitats, there were perhaps less than 300 pairs remaining, making it Europe’s rarest passerine.
Juvenile Azores Bullfinch (© Ricardo Ceia)
Since the turn of the century, SPEA has coordinated three EU funded projects aimed at saving the Azores Bullfinch’s remaining strongholds in the east. The first two projects, which took place between 2003-2008 and again between 2008-2013, were geared towards maintaining and restoring pockets of the species’ habitats, by means of clearing invasive plants, establishing fruit tree orchards, and planting native species in core areas and the buffer zones around them. SPEA also strove to raise awareness among Azoreans of the bird's plight, all of which helped the São Miguel Natural Park to be classified in July 2008, which now protects a significant part of the species’ habitat.
So far, these projects have succeeded in allowing the recovery of over 300 hectares of laurel forest and peatland habitat; bullfinch numbers have bounced back accordingly. In 2010, the population recoveries were such that the species was downlisted from Critically Endangered to Endangered, a major milestone in saving a bird that was once seemingly on an inexorable march towards extinction.
The third and current project, LIFE Terras do Priolo (meaning “Land of the Azores Bullfinch” in Portuguese) , is working to build on the promise of these established patches of habitat by beginning work to join them together into one large contiguous area. “The problem with these patches is that invasive plants, which are dominant outside of them, are able to re-enter the area and this obliges continuous maintenance of the areas”, says de la Cruz. “For this reason, the building of larger continuous areas was considered necessary in order to reduce the re-entering of invasive plants.”
Joining up the restored patches of vegetation will not be a simple process, however; it involves revisiting areas previously considered not cost-effective to restore. “The Tronqueira Mountains have very steep slopes with loose volcanic soils and several water lines that are mostly not permanent and torrential”, says de la Cruz. “In previous projects, we selected areas with more gentle slopes and this resulted in a series of restored patches scattered in the mountains.”
The present Terras do Priolo project is also working to construct a ring around the mountain ridges to stem the further spread of the invasive plant species that have brought the Azores Bullfinch to the brink. Whether or not this ring will be effective depends on how well the interior areas can be controlled; this will involve a great deal of intensive conservation effort in the decades to come. So, although the Azores Bullfinch population continues to grow, and is edging towards the symbolic 1,000 mark, we should not become complacent; birds such as this, with such particular needs and such tiny ranges, will likely always be at risk of slipping back towards extinction, and it is essential the good work made possible by the EU's LIFE programme is allowed to continue.
Nevertheless, with the 2016 Red List delivering news that the Azores' beloved 'priolo' has now met the criteria to be eligable for a second downlisting in under a decade - this time to Vulnerable - for now we have every reason to feel bullish about the Azores Bullfinch’s future.
The new factsheets for all 11,121 bird species assessed in the 2016 Red List update can be found at http://datazone.birdlife.org/species/search. For an Excel spreadsheet containing the complete 2016 checklist, and another listing all the changes since 2015, please visit http://datazone.birdlife.org/species/taxonomy and download the zip file.
BirdLife International’s Red List Assessments are sponsored by Zeiss and supported by the Tasso Leventis Foundation, to whom we are very grateful. Thanks also to the A. G. Leventis Foundation for supporting our taxonomic work that underpins the Red List.