Risk of rat infestation on Filfla could kill seabird population

The accidental introduction of rats on the small, uninhabited island of Filfla could have a devastating impact on the fate on the largest Mediterranean colony of European Storm Petrels (Hydrobates pelagicus), an offshore seabird which breeds in the islet.

The report strengthens the case for stricter controls on sea craft around the small islet.

According to a report on bird populations, rodents could be easily introduced should any sea craft containing rats - intentionally or accidentally - allow them to run loose in the vicinity of the islet.

Rats are known to have relatively high breeding success and may likely invade the islet and drive this species to extinction, the report warns.

This is because rodents can predate on hidden eggs in burrows, consequently resulting in a drastic reduction in fecundity.

All three species of off shore feeding birds frequenting Maltese coastal waters have suffered a decline in number of breeding pairs throughout the past decades, the report compiled by biologist John J. Borg says.

The report submitted to the Malta Environment and Planning Authority is a requirement of the Islands of the EU Marine Strategy Framework Directive.

The report attributes the decline in the number of seabirds to the different pressures each species is subject to, but this decline is deemed to be reversible should the current pressures be addressed.

In fact the numbers of Puffinus yelkouan are recovering following implementation of conservation measures at Rdum tal-Madonna.

The measures included the eradication of rats with the aim of increasing breeding success and reducing rates of premature mortality and legal amendments to modify recreational activities and potential infrastructure to reduce human disturbance at nesting sites and avoid further loss of breeding habitat.

Pressures on bird populations

Offshore feeding birds occur regularly in Maltese coastal waters and aggregations of seabird species are regularly observed on coastal waters during spring and autumn migrations or during winter.

Coastal cliffs provide shelter and a breeding habitat to offshore pelagic feeding birds, namely Scopoli's Shearwater Calonectris diomedea, Yelkouan Shearwater Puffinus yelkouan and the European Storm Petrel Hydrobates pelagicus. 

Malta is deemed to be an internationally important breeding location for all three species. Coastal cliffs are also important for the Yellow Legged Gull, an inshore feeding bird which breeds and occurs in significant numbers around the Maltese Islands.

The main pressures on populations of Scopoli's Shearwater consists mainly of disturbance from human activities.

"Encroachment by development onto the coastline is considered to be a major source of disturbance, along the Southwestern cliffs of mainland Malta."

The breeding population at this site has declined due to abandonment of nests along the cliff tops as a result of disturbance, particularly through light pollution, from the main industrial area developed along the coastal cliffs in the 'Ħal Far' area.

Biological disturbance, mainly through predation by rats and domestic animals such as cats and dogs, is also considered to be a significant pressure on this species.

Fishing practices may also put pressure on Scopoli's shearwaters. Longline fishing is considered to be a significant pressure on seabirds in the Mediterranean region.

The colony of Puffinus yelkouan at Rdum tal-Madonna is recovering after years of decline, also attributed to human disturbance - particularly at Rdum tal-Madonna, which hosts the main colony of this species.

"Disturbance is mainly associated with an increase in recreational activities at this site since the late 1990s, which led to a significant number of rodents being attracted by litter and regularly predating on chicks and eggs." 

In addition to predation, death of adult birds between 2000 and 2006 was also attributed to direct persecution. Missing long auxiliary feathers were found on dead specimens, suggest that feathers are used as baits for line-fishing.

But thanks to targeted conservation measures undertaken between 2006 and 2010, the situation improved. One of the positive developments is that no further records of chicks or eggs being eaten by rats took place from 2007 onwards. 

Other threats to Puffinus yelkouan include illegal hunting and ferreting. Such threats induced Yelkouan Shearwater to choose deeper, safer cavities. 

The resident population of European Storm Petrels breeds regularly on Filfla.

The current population of storm petrels in Malta is estimated at 5,000-8,000 pairs on Filfla. Until the mid 1980s, the Filfla colony was estimated at 10,000 pairs. During the past decade, however, only 5,000-8,000 pairs of storm petrels were estimated at their colony. The decline in the population of this species on Filfla is mainly attributed to the loss of nesting sites after storms washed away some of the rubble screes below the cliffs, as well as to an increase in predation by the Yellow-legged gull.