Justification of Red List Category
An estimate of 9,000-16,000 individuals is given by Woods (2000). In 2008 this estimate had remained unchanged at 9,000-16,000 individuals or 6,000 pairs (12,000 mature individuals).
No new data on population trends have been provided since Woods (2000). This species is assumed to be stable, owing to lack of current direct threats.
Troglodytes cobbi has a very scattered distribution in the Falkland Islands (Malvinas). Surveys in 1983/1984-1992/1993 indicated breeding on 12 offshore islands and islets, and estimated the total population at 1,300-2,400 pairs (Woods and Woods 1997). In later years, surveys indicated breeding on 35 islands (N. Huin in litt. 2007), and estimated the total population at 4,500-8,000 pairs (R. W. Woods in litt. 1999, Woods 2000). In 2008 it was estimated to breed on 70 islands, half of which are smaller than 50 ha (Woods and Otley 2008). However, since 2008, more islands have been surveyed for presence of T. cobbi and the total number of confirmed breeding islands now stands at 122 (Poncet 2011, S. Poncet per C. Stringer in litt. 2016). The species’ Area of Occupancy has not been calculated, but the total area of the islands where it is known to breed is approximately 256km2, with the species occurring up to 1.6km from coastal tussac grass (Woods 1993). Despite the discovery of additional breeding islands, estimates of population density led to the conclusion that the 2008 population estimate of ca. 9,000-16,000 individuals or ca. 6,000 pairs remains realistic (Poncet 2011). Most of the islands are in small groups, separated by up to 64 km of sea, and there is no evidence to suggest an interchange between these island populations (Woods 1993, Woods and Woods 1997). However, it is likely that dispersing immatures are able to cross small bays (R. W. Woods in litt. 1999). In 1983, sample plots on Kidney and Carcass Islands produced population densities of four territorial males per hectare in optimum habitat and two males per hectare in less suitable conditions (R. W. Woods in litt. 1999).
Optimum habitat is dense tussock-grassland, growing from the high-water mark behind boulder beaches with accumulated dead kelp in which invertebrates thrive. The species is also found in rushes and among rock outcrops up to 1.6 km from coastal tussock on islands with no introduced predators. The nest is usually well-hidden in a gap amongst tussock stems or a tussock pedestal, or in a rock-crevice. Eggs are laid between early October and December, and there are probably two broods per season (Woods 1993, Woods and Woods 1997).
The species is threatened by the potential introduction of mammalian predators to its breeding islands, especially rats (probably brown rat Rattus norvegicus as well as black rat Rattus rattus) as well as mice (Mus musculus) and Patagonian foxes (Lycalopex griseus) because it feeds at ground-level in exactly the habitat used by foraging rats (R. W. Woods in litt. 1999, FC and FIG 2008). Its present distribution is inversely related to the presence of such predators, whose impact may have increased with the historic destruction of its habitat (Woods 1993, Woods and Woods 1997). Rats and probably feral cats have destroyed entire populations (Woods and Woods 1997), with extirpation of this species taking place c.20 years after the arrival of R. norvegicus (FC and FIG 2008). Grazing pressure and uncontrolled fires are also potential threats (G. Munro in litt. 2007).
Conservation Actions Underway
In 1998, Double and Outer Islands, off Spring Point, West Falkland, were acquired by Falklands Conservation (Woods 2000), and rat eradication started in 2000, covering these islands and two others, Top and Bottom Islands at Port William (R. Ingham in litt. 2000). In total, rats have now been eradicated from 22 islands (G. Munro in litt. 2007). Of the remaining islands within the range, 162 are known to have no introduced land predators, 75 have confirmed rats and/or mice present and a further 553 have not been surveyed (most of them are small or tiny islands) (N. Huin in litt. 2007). Following rat eradication projects several islands have been subsequently re-invaded; although others have been declared rat-free (Poncet 2011, Wolfaardt 2011, Poncet and Passfield 2017) and it is likely that T. cobbi has commenced breeding on one such island nine years after rat eradication (Woolfaardt 2011, S. Poncet per C. Stringer in litt. 2016).
13.5 cm. Smallish, dark wren. Uniformly dark chestnut-brown upperparts. Greyer head. Pale greyish-buff underparts. Wings and tail barred dark brown and pale buff. Slender blackish bill. Voice Harsh chiz notes. Song is complex and variable, mixed phrase of trills and whistles with harsh notes.
Text account compilers
Westrip, J., Symes, A., Gilroy, J., Sharpe, C.J., Stringer, C., Pople, R., Pilgrim, J.
Woods, R.W., Munro, G., Stringer, C., Ingham, R., Poncet, S.
BirdLife International (2020) Species factsheet: Troglodytes cobbi. Downloaded from http://www.birdlife.org on 13/07/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 13/07/2020.