Working for birds in Africa

Is the endangered Long-billed (Moreau's) Tailorbird Orthotomus [Artisornis] moreaui safe in the East Usambaras?

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Long-billed (Moreau's) Tailorbird, Tanzania
N. J. Cordeiro

Le statut de la Couturière de Moreau Orthotomus [Artisornis] moreaui, espèce ne se trouvant que dans les Usambaras orientaux en Tanzanie et dans le nord du Mozambique, est considéré comme Critique [4, 13]. En vue de cette situation alarmante, une étude de population a été entreprise dans la Réserve naturelle d'Amani, Tanzanie, en juin 2000. Les résultats indiquent qu'il existe une petite population de 150-200 individus répandus sur le plateau des Usambaras orientaux, principalement à l'intérieur de la Réserve. Des précisions sur la population estimée sont présentées, ainsi que des observations générales faites pendant la période de janvier 2000-mars 2001; celles-ci mettent à jour les connaissances du statut actuel de l'espèce, son écologie et sa protection.

Background

Reg Moreau discovered Long-billed (Moreau's) Tailorbird Orthotomus [Artisornis] moreaui in 1930 in Amani, within the East Usambara Mountains, Tanzania [ll]. It was not recorded from the area again until the 1970s when several new observations were reported [3, 5,14,16]. Despite intensive searches in other Eastern Arc mountains, the species remains known only from the Amani and Nilo areas of the East Usambaras [4-6]. An isolated population on the Njesi Plateau, in Mozambique, has not been searched for since 1945[1, 2,13]. The species is currently listed as Critical [13]. Since the 1970s very few individuals have been observed in the Arnani area. Cordeiro [6] recorded it in Nilo Forest Reserve, 20 km north of Amani, in 1994. During the last 20 years fewer than 30 sightings have been made in Arnani and Nilo, some possibly of the same individuals [4-6]. The few documented observations suggest that it resides in forest edge and large forest gaps [4, 5,14], but its precise habitat requirements, biology and status are poorly understood.

Distribution and population estimate

Earlier records of the Long-billed Tailorbird centred on the Amani area [4,5], with recent reports from further north on the same plateau, as far as Mt. Nilo [6]. Prior to this new assessment of the distribution and abundance of the species, no population estimates had been made, though the lack of records in the 1980s and early 1990s suggested a population decline in the East Usambaras [4]. Given hindsight, insufficient attention had been paid to the correct habitat and most visits to the plateau had been rather short. In June 2000, we conducted a pilot count of the tailorbird. Preliminary encounters with the species during an ongoing bird census conducted by NJC, covering a large part of the nature reserve and its environs, indicated that it prefers forest edges above 800 m [7]. Therefore, we concentrated subsequent efforts along forest edge on the plateau of Amani Nature Reserve, at 800-1,050 m.

Using playback of the most common vocalisation at 200 m intervals along the forest edge, a total of 84 point samples were made and 44 individuals were counted at 27 of these. As one-third of suitable habitat was sampled, we assume a conservative total estimate of 150-200 individuals. More than 50% of our records were of lone individuals. Because of this our estimate is likely to be low as the species regularly occurs in pairs, with one individual being more elusive than the other. Several large gaps within the forest and the large area of forest north and east of Monga and Derema to Nilo were not sampled. Thus far we have found the species to be most abundant in the Sangarawe to Monga areas, with fewer individuals west, east and south of these general locations.

The new population estimate, while conservative, may indicate an increase based on previous data [4,13]. Furthermore, the many observations at forest edge may contradict the previous suggestion by Collar & Stuart [5] that it is susceptible to forest disturbance. It is probable that increased knowledge of its distinctive vocalisations has heightened the species' detectability. Additionally, its apparent favoured habitat was investigated more thoroughly during our work than previously. Since the population study, Long-billed Tailorbird has been discovered at further forest-edge sites and within some large gaps inside forest (NJC, B Munisi, C McBride and M Joho pers obs 2000-2001).

General ecology

Habitat requirements

Previous reports have suggested that this secretive species is restricted to forest edge and clearings with vines and climbers [l2]. It was also believed to prefer dark forest undergrowth [12], and more recent reports indicated that it could be partial to wetland habitats at the forest edge (W Newmark pers comm). Our observations suggest the species is a retiring bird of dark undergrowth with a high density of vines and climbers, but as previously noted [6,14], it also ventures into sunlit and open areas within suitable habitat. While largely restricted to forest-edge habitat, possibly near water, we have also discovered the species in a large wetland glade and large gap deep within primary forest, indicating how poorly known its habits remain. Furthermore, Collar & Stuart [5] noted the population in Mozambique had probably been mistakenly reported by Benson [2] to be a canopy dweller. It should be noted that Benson [2] indicated the canopy to be of a mean height of c8 m, ie well within the range of our encounters of the species. In addition to its primary forest-edge and gap habitats, we have also found it in dense Lantana thickets adjacent to forest, indicating an ability to persist in relatively disturbed habitats with sufficient shelter and food.

Foraging behaviour

We have observed the species gleaning foliage of vines and dense shrubs, being rapid and deliberate in its movements through vegetation. While foraging, individuals largely remained in the understorey below 3 m. However, they occasionally work areas at 4-20 m. All observed prey have been invertebrates, many skilfully obtained by deep piercing of densely packed foliage. McEntee, Cordeiro & Moyer (in prep) have conducted a short, but in-depth quantitative study of the tailorbird's foraging ecology, which will be reported elsewhere. Unlike Stuart & Hutton [14] who reported it to be an occasional member of mixed- species foraging flocks, we have not observed it in such groupings in over 150 observations of at least 12 different pairs.

Breeding

There is no previous breeding information for the species. One of us (NJC) found two recently fledged young with parents in mid-October 2000, indicating that eggs are laid in September. Vocalisations of territorial pairs were more frequent in October- November, suggesting breeding activity during this period. The two young were observed until mid-March 2001, being fed by both parents until early February; subsequently parents and young have foraged separately, but still within a group. These are resident around the Amani Nature Reserve headquarters and will be monitored until July 2001. No definite nests have been located by us, though one believed to have been constructed by this species was discovered by R. Stjernstedt in Amani [9]. While the description in Birds of Africa implies that the nest was attributable to this species, Fry [9] opined that photographs do not indicate the sewn leaves to be of the same plant in which it was constructed. We believe more definitive evidence of the species' nest is needed.

Vocalisations

Sclater & Moreau [12] described the species' vocalisation as 'percussive and mechanical, like a series of deliberate blows with a mallet on an iron peg'. Zimmennan et al [18] provide a very adequate description of one of the commonest calls: tcbeu-tcbeu-tcbeu-tcbeu-tcbeu. Vocalisations are varied, with at least 2-3 variations on the above, and one highly specific rasping call given when contacting conspecifics.

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