The extent of occurrence is unlikely to be the relevant approach for quantifying this species’s range. It has a patchy range (see ebird and Birds of Africa Vol VII) with large gaps between observations at coastal sites. For example, between Pointe Indienne in Congo and Sette Cama in Gabon there are no observations, despite adequate numbers of observers visiting both Mayumba and Gamba (both Gabon).
Criterion B. Area of occupancy may be a suitable approach to use given its fragmented and narrow distribution. See ebird and Birds of Africa to estimate this. The coastal strip which this species inhabits can be quite narrow, for example 100-200 metres in some places. It is not strictly a mangrove or forest species as it also requires open habitats which constrains habitat use further. I would suspect that its AOO is <2,000 km2. Guesstimated calculation: from Libreville to Boma, the full length of its range it is 1,155 km, it has a fragmented range and is found only within a few hundred metres of the coast. Therefore, an estimate could be: 1,155 km x 25% (guesstimated occupancy due to fragmentation) x 500 m (guesstimated occupied coastal strip) = 144 km2.
It is known from c. 10 sites although some sites may be quite large. However, you already state the area of habitat is declining and it may therefore be VU.
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