Archived 2012-2013 topics: Rodrigues Warbler (Acrocephalus rodericanus) and Rodrigues Fody (Foudia flavicans): downlist both to Near Threatened?

BirdLife Species Factsheet for Rodrigues Warbler BirdLife Species Factsheet for Rodrigues Fody The known population of Rodrigues Warbler Acrocephalus rodericanus declined to as few as 8 pairs and one singleton in 1979, but a slow increase followed in the 1980s and 1990s, with at least 150 birds estimated in 1999. The population was believed to have remained relatively stable until at least 2004 but more recent surveys suggest an extremely rapid recent increase in both range (area occupied increasing from 269 ha to 2,140 ha) and population density, with estimates of over 3,000 individuals (3,100-3,900) by 2010 and close to 4,000 by 2012 (Steward 2010, V. Tatayah in litt. 2011, 2012). The species is currently classified as Endangered under criterion D as its population was estimated to number fewer than 250 mature individuals, however the recent population estimates suggest that the species is likely to have considerably exceeded this threshold for at least five years and therefore warrants downlisting. If it is also estimated to have numbered >1,000 mature individuals since at least 2008, it furthermore fails to qualify as Vulnerable under criterion D1. However, it is currently also listed as Vulnerable under criterion D2 – having a very small range and subject to plausible future threats which could cause it to become Critically Endangered or Extinct within a short space of time. Rodrigues Fody Foudia flavicans is currently listed as Vulnerable under criteria D1 and D2 (a population of fewer than 1,000 mature individuals, and having a very small range within which plausible future threats could cause it to become Critically Endangered or Extinct within a short space of time). Its population has apparently also increased extremely rapidly, with an estimated population of over 8,000 individuals by 2010 (V. Tatayah in litt. 2011, 2012). If such figures can be confirmed, it is very likely that the population has numbered over 1,000 mature individuals for at least five years, and therefore no longer qualifies as Vulnerable under criterion D1. The key question for both species therefore becomes whether sufficiently severe plausible threat(s) remain such that either or both should be classified as Vulnerable under criterion D2, or whether classification as Near Threatened is more appropriate. Cyclones remain a natural threat, but following an increase in the extent of suitable habitat and expansion in both their ranges and populations, both species may now be able recover much more readily (V. Tatayah in litt. 2011). It may therefore be appropriate to reclassify both species as Near Threatened (nearly meeting criterion D2) – both remain restricted to a tiny range and there are plausible future threats which could cause population declines, but none which are likely to cause either to become Critically Endangered or Extinct within a short space of time. Comments on the revised population estimates, and the potential severity of future threats to both species are welcomed. Reference: Steward, P. (2010) Rodrigues Warbler Acrocephalus rodericanus census: Onwards and Upwards. University of East Anglia.

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2 Responses to Archived 2012-2013 topics: Rodrigues Warbler (Acrocephalus rodericanus) and Rodrigues Fody (Foudia flavicans): downlist both to Near Threatened?

  1. Peter Steward says:

    I agree with the downlisting of the Rodrigues Warbler to Vulnerable status at the current time. The primary factor in the amazing recovery of this species is reafforestation of the island primarily due to catchment protection schemes and to a much lesser extent a (probable) decline in agriculture and enhancement of indigenous habitats by the MWF. The species appears to have a catholic habitat preference and can be found at high densities even in pure stands of exotic woodland, although it does exceptionally well in indigenous forest too. Its behaviour as a habitat generalist is probably a major factor in the survival of this species; most other Rodrigues endemics are now extinct. Given the size of the island this species will always be vulnerable to extinction.

    Note the range of 2140 ha is the area of warbler occupied woodland and does not include unsuitable habitats such as farmland that would fall within a convex-hull polygon drawn around the occupied woodlands. At a guess the total area occupied including all habitats would be 3000-4000 ha, but could be higher.

    Any loss of forest habitat on the island should be considered an early warning sign for future declines in this species. Although this is unlikely given the benefits islanders percieve from the reafforestation of water catchments.

    The currently high intensity methodology for censusing this species (Showler, 2002; Steward, 2010) was designed for low population densities and there is a danger now that this method could over-estimate numbers. The next complete census should consider switching to a more robust methodology, the original point grids from 2002 could still be incorporate so as to allow comparison between surveys.

    Please contact peetmate@gmail.com for a copy of my MSc thesis.

    ##########ISSUE WITH FODY ESTIMATE IN ORIGINAL POST##########

    According to David Norfolk figures, a masters student who conducted a repeat of Andrew Impey’s 2002 census, the number of Rodrigues Fodies was as follows: 800 breeding pairs; 47 sub-adults; and 59 juveniles. That is approximately 1700 individuals not 8000. Norfolk suggests the fodies range had increased from 1000 ha in 2002 to 7000 ha by 2010. However, this figure of 7000 ha is likely to be incorrect, as Figure 6 of Norfolk’s thesis shows a range that covers less than half of the total island area which is only 10900 ha. Comparing the distribution maps in the 2010 theses the fody appears to occupy a similar range to the warbler. Roughly measuring the range illustrated in Norfolk (2010) using the Openlayers plugin and googlemaps in QGIS gives a polygon of ca. 3500 ha, so the range (all habitats not just woodland) should be considered to be 3000-4000 ha.

    I certainly encountered the fody less frequently than the warbler in my 2010 census and would extremely surprised if there are more than double the number of fodies than warblers.

  2. Andy Symes says:

    The following is an edited summary of relevant paragraphs taken from email discussion between Peter Steward, Prof. Carl Jones and Dr Vikash Tatayah in July 2013:

    Rodrigues Warbler

    Comments from Carl Jones:
    I have no problem with (the) warbler numbers.

    Comments from Vikash Tatayah:
    It does appear that we are able to downlist both species to Near Threatened, even if we are conservative on numbers

    _______________________________

    Rodrigues Fody

    Population estimates

    Comments from Peter Steward:
    As there are many large assumptions when reaching an estimate of 8000… why not be precautionary and say the figure is in now in the region of 4000-8000 birds… Then highlight the issues with the previous survey methodologies and non-breeders and the need for these to be addressed.

    Comments from Carl Jones:
    The problem with the Rodrigues Fody is that its social behaviour is complex and it is difficult to extrapolate from other fody species. Compared with the Mauritius Fody the Rodrigues Fody appears to have many more floating birds. Rodrigues Fodies are long-lived with poor dispersal and in the core areas of their distribution around Solitude there are more birds than territories to occupy.

    To come up with an accurate figure of how many birds we have we would need to colour ring enough birds to be able to get mark re-sighting data and to know enough about their social behaviour to interpret what we are seeing since the detectabilities of territory holding breeding birds and the non-breeding birds are different.

    Previous surveys likely to have underestimated numbers of non-breeding, unpaired birds, and thus their results may be best treated as minimum estimates.

    I believe the main problem arises since (previous surveys) were during the non- breeding season when the rate of fody detectability is low. I have fallen into this trap myself thinking that fody numbers were down only to visit a few months later when the birds are in colour and breeding to find the fodies to be very much commoner than I had supposed. I suggest that during the non-breeding season the fodies are only about half as detectable as they are when breeding.

    The social behaviour of the fody is complex and we do not yet understand it. There are different classes of non-breeding birds, pale billed juveniles and horn and black billed adult grey birds. There are also non breeding adult males that are only in very dilute or patchy yellow colour. Outside of the breeding territories there are non-breeding birds that are mobile, vocal and demonstrative that form mixed flocks with Madagascar Fodies, waxbills and the odd warbler and these move around either between territories or around the outside of them often along streams. These birds are grey birds and dilute or patchy adult males. They are only occasionally detected and are highly mobile. The techniques used for counting the fodies have to date been inadequate since we do not know enough about the social behaviour of the species. We need a study on the social organisation of the fody throughout the year first and then we will be able to design a method of surveying the population that would give use realistic figures.

    The fodies are now many times more common than the warblers….The number is certainly several thousand birds and is derived by looking at the average density and multiplying this up by the area the birds are occupying. In core areas we have several per hectare and an average of 2-3 per hectare seems a little low, but since the birds are usually in clusters of territories there will be some seemingly suitable areas with few birds. The staff in Rodrigues agree with this number as a minimum estimate (and there) is a good chance it is actually a bit higher than this. The number of floating birds is always high and difficult to count or reliably estimate.

    The 8,000 bird estimate is not based on a direct count but on the relative abundances of warblers and fodies, and should be regarded as a maximum. I came up with this estimate after discussion with our staff on Rodrigues during late 2010, after some fieldwork on the fodies that were in breeding plumage and several times more common and detectable than the warblers… Yes let’s go with the figure of 4,000-8,000 which correctly conveys the uncertainly of the population figure, but clearly shows the population is increasing and in good shape.

    Status/trends

    Comments from Peter Steward:
    Since methods in 1999 and 2010 were the same and therefore comparable, estimates of 334 (1999) and 804 pairs (2010) give an average annual growth rate of around 8% (assuming the age structure / relative proportion of total population for the non-breeding population remained similar)

    Comments from Carl Jones:
    ..the fodies occur in dense clusters in very small territories 0.1-0.4 ha and there are birds packed in throughout the central part of the island wherever there is suitable habitat and also down the northern valleys to sea level. The population has been growing and is still increasing as the amount of forest cover is increasing in area and the trees are increasing in size.

    Comments from Vikash Tatayah:
    It does appear that we are able to downlist both species to Near Threatened, even if we are conservative on numbers.

Comments are closed.