Discussion

The interpretation of the fossil deposits rests on the assumption that the birds represented were living in the immediate environment of the fossil site at the time of their death.  In no site in New Zealand is there evidence that brown bones were washed a significant distance, and so we can assume that the birds lived in the habitats surrounding the site. Secondly, it is also assumed that the while presence of one or two individuals may represent a random event such as a storm blown bird, the presence of several birds suggests that a population was present in the area of the site. This further necessitates acceptance that the birds were at home in the surrounding environment.

 In this review of the distribution of Brown Teal fossils, I have compiled data for 641 birds from 73 sites of mostly Holocene age.  Finsch’s duck excluded, Brown Teal were the most abundant anatid in all regions of New Zealand. Because of the difficulties of identifying the exact habitat that surrounded a fossil site during the period of deposition and as fossil sites, with few exceptions, do not have more than one or two individual Brown Teal, a tight correlation between habitat type and relative frequency of anatids cannot be made. However, some useful insights into the variety of habitats Brown Teal used were obtained from amalgamating faunas from relatively small geographic regions likely to have had similar vegetation and climatic characteristics.

The broad trends revealed from the regional analyses

Brown teal were the most abundant species in all lacustrine habitats for which fossils are available, so they were formerly common on lakes. The data, especially that from sites such as Lake Poukawa and Pyramid Valley, supports the historical observations that Brown Teal preferred deep and quiet waterways with overhanging vegetation and dense kahikatea swamps. However, Brown Teal used to occupy a very much broader range of habitats than lakes and rivers, a range that was wider than that of any other anatid in New Zealand. Such additional habitats included coastal dunes, lagoons, and swamps, inland forests as diverse as wet rimu forests and seasonally dry matai forests in low rainfall areas, to montane silver beech and dry mountain beech forests up to 800 m altitude. Use of such forests was not restricted to swamps or streams within them, as fossil sites at times kilometres from any wetland indicate use of forest habitats far removed from wetlands. Generally, within a region, and in all habitats that Brown Teal occur, there is no predilection for water evident in the data. Thus, the historical observations that suggested a preferred habitat of quiet waterways etc are doubtless the result of ease of observations from waterways compared to those made deep within forests.

 The only places Brown Teal may have usually avoided were the subalpine zone above the tree line and perhaps also the driest forests of eastern regions such as on the Haurangi Range in the Wairarapa. In the dry eastern regions where mosaics of forest, grassland, and shrubland occurred widely during the Holocene, Finsch’s duck dominated terrestrial sites and was apparently very abundant. As such it may have competed with Brown Teal excluding it from the more terrestrial habitats in these regions.

 That the decline in Brown Teal numbers was first noticed not long after the introduction of mustelids suggests that predation by these predators was at least partly responsible for the decline of Brown Teal. However, that the species went extinct on both Chatham Island and Stewart Island, and are declining on Great Barrier Island, all places where mustelids are absent, suggests they are not the sole cause. The main predators on Stewart and Chatham Island are cats (Felis catus), Norway rats (Rattus norvegicus), and ship rats (R. rattus) indicating that Brown Teal populations cannot tolerate predation by one or other, or a combination of, these predators.

The fossil data cannot be analysed with statistical rigour that might lead to significant correlations with habitat types on a site by site basis. However, when site data is amalgamated into regions, wherein constituent sites have similar climate and vegetation variables, patterns if present should be evident. For example, we then see that Finsch’s duck is a common and abundant species only in areas with mosaics of shrubland and forest – mainly the drier eastern regions. The dominance of Brown Teal relative to other anatids (exclusive of Finsch’s duck) in composite faunas from each of the regions for which data is available, shows that Brown Teal was at home in a wide variety of habitats. The principal conclusion from these data is that Brown Teal did not have a preference for any specific habitat. It was a habitat generalist that could use any forest or wetland habitat below about 800 m, with the exception perhaps being the dry forests in areas receiving less than about 1000 mm rain annually. Therefore, the decline in Brown Teal populations throughout the last 100 years cannot be linked solely to loss of habitat, but conversely any of a wide range of surviving forest and wetland habitats should be suitable for the survival of populations of Brown Teal if mammalian predation is removed.

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