VI.3.1. Limits regarding the study scope
Due to all of the constraints affecting our green patches
sampling, the patches selected show trends only regarding public areas within
the most urbanized part of Bangkok. Other green patches alike zoos, vacant
lands, privates gardens or street trees haven't been taken into account
although they can be considered as well as green patches and they include
avifauna potential habitat. It is therefore important to ask ourselves the
question of the chosen patches representativeness as they represent only a part
of the green patches within the study area. Furthermore, as the field work
depended on existing structures, the patches didn't range in area and land
cover types and the representativeness of the patches panel can also be
questioned.
VI.3.2. Limits concerning the bird data
collected
A second important fact concerning bird counting is the
difficulty of getting relevant abundance data. Indeed, it has never been
difficult for an ornithologist to create a list of the bird species recorded.
However, calculating species abundance is difficult considering such a mobile
animal. Their mobility could lead us to believe that they could be everywhere
although this is not the case.
It is true that birds are easily seen compared with other taxa
but while counting multi bird species, many studies tend (us also) to assume
them as similar. However, identifying the proximal factor of the urban bird
diversity is relatively difficult as well as studying urbanizations gradients
and biological response because they are far from being linear (MACDONNELL and
HAHS, 2008). Indeed, many parameters affect the bird species differently
according to one species eco-ethology: detectability, response to the
environment changes, adaptation to human disturbances...It is therefore a limit
to assume the anthropic effect or the habitats as equal in the overall patches
while those factors have variable influences for each species. Statistical
models for individual species would be best understood from the perspective of
each species' natural history and habitat preferences but as we focused on the
overall bird distribution and because the number of species was important, we
chose not to proceed in this way (PEARSON, 1993).
Nevertheless, as the study was made in order to implement the
basis for long-term monitoring, the data obtained will be compared to future
data collected in a similar way. Therefore, while forgetting the bias brought
by another observer, the abundance scores specified will allow future surveys
to detect large scale changes in the abundance of individual species.
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Concerning the identification process, the fact that the field
work was a first ornithological survey experience and, in a foreign country,
and despite the great motivation put into the bird visual and sound
identification, a professional birdwatcher would surely have seen more birds.
Hence, the species identifications we made were accurate concerning the
abundant species while the multiplication of visual encounters allowed us to be
sure of the species recognition. On the other hand, non-abundant less
detectable species can lead to identification mistakes.
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