I suspect, but don't know, that one difference between Australia and
North America is that our raptors do not migrate in the same way that
North American raptors might (?). If true, this would mean, if food is a
limiting factor, that Australian raptor numbers are determined by when
food is at a minimum. In this area that would be when many of our
passerines have headed to the coast, overseas or downhill.
I am not sure of the impacts on raptor numbers, but there is also a
significant degree in Australian avifauna of birds wandering about
regionally in response to boom and bust conditions.
The Botanic Gardens is set in Black Mountain Nature Reserve. Most of
Black Mountain has very poor soil. A recent talk by Stuart Rae
demonstrated that this reserve has a very low density of Tawny
Frogmouths. So, while the Bot Gardens has an artificially raised number
of small birds, the surrounding areas probably have low bird densities.
Australian soils more generally are also poor compared with US soils
more generally, so that might have systemic effects on bird densities.
As always, I suppose, there is no substitute for comparative statistics,
of which I have none to hand.
regards
Con
Adam James Searcy wrote:
Hello COG,
A brief inquiry; I have been working in the National Botanic
Gardens since mid-August, and either myself or my boss Tonya Haff are
in the gardens (usually both of us) 6-7 days per week, for 6-8 hours
per day. We have both noted what seems to be a much lower raptor
density in this area than expected, when compared with similar areas
in North America (where we both hail from) and elsewhere (areas which
also have several different sized /Accipiter/ species, as are present
in this area, and numerous other raptor species). There are abundant
prey species in the gardens, and it seems that they could/should
support at least a pair or two of Sparrowhawks, a Brown Goshawk or
two, associated juveniles, etc. While I have observed these species
in the gardens and elsewhere, they are definitely less conspicuous
than I would expect, both in the gardens and elsewhere. Note that we
are out doing observational work with small passerines, and they or
the surrounding Honeyeaters etc. are generally vocal enough to alert
us of any nearby aerial predators (and we are looking about for such
things anyway...)
Any comments on this situation? Also, have raptors undergone
declines in this area and Australia in general? I'm sure the
Australian Bird Atlas will have more broad-scale information on trends
in raptor populations, but information from anyone with extensive
local experience would be welcome. Anyone with comparative experience
in both N America and here would obviously have a better grasp on the
situation. Cheers,
Adam Searcy
Macquarie
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