I strongly agree that we know very little about movement in Australian
landbirds and would like to add a few points to the discussion. I am
currently studying partial migration in rainbow bee-eaters and I just
can't help myself...
Without getting too hung-up on definitions it may help to clarify that
'partial migration' is not really a question about migration distance,
per se. The term is generally applied to species in which one or some
members of a population remain resident, while the remainder migrate, or
some sub-populations within the entire distribution of a species remain
resident, while the remainder migrate. Interestingly, the latter appears
to apply to rainbow bee-eaters at least in northern Australia, yet they
remain a 'true migrant' in the south. Geoffrey's point about observing
'migrants' or 'residents' at the edge of their distribution is valid and
may have helped to create confusion around whether an individual bird
(or population) is in fact present all year round.
There are many interesting questions that the study of partial migration
in a particular species might pose:
- why migrate? why stay? why have two strategies?
- who came first - the migrant or the resident?
- what is the evolutionary direction - more migrants or more residents?
- how flexible is the adaptation to migrate or remain resident?
- will climate change produce more migrants or residents?
There are many more, however, check out Ken Chan's great work on
Silvereyes which apparently switch from being migratory to taking up
residency in successive years (partial migration) - now that is cool!
Incidentally bee-eater arrivals in the ACT (COG Atlas data) seemed
fairly 'tight' (10 days either side of 5 October), yet there has been a
drastic decline in arrival dates since 2000 with no records for many
years (2000-2, 2004-6). This could be an artefact of data collection but
I am checking out this trend for NSW and Vic.
Geoffrey Dabb wrote:
Like Peter Ormay, I think Martin’s work on this is interesting and
valuable - including his note on ‘tight-arrival’ migrants in this
months Gang-gang. However, I sometimes think it can be misleading to
focus, like the figurative eagle-eyed contributor to The Times, on the
‘first arrival’ of migrants - in relation to so-called partial
migrants. For one thing it diverts attention from the true migrants,
whose movement variation might conceivably reflect climate shift and not
just local food availability.
Clearly enough, the GBS records movement of birds through the suburbs.
Thus there is clearly a seasonal element to the /garden/ appearance of
Grey Fantails for example. With such birds there is quite likely (a) an
altitudinal element and/or (b) a vegetation element eg to/from woodlands.
Taking ‘Canberra’ as including its woodlands, it seems to me that
insufficient work has been done to be certain of the annual movement
pattern of birds that are present in our area during Winter, although in
much reduced numbers: eg Noisy Friarbird, Dusky Woodswallow,
Olive-backed Oriole (remember Graeme Clifton’s report in July). The
second BA atlas, with the first COG atlas, is a start. Other movers
that can be present in Winter in reasonable numbers include Welcome
Swallow, Black-faced Cuckoo-shrike, Rufous Whistler, Yellow-faced and
White-naped Honeyeaters and Fan-tailed Cuckoo. I have difficulty with
the idea that any of those birds have ‘just arrived’ when they are
reported in Spring.
The term ‘partial migrant’ seems to me to be confusing shorthand. It
might suggest that some members of a population migrate only a short
distance, or that some members do not migrate at all. With regard to
the species in question it may be that Canberra is properly regarded as
at the fringe of the Winter range and a few members of more southerly
populations range about our area.
To me the ‘true migrants’ among our land birds are those that are absent
from South-east Australia in Winter, eg Koel, Bee-eater, Dollarbird.
Significantly, it is these that seem to have ‘tight’ arrival schedules
(which makes all the more interesting the ‘early Koel’ reports). I
would also include the Painted Honeyeater, which you can set your
calendar by at Cocoparra.
*From:* martin butterfield
*Sent:* Monday, 1 September 2008 5:04 PM
*Subject:* [canberrabirds] Dusky Woodswallows
This afternoon a flock of about 6 Dusky Woodswallows were hawking along
Whiskers Creek Road (about 200m outside my GBS site).
In terms of GBS observations this coincides nicely with the expected
upturn in number of observations. A graph is attached: note that this
shows the raw number of observations rather than the F% value, since the
two lines are similar and this one is slightly simpler to plot. I have
also extended the graph to show that the observations (after the initial
'surge") are less consistent than was the case for the Noisy Friarbird.
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