Newsgroups: bit.listserv.biosph-l
Date:         Sun, 13 Sep 1992 00:17:48 GMT
Reply-To:     Terence Sommerville <TSOMMER@IRLEARN.UCD.IE>
From:         Terence Sommerville <TSOMMER@IRLEARN.UCD.IE>
Subject:      Rapid Loss of Migratory Songbirds in Eastern North America

There was an interesting article in the May (92) issue of Scientific American
on 'Why American Songbirds are Vanishing' by J. Terborgh, p 56-62.

It discusses reasons why there has been such a dramatic decline in numbers
of songbirds, particularly in the Eastern half of the US in the past few
decades. Numbers are down by about 70% and some species have become extinct
to some regions, while the Bachman's warbler once common throughout the US,
was last seen in the 60's. Up to this time their numbers had fallen rapidly,
because 90% of the forests in Cuba, (it's wintering habitat) had been
converted to sugarcane. The last sightings were all of males singing for days
on end looking for females, and finding none. Clearly the vastness of the
south-easstern US had spread their by then, few numbers very thin.

Songbirds migrate every year, usually arriving in late spring and leaving
in early autumn/fall, thereby leaving themselves rather a short space of time
to build nests, lay eggs and raise their young. They  are often referred  to
as neotropical since they migrate from the near tropics - mexico, Central
America and from northern South America.

I will not re-write what the above reference says above, (it is well worth
a read), but that to say that it was initially thought that the decline was
due to forest fragmentation and pollution in the US. While both of these have
a role to play it was discovered, that the main cause in the US at least for
the decline was the massive levels of predation and parasitism by other birds
such as the Cowbird, (does same as Cuckoo does in Europe). The worst areas
were in the urban and semi-urban environments, where many predators, e.g.
- racoons and cats live. Additionally parasitism (of nests) by other birds
recently have had extremely high levels in some areas (in some cases 100
percent). The only places where songbirds are safe is in regions which have
large tracts of continous forest such as the Great Smoky Mountains Park and
the Adirnobacks in NY state, where predators are not as common and for the
parastic periods which normally prefer open ground; it is beyond their
range. (which can be several kilometres)

Because of the above problems the songbirds are not producing enough young
to maintain their populations as they must produce at least 0.8 sucessful
hatchlings per year to maintain the status quo. For some birds this had
fallen to as low as 0.1 Consequently each year as they returned there were
fewer and still face the problem of most sites being unsafe.

While these reasons have been given as the main reasons for the decline,
(change in the ecosystem dynamics), the loss of rainforest habitat has also
been a factor ticking away in the background, but evidence is mounting that
the threshold has been reached or is near. Recent estimates of forest loss
by FoE and others show the the rate of loss has increased since the 1990's.
(Note not decreased, despite all the political attention). Therefore in the
next very few years there will be huge changes in the habitats of the
songbirds in their winter habitats as the forest become extremely fragmented.
Measurements made by radar over the Gulf of migrations, with data from 87-89
and compared to ones made in the mid 60's have recorded 50% fewer birds.

We can therefore expect that the decline will move into a new phrase of
rapid loss. The main things to be learnt however are the importance of
maintaining large tracts of continous (old growth) forest, and of helping
to recreate such tracts in areas where it is most feasible. In the case of
ducks in this story, they have been suffering their own problems, with water
quality and loss of habitat.

Where we can help in the tropics is by giving money to the 'Belize fund'
that I mentioned in a previous posting a few days ago, or to get invloved in
Debt for Nature Swaps as discussed recently on this list. I must stress that
doing so will make a difference. (Or at least it is better to try and fail,
than to not try at all.)
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