I have received the following short item from the author ...
Tomas Roslin [University of Helsinki]
So near and yet so far ? habitat fragmentation and bird movement
TRENDS in Ecology & Evolution Vol.17 No.2 February 2002
Habitat fragmentation shreds continuous habitats into pieces and to move between
these, organisms must venture through new habitats. It is easy
to imagine how, for example, an aquatic snail might find it difficult to cross a
dry road bank running through its wetland habitat. But what about
migratory birds, which regularly cross thousands of kilometres of inhospitable
terrain during their migration? Will they be affected by the finer
details of the landscape in which they breed?
Two new studies add fuel to the contention that they do. The first used a smart
design to reveal landscape effects on bird movements. Bélisle and
co-workers [1] caught mated males of two migratory bird species (the
black-throated blue warbler Dendroica caerulescens and the ovenbird Seiurus
aurocapillus), and a resident species (the black-capped chickadee Poecile
atricapilla). They then moved the males 1?4 km away from their
territories, and quantified the effect of the intervening landscape on how many
males returned and how long it took them. In all three species, the
effects of habitat fragmentation were strong and clear: in a more fragmented
habitat (one with less forest cover), fewer males returned, and the ones
that did spent more time making their way home.
The second study showed why male birds might actually want to travel between
fragments. Norris and Stutchbury [2] used radio tracking to
follow the movements of the males of the migratory hooded warbler Wilsonia
citrina. The primary reason why these males leave their own forest
patch appears to be to try and acquire extra-pair copulations, rather than to
search for food for their own offspring. The costs of such amorous
adventures are probably much higher in fragmented than in continuous landscapes:
males spent more time making forays and travelled greater
distances in fragmented habitat than in continuous forest.
These two studies suggest that not only is habitat fragmentation likely to have
a major impact on the exact paths travelled by breeding birds, but
it is also likely to affect important population processes. If birds find it
difficult to travel through fragmented landscapes, they might fail to
explore the landscape efficiently and so settle in poor quality territories.
And, if territorial establishment depends on the availability of extra-pair
partners, then some species might avoid settling in extremely fragmented
landscapes altogether.
1 Bélisle, M. et al.(2001) Influence of forest cover on the movements of forest
birds: a homing experiment. Ecology 82, 1893?1904
2 Norris, D.R. and Stutchbury, B. (2001) Extraterritorial movements of a forest
songbird in a fragmented landscape. Conserv. Biol. 15, 729?736
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