The reasons are likely multiple,
among them easy access to nesting sites and food, and a learned tolerance of
humans. However, in our recently
published study, we raised the possibility that ease of flight
might also make city centres an attractive option for these birds.
Turbulant skies
As terrestrial animals, it can be
difficult for humans to imagine what it is like to travel in a medium that is
also moving. If you swim in a pool alone, it feels easier to slip through the
water because it hasn't been churned up by another swimmer. This is just
small-scale turbulence: add on top of this how it feels to swim in the sea,
where the tide can pull you back as you try to return to the shore, and you
will begin to understand what it is like to be a bird.
Now imagine that you have to swim
through the sea to get to work every day. Sometimes the currents would be with
you, sometimes they would be against you, this and the choppiness of the water
will have a huge effect on how hard you have to work. If you had to do this
every day, you would get pretty good at predicting the sea state and current
direction. Flying animals face this all the time: the air is hardly ever still
and this has a profound effect on flight behaviour.
In our study, we looked at how
gulls use the rising air generated by buildings to fly without flapping. Using
the seaside city of Swansea as our research location, we found that gulls
actually alter their flight paths in certain wind conditions, to take advantage
of updraughts occurring around a line of hotels bordering the bay.
Such energy-saving strategies are
already well-recognised in birds that are undertaking their vast annual
migrations, but are less well-studied for birds moving around on a daily basis.
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