Dear Tweeters,

Back in Ontario, I spent a couple of summers working with colonial
waterbirds in the Great Lakes. One of my co-workers had done his M.Sc. on
Black-crowned Night-Herons. One of the things he did was to conduct observations
using an army infra-red telescope to see the herons in their element, darkness.
As Dennis mentions, they are voracious predators. The herons would stealthfully walk through the colony (mainly Ring-billed Gulls) and eat all the chicks in
sight. He personally felt that NIght-Herons were the main controlling agent
of the colony numbers, unfortunatelly the success of the herons may have
driven away several large Caspian Tern colonies from Lake Ontario.
In the Great Lakes, bird colonies are very dynamic. Most of the new colonies
that have appeared in Lake Ontario have set up on landfilled spits. The
general order is that Ring-billed Gulls set up first with a few Herring Gulls
nesting along the edge of the colony. If there is a high sandy area in the
colony Caspian Terns may set up shop. The colonies grow rapidly for several
years until the trees get big enough to support the nests of Black-crowned
NIght-Herons and then colony growth may slow down, probably due to the herons.
As the trees grow, more herons set up shop and cash in on the abundant food
supply which is the gull colony. The colony in Toronto had over 600 Black-
crowned Night-Herons nesting a few years back. The newest addition to the
colonial waterbird fauna of the Great Lakes is the Double-crested Cormorant
which has had a population explosion in the last decade or so. The cormorants
nest on the same trees that the herons use and quickly drive the herons away,
mainly by defecating on them!
It will be interesting to see if the increase in Cormorants will cause a
decrease in herons and ultimately an increase in gull numbers. We will have to
wait and see what happens.

Al Jaramillo
jaramill at sfu.ca
Vancouver, BC.