Sarah Keenan, a pupil at Hoon Hay Primary School, asks :-
Why do monarch butterflies gather in parks in late autumn?
Simon Pollard, a zoologist at Canterbury Museum, responded.
In the cooling autumn temperatures of late April and May, monarch butterflies are often seen preparing to spend the winter. While some of these monarchs will have spent all their life locally, adults also fly in from inland areas because the coast is warmer in winter. Stimulated by a drop in temperature, the monarchs look for areas that are protected from wind, have trees suitable for roosting and are close to sources of nectar from winter blossoms. Swarms of monarchs are often seen hanging, like the insect equivalent of Christmas decorations, from the leaves and twigs of trees. What makes a good roosting tree for a monarch may depend on how well they can cling to the tree's foliage with the claws on the ends of their legs. A monarch cooled down by the coldness of winter does not want to lose its grip!
Although the butterflies tend to remain still when it is cold, once warmed into activity on sunny winter days, they will fly around and drink energy-rich nectar. Cold snaps do tend to kill off some of the over wintering butterflies, but the survivors disperse from their winter retreat once the temperature rises again in Spring.
According to George Gibb's excellent book, The Monarch Butterfly (published as part of the Mobil New Zealand nature series) monarchs especially like roosting on cedars, oaks, totara, kanuka and macrocarpa, because they can cling safely to these trees. It seems likely that monarchs have been gathering in parks because it's a good place to spend the winter.