Michael Viney responds to queries and observations on nature.
If crows eat rats killed by poison do they die too? Corn is dressed with insecticides and herbicides but birds seem to eat it and we don't see lots of dead birds at seeding time.
Marion Rowe, New Ross, Co Wexford
Rats are usually killed by an anticoagulant which would not effect the flesh. However birds can accumulate pesticides in their system which may harm them eventually. The Soil Association says that a quarter of our food has chemical residues, at levels which the authorities say "do not pose any unacceptable risk to human health".
In Glendalough, on the old mines track we saw several blobs of clear jelly on the ground which looked similar to frogspawn without the embryo.
Dieter Gerhardt, Callan, Co Kilkenny
Hibernating frogs can contain partially developed spawn which makes a predator sick.
At Ballylongford on the Shannon estuary I watched two ravens make a sustained attack on a little egret. It seemed that they intended to kill it if they could. Perhaps they regarded it as an alien threat?
Mark Helmore, Burrin, Co Clare