Actually it's squirrels in the plantpots at the moment. Wish they'd eff off. :-)
Well, a while ago I posted about a problem my mother was having with filter flies! And they got into her kettle - so she always draped a teatowel over it. Not very satisfactory.
The little slider across the kettle spout seems also to reduce the amount of steam/clouds of condensed steam that appear above the spout (as against our previous 'open' kettle). A plus for that reason even if you don't have any flying creatures, breadcrumbs, bits of tea leaf or anything else to keep out of your kettles!