Having fed birds in our south-of-England back garden all through the winter with several-times-a-day visits by 7 Long-tailed Tits, we now have what seems to be a resident pair who visit our suet log feeders daily, appearing even when we are refilling another feeder only 4 feet away, so they are very unafraid (last summer's young?). One (or both?) visit our window sills several times a day, tapping on the glass--something we have never experienced before. Is he/she trying to collect the spider webs in the houseplants lining our window sills for nesting material? Or...any other ideas? What would happen if we opened the window?