victoria gill reports. a welcome dose of nature on our doorsteps, window ledges and in our gardens. in the uk, we spend more than £250 million every year on food for our feathered friends — but there's a limited number of familiar resident species that eat the thousands of tonnes of peanuts, seeds and suet that we put out for the birds. and scientists now say that this constant supply of supplementary food could have boosted the population of those species at the expense of others. an ongoing rise in the population of great tits and blue tits, for example, could be costly for some of the birds that we don't often see in our gardens. 0ur worry as ecologists is that by providing, sort of, unlimited food for species like blue and great tits, we impact upon the species which compete with blue and great tits. so, there are two other species of tits, which many people might not have heard of. 0ne's called a marsh tit, another�*s called the willow tit, and both of them are amongst