...but it's a starling (Sturnus vulgaris). The Cambridge Swift Tower is now (30th April 2013) into its 3rd season, with no swifts yet, though some swifts have already been seen in the area after their migration back from Africa. Let's hope some of them take up residence this year. Meanwhile the ever-versatile starling has managed to fit through at least one of the entrance holes - size is critical if swifts are to be admitted but not starlings, but it doesn't really matter unless starlings occupy every hole. In any case, unless they have a 2nd brood, starlings will finish raising their brood and still leave time for swifts to move in.
See here for news from later years, with swifts finally nesting in the tower from 2014 onwards.