A method to find out which species is more likely to start egg laying in a certain nestbox is described. Nestboxes were visited daily and the behaviour of the birds (Great, Blue and Coal Tits) that appeared around the nestbox was observed. The birds' response consisted mainly of giving alarm calls and showing inquisitive behaviour (i.e., approaching the observer and looking at close distance). Proximity of the individual (or the pair) to the nest and inquisitive behaviour of the female bird were found to be associated with nest ownership. Alarm calling and inquisitive behaviour of males were less selective and therefore not diagnostic. Nocturnal check of nestboxes provided useful information about the nest owners, but they were more reliable at late nest-building phases, when the female was closer to egg laying. It is suggested that this observation method may be used in field studies of species breeding in nestboxes or even natural cavities. This work emphasises the importance of observing small details of behaviour as symptoms of the condition 'inside' the individual (motivation).