<strong>I. Introduction</strong> The family Paridae contains five important genera &ndash; Parus, Aegithalos, Psaltiparus, Auriparus and Remiz. Each of these genera includes a group of vary similar species, but the different genera appear to be widely separated from each other. The objects of the present study were:- To ascertain the annual cycle of behaviour of the Great Tit (Parus major), and to attempt an analysis of some aspects of it.</li> <li>To compare the behaviour of the Great Tit with that of other tits, particularly with other species of Parus. To this end parallel observations were made on the other members of the family present in the study area, and the relevant literature was reviewed. Comparative study is of value in facilitating the interpretation of behaviour patterns, and in indicating relationships between species and genera. The study was carried out in Wytham Wood, Berkshire. <strong>II. Methods</strong> A field study was made on a wild population of tits. Birds were trapped and ringed so that individuals were recognisable in the field. <strong>III. Flocking</strong> Outside the breeding season Great Tits live in flocks which show a moderate decree of integration and which move irregularly over a limited area at rates of up to 200&nbsp;yds. per hour. The flocks show two types of movement:- (i) Drifting movements occurring during feeding. (ii) Integrated movements, during which the birds stop feeding and move for some distance. In the latter the flocks are integrated by: (a) The 'Twink' call, which brings other individuals into a ready-to-fly mood. (b) The situation "other-bird-flying-away" which releases flying. The tendency to integration of the flock is balanced by certain disruptive tendencies, the most important of which are due to aggressive behaviour. <strong>IV. Winter Fighting</strong> Fighting over food is common in winter flocks. The most important behaviour patterns are:- (i) The supplanting attack, in which one bird flies at another and dispossesses it of its food. (ii) The wings-raised, and (iii) the head-forward postures, used to threaten nearby individuals, (iv) Avoiding behaviour: tits with food avoid others which approach too closely. (v) Flying away with food particles too large to be eaten immediately. The study of such behaviour can be amplified by allowing, the tits to feed in front of a mirror: they then behave towards their image as they would towards a rival. The image may provoke attack, or flight, or intermediate responses. Supplanting attacks are most common in mid-winter, and occur sufficiently often for such behaviour to be of definite ecological importance. <strong>V. Movements</strong> Adult Great Tits are completely sedentary, and remain on or near their previous year's territory the whole year round. Juvenile Great Tits disperse soon after becoming independent of their parents: they may wander up to several miles from their birth-place. Many juveniles hare settled in a restricted range by the Autumn, though some continue to wander until the nesting; season. The winter range is approximately 10&ndash;20 acres. <strong>VI. Pair Formation</strong> The Great Tit flocks begin to break up in January, but the process is gradual, and in cold weather some flocks say be seen as late as April. Some birds are paired during the winter, but most pair formation takes place when the flocks break up. The mechanism is not yet fully understood, but it appears that the male makes supplanting attacks on other members of the flock, and that his further behaviour depends on that of the attacked individual. Once pair formation has occurred, there is a marked difference in the behaviour of the sexes, the males being much more 'alert' and lass retiring than the females. Certain courtship displays appear at this time of year. <strong>VII. Territory</strong> Soon after the pairs separate out from the flock, the male Great Tits start to use certain sites in preference to others for singing. Fighting against other males is more intense in the neighbourhood of these sites (preferred stations) than elsewhere. In this way an area round the preferred stations (the preferred area) comes to be defended from other Great Tits. The boundaries of this area become defined through skirmishes with other pairs. The preferred area is at first unstable and its boundaries vague: it changes only gradually into a territory in the classical sense. Territorial behaviour is thus much less rigid in the titmice than in most species which have previously been studied. The average size of Great Tit territories in the study area was 2.0 acres. Territories seem to differ considerably in size for the same species in different areas, and also for different Parus species in the same area. The biological value of territorial behaviour is difficult to assess. <strong>VIII. Song</strong> Each Parus species has a characteristic song, though in each species there is considerable variation. The song period is roughly similar in all the Parus species studied: song starts in December, becomes common and well marked in early spring, and dies away at about the time when the young leave the nest. Song nay also be heard from juvenile birds in late summer, and during the autumn period of sexual behaviour. The singing of a male attracts females and warns off other males. The stimulus situations in which it occurs are similar to those in other Passerine species. <strong>IX. Reproductive Fighting</strong> Reproductive fighting occurs in relation to the defence of a female, a territory and a rest-site. The most important behaviour patterns are: (i) The supplanting attack, (ii) The head-up posture, (iii) Upright flight, and (iv) Displacement pecking. Although the behaviour is complex, it can be understood on the assumption that each of the combatants is activated by two drives &ndash; a fighting drive and an escape drive. The fighting drive is expressed primarily in the supplanting attack:, and the escape drive in flying away. In a skirmish the two drives are in approximate equilibrium. The strengths, and relative strengths, of these two drives determine which of the several possible responses will be given to each stimulus situation. In the Great Tit the head-up posture is very elaborate, and is accentuated by a black ventral stripe. In the Blue Tit the posture is less elaborate, and there is no corresponding structure. Although the displays used in intraspecific reproductive fighting are very similar in the different Parus species, they do not appear in the interspecific disputes over nest-sites. <strong>X. Copulation and Courtship Feeding</strong> In all Parus species it is usually the male who initiates copulation, though the female sometimes does so. The precopulatory display consists of a rapid shivering of the wings, a crouching posture, and a high pitched 'zeedling' note. This is given by both sexes. Courtship feeding often follows copulation. It becomes very common during the egg-laying period, the female begging almost continuously all day. The male continues tc feed the female during incubation. The begging display of the female is similar to the precopulatory display, and to the begging of the young. Wing shivering seems to be a ritualized intention movement of flight. <strong>XI. Nest Site</strong> All Parus species build in holes. The different species have slightly different requirements for nest-sites, though there is a very large overlap. During the pre-nesting season the male inspects suitable sites, and his display causes the female to shiver her wings and inspect the hole.