The thesis is divided into three parts. In part I the ecology and behaviour of the Jungle Babbler Turdoides striatus are described and in part II comparable data on the Common Babbler T.caudatus are given. In part III some observations on other species of Turdoides and on the Long-tailed Tit Aegithalos caudatus are described and a general model for the evolution of group territoriality and co-operative breeding is presented. A description of the habitat in which Turdoides studies were carried out is given in an introduction which includes data on seasonal cycles and a brief outline of the ecology of the resident avifauna. A summary is also given of the biology of Turdoides species, which live in permanent exclusive groups of 2-20 birds, holding common territories throughout the year. In most cases only one pair breeds during the season, but many non-breeding group members take part in nesting activities, particularly feeding the nestlings. The aims of the study are summarised in two questions: "What circumstances make it advantageous for more than two birds to share a territory?" "What circumstances make it advantageous for birds to assist in rearing conspecifics which are not their own offspring?" Part I contains four chapters. The first deals with the population dynamics of the Jungle Babblers within the main study area on the outskirts of Delhi, India. Methods used to estimate recruitment, survival, interchange between groups, group structure and the degree of relatedness between group members are described. Territories are classified into three types on the basis of the vegetation that they contain and differences are found between these categories in the number of young reared and the number of birds per group. Large groups, which occupy territories mainly in woodland, probably produce most of the recruitment into the adult population. Birds from these groups may move into territories outside woodland to breed, but breeders in woodland territories are probably recruited from territories of a similar type. Although woodland groups are large and have a high breeding success there is no evidence that the number of non-breeders in the group influences the reproductive success of the breeding pair directly. In Chapter 2 the territorial behaviour of the Jungle Babbler is described. Groups do not cover their home ranges evenly, but tend to concentrate their activities in a small portion, usually comprising closed-canopy woodland with a dense understorey, known as the 'core area'. This was arbitrarily defined as the area within which 50% of the observations of a particular group were made and it comprised about 17% of the total home range recorded. Use of different parts of the home range varies with the time of year and territorially appears to be most marked at the start of the two peaks of breeding activity. During the winter groups often forage together for periods of several hours. The size of core areas is correlated with the number of birds in the group. Groups which changed their numbers from one season to the next showed corresponding changes in the extent and quality of their core area. Taken in conjunction with evidence from Chapter 1, this suggests that large groups tend to be stable and persistent, while small groups are vulnerable to encroachment by others and hence tend to be ephermeral. Chapter 3 describes five aspects of intra-group behaviour in relation to the age, sex and breeding status of the participants. These are allo- preening, sentinel behaviour, leadership, play and roosting. A rough concordance was found between hierarchies based on participation in allo- preening, sentinel and leadership behaviour, but among non-breeding adults differences were found between the sexes. If these three behaviour patterns function partly to express the social status of the participants then differences between males and females are probably related to their respective breeding strategies; males striving to become breeder in their natal group, while females normally move elsewhere to breed. Play behaviour occurs mainly among first year birds, particularly between 2-4 months after fledging, and may be-associated with the establishment of dominance relations among siblings. At the same age they show various signs of social indiscipline which may contribute to the unsettled behaviour shown by some groups while going to roost. The behaviour of non-breeding birds during nesting is described in Chapter 4. All non-breeders more than one year old contribute to feeding the nestlings and driving off potential predators, but a greater share is taken by males than by females. Like the differences in other aspects of behaviour, this difference between the sexes may be attributable to differences in their strategies for breeding. Non-breeders also take some part in incubation, but do not participate in nest-building. The introduction to part II explains how limitations on the visibility of Common Babblers in the field led to an emphasis on different aspects of their behaviour. Chapter 5 covers population dynamics and movements, and also deals with seasonal weight changes. These suggest that some birds which were trapped in mid-winter may have been close to the minimum weight guaranteeing survival and it is possible that the availability of food at this season may be a factor controlling the population. The proportion of unrelated birds found in groups of Common Babblers appears to be higher than estimated for Jungle Babblers. Females leave their natal groups in their first summer and a few males were observed to breed at one year old. The population of Common Babblers in the study area declined throughout the period of the study and this may have affected observed rates of immigration. In some Common Babbler groups only one pair nests during the season, but in others the same male nests successively with two different females and in a third situation two pairs nest simultaneously. There are significant differences between groups exhibiting these three patterns in the number of birds per group and in the density of birds within the territory. In Chapter 6 the relationship between group size, population density and territorial area is described for the main study area and also for populations in the Salt Range, Pakistan. The results of some inconclusive removal experiments are described. Behaviour at the nest was studied in greater detail for the Common Babbler than for the Jungle Babbler and the results are described in Chapter7. Incubation is carried out mainly by the breeding female. Non-breeding females do not usually participate in feeding the nestlings, being driven from the vicinity of the nest by the breeding female. A correlation was found between the number of non-breeders assisting at the nest and the number of nestlings in the nest on day nine, but a causal relationship could not be established. In part III, Chapter 8, brief observations on five other species of Turdoides are described. A comparison between population densities and group sizes for all the populations of Turdoides studied reveals that there is a close correlation between these parameters. The variance in group size within populations is also similar, suggesting that factors controlling group size operate in a similar way on populations in different habitats and at a wide range of densities. The behaviour and ecology of the Long-tailed Tit, studied over a period of nine months in Wytham Wood, Oxford, are described in Chapter 9. In winter stable groups of 10-25 birds hold territories. In February these groups break up into pairs during the day although members of the same winter group continue to roost together.