A comparison of two populations of the grey-crowned babbler (Pomatostomus temporalis)

Abstract

Group-breeding is defined as breeding behaviour in which more birds than a mated pair attend a nest. It is interesting because, firstly, it may consist of actions that cannot be interpreted by classical views of natural selection and, secondly, the environmental conditions that favour this behaviour are not known in detail for any species. These two problems are examined for the Grey-crowned Babbler Pomatostomus temporalise. The Babbler lives and breeds within highly coordinated territorial groups of usually three to ten birds. For theoretical and empirical reasons, it was concluded (in the Introduction) that large groups breed more successfully than small groups or unattended pairs as a result of the active participation in nesting of a greater number of non-laying attendants, relatives of the breeding pair.The empirical work presented in the thesis is mostly concerned with the second problem, that is, the causation of group-breeding in Babblers. It is divided into three parts: (i) methods of ageing and sexing, (ii) group dynamics and some other features of the Babbler's biology (e.g. territoriality) and (iii) quantitative analyses of behaviour. Methods of ageing and sexing Babblers were necessary to analyze properly most data from the latter sections. The last two parts were chosen because they allowed the examination of a wide range of aspects of the Babbler's biology. This was especially desirable as no a priori hypothesis could be formulated from an analysis of group-breeding species in general or from previous studies of the Babbler in particular.Babblers can be aged by calls, plumage, colour of iris, colour of gape and pneumatization of the skull„ They have dark brown irides for about their first year, then brown followed by brownish-yellow irides for a year to year and a half and finally yellow irides for the rest of their lives. They can be sexed by possession of a brood-patch, shape of the cloaca, nesting behaviour, vocalizations and size of some body measurements. In the last case, discriminant analysis of body weight and sizes of tail, bill, tarsus and wing could be used to sex Babblers with a high degree of accuracy.The remaining two sections are alike in that they compare many features of two populations located at the same latitude but 250 km apart in southeastern Queensland. In the section on group dynamics, the two populations were found to be similar for average number of attendants per group; average body weight; coloration; average territory size; average lengths of bill, tail, tarsus and wing; waking and roosting behaviour (except time of roosting); sex ratio; and longer membership within the same group of older birds than younger ones. They were unlike for average group size, seasonal fluctuations in group sizes, breeding success, number of territorial disputes, frequency of interspecific interactions, age structure, duration of group-membership by all birds, time of roosting and timing and duration of wing moult. Most differences were attributed to variations in eastern and western localities in climate and in the levels of some resource or resources, particularly food.In the last section, the Babbler's behaviour is divided into 63 activities and five categories (viz. foraging, maintenance, interspecific, social and reproductive behaviour). The specific activities are/examined in detail, using univariate statistics. The categories are analyzed by multivariate statistics for, firstly, the effects on behaviour of variables that are inherent to groups (i.e. group size and age and sex of members) and, secondly, the effects of some environmental variables (e.g. temperature). In addition, the two populations are compared and seasonal patterns examined.A Babbler's age apparently determined to some extent how much time it spent foraging, maintaining itself, socializing and breeding and perhaps also how frequently it performed actions of all kinds. These findings were related to differences among the age classes in status and experience and in opportunities to behave in certain ways. A Babbler's sex probably influenced the amount of time it spent foraging, maintaining itself, socializing, breeding and in interspecific interactions during the breeding season. Sexual differences in nesting duties were indirectly or directly responsible for all these patterns, and some others. The amount of time a bird spent in foraging, maintenance, social and reproductive behaviour, and also the average distance it flew per behavioural sample, varied with the size of its group. These differences were attributed to variations among the various sized groups in age structure, quality of their territories and breeding efficiency.Of the environmental variables, number of attacks on Babblers by other bird species and number of arthropods probably had the greatest effects on behaviour, at least in the eastern population. For example, values for four of the five categories of behaviour varied significantly with the number of attacks per behavioural sample in the east, and the amount of time spent foraging by eastern birds correlated negatively with arthropod numbers. The remaining environmental variables can probably be ranked, in order of decreasing importance, as time of day, temperature, cloud-cover, windspeed, ground moisture and humidity.The eastern and western populations differed significantly in amount of time, in frequency and in duration of many categories and activities. For the categories, the most important differences between east and west were duration of foraging actions and frequencies of maintenance, social and interspecific actions. Along with variations in average distance and in frequency of flights, these populational differences were attributed to the greater patchiness and concentration of accessible food in the west, and also to greater densities of competitors and predators in the east.Most of the seasonal patterns of behaviour displayed by the two populations were not confirmed by tests of significance. They were, however, generally consistent with other data and with patterns displayed by some other bird species. They showed that the amount of time spent foraging by all birds in both populations had two annual peaks, one during late autumn or early winter and one during summer. The amount of time spent in interspecific behaviour was greatest during winter, in social and maintenance behaviour greatest during late summer or early autumn and in reproductive behaviour greatest during late spring or early summer.Finally, a hypothesis on the evolution of group living and breeding in the Grey-crowned Babbler is presented. The Babbler experiences scarcities of food perhaps year-round but at least during late autumn and early winter, when prey are few, and during the nesting season, when prey are numerous compared with winter but members of groups have dependent nestlings and fledglings to feed as well as themselves. In addition to food shortages, numerous predators, which not only kill young birds but also interfere with the activities of all group members, and perhaps also brood parasites favour the selection of groups over unattended pairs,