Nest Structure and Related Building Behaviour in the Rook Corvus frugilegus L

Rutnagur, Richard Sohrab (1990) Nest Structure and Related Building Behaviour in the Rook Corvus frugilegus L. PhD thesis, University of Glasgow.

Abstract

1. The functions of animal artefacts and their importance in ethological study are discussed. The prominent role of nest building in the breeding biology of birds is then reviewed, emphasising its influence on avian reproductive biology. The central aim of the study, namely to establish the cost of nest building is introduced and the reasons for choosing the rook as a model species are stated. The natural history of the rook is briefly described and the approach to studying the cost of nest building in this species outlined. The Cheap and Expensive hypotheses which form the basic model for the study approach are defined and the chapter concludes with a plan of the thesis. 2. The design features of rook nests and building materials used were studied in order to examine the extent of variability in the size and proportions of the whole structure. In addition, comparative data on twig material from nests and the rookery floor were analysed, i) to establish the principal source of nesting material and ii) to test for the preference of certain types of twigs for use in nest building. The conclusion is that rook nests are all built to the same design of a platform of twigs with a central, lined cup. However, it is shown that nests exhibit considerable variation in their size and weight: reasons for this are proposed. There is shown to be a significant preference for twigs in the size category 40. 1-50 cms. , as well for larch and highly branched twigs. The reasons why these may be preferred building materials are discussed. 3. Fluctuations in the number of nests within five colonies were recorded over the annual cycle. Nest numbers increased dramatically between early March and early April at all sites and then declined in every month after May. It was noted that the proportion of nests lost from within the colony varied between sites. There is no evidence, based on changes in nest state, to suggest that nests are refurbished outside the building and breeding seasons. These conclusions support the Cheap hypothesis. Data from the major study site at Milton Farm, show a differential rate of nest loss within the three major nest-containing tree species. Furthermore, disproportionately high numbers of nests were recorded in some species and disproportionately low numbers in others. On this basis, it is suggested that there may be a preference for nesting in certain types of tree. 4. The autumn resurgence of sexual behaviour was studied to quantify the extent of refurbishment. Repairs to the nest structure outside the breeding season would be regarded as being supportive of the expensive view of nest building. No evidence was found to suggest that nests were repaired or built anew in the autumn period, and the data therefore substantiated the predictions of the cheap perspective. The two alternative hypotheses for autumn sexuality, namely as a period of courtship or nest site prospecting were investigated, though it did not prove possible to resolve between these adaptive explanations. Recommendations are made for further study on rook behaviour during this period to confirm the functions of these behaviours and establish their role in the species' breeding biology the following spring. 5. Studies conducted at Milton Farm in the spring were aimed at establishing the effect of variation in building behaviour on reproductive performance. Predictions from the Cheap/Expensive model were made with regard to nest reuse, mode of material assimilation and choice of nest site, to examine their effect on reproduction. After the 1988 breeding season, the lack of any evidence supporting nest refurbishment outside the spring season and the absence of an observed effect of building upon reproduction were two major conclusions in support of the cheap view of building. However, the conspicuous and frequent theft of nesting material and the increasing attendance at the nest site in relation to laying date were two anomalies to a total adoption of the cheap view. This prompted an experiment to remove a large proportion of twigs from the woodland floor in the vicinity of the rookery prior to the 1989 building season. The intention was to increase competition for nesting material and consequently, the cost of building. An increased incidence of stealing, a lower peak in nest numbers within the colony and a significantly lower fledging rate were recorded in the experimental year compared to the previous year. In the light of such findings, the effect of the perturbation experiment in increasing the cost of nest building to the rook is discussed. 6. The empirical evidence from this study is reviewed with the general conclusion supporting a low cost to nest building. The suitability of the rook as a model system is reviewed before making suggestions for further research on this species.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Zoology, Ecology, Behavioral sciences
• Date of Award: 1990
• Depositing User: Enlighten Team
• Unique ID: glathesis:1990-78045
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jan 2020 15:42
• Last Modified: 30 Jan 2020 15:42
• URI: https://theses.gla.ac.uk/id/eprint/78045