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SUMMARY The calculations of Odum (1960) on the fuel reserves of small birds are shown to be misleading, since they are based on the figures of Pearson (1950), which are not precise enough for the conclusions that have been drawn from them. A discussion of the energy consumption of small birds shows that fuel reserves are likely to be adequate for flights of several hundred miles. Water loss is discussed and is suggested as more likely to be the factor in limiting duration of flights.

SUMMARY1. This paper is an attempt at a comprehensive classification of the habitats of British birds which should be useful both for precise statement of the habitat in ecological studies and in co-operative enquiries such as the nest record scheme.2. The classification is based largely on the vegetational background.3. It uses a method of continuous subdivision, so that anyone, however inexpert, can make some use of the system, and more detailed records can be added to those that are less detailed.4. It will be possible, when the scheme has been adequately tested in the field, to produce a numbered revised version which should economise space in records. The present numbering and lettering are for clarity only and should not be quoted. INTRODUCTION AND ACKNOWLEDGEMENTS