Hans-Guenther Bauer

Mapping avian distributions: the evolution of bird atlases

Capsule An increasing proportion of atlases now map patterns of abundance but they are still a minority even though they require no more input of time or fieldworkers. Aims To examine quantitatively the evolution of bird atlas methods, from their inception to the present day, to document the most frequently used methods and to quantify temporal changes in them, and so identify broad patterns that might be of use in the planning and interpretation of future atlases. Methods A database of over 400 atlases was compiled, and a number of variables extracted from each. Temporal trends within, and relationships between, these variables were analysed. Results Atlases have become significantly reduced in scale over time, covering smaller areas in shorter periods of fieldwork, but at higher spatial resolutions and with increasing numbers of observers per unit area. The number of participating fieldworkers and the size of the region being covered together explain over 70% of variation between atlases in spatial resolution. The number of atlases that have mapped abundance or relative abundance, rather than simply occurrence (presence/absence) or breeding status, has increased significantly over time but remains relatively small. However, such atlases were no more expensive in terms of length of the fieldwork or preparation periods or the number of observers deployed per unit area. There is evidence of a sharp decline in the number of new bird atlases being initiated. Conclusions There have been significant changes in the way atlas surveys are undertaken, but no standardized method has evolved. This leads to flexibility that allows atlas surveys to be undertaken over areas varying by six orders of magnitude using numbers of observers that vary by five orders of magnitude.

Depth-specific and spatiotemporal variation of delta C-13 and delta N-15 in Charophytes of Lake Constance: implications for food web studies

Macrophytes are at the base of many lake food webs providing essential food resources for animals at higher trophic level, such as invertebrates, fish and waterbirds. However, data regarding the spatiotemporal variation in isotopic composition of macrophytes are generally missing. We measured the carbon and nitrogen stable isotope ratios of Charophytes at Lake Constance, where they constitute a major food source for waterbirds. Our data reveal seasonal and site-specific differences as well as depth-specific variations in isotopic carbon values within the littoral zone. Charophytes were enriched in (13)C at sites of higher productivity: the δ(13)C values were high in summer, at shallow and at relatively nutrient-rich sites, and comparatively low in winter, and in deeper and nutrient-poorer sites. In contrast, no temporal or spatial trend was found to explain the variability in the isotopic nitrogen values. These results imply that the seasonal timing of food intake (relative to turnover rates of consumers tissue) and the potential depth of foraging need to be taken into account when calculating the relative contribution of energy sources to diets of consumers such as waterbirds.