Fine-scale selection of nesting habitat in Little Crake Porzana parva and Water Rail Rallus aquaticus in small ponds

Capsule The most important factor determining fine-scale selection of nesting habitat in Little Crake and Water Rail is water depth. Aims To evaluate factors affecting nest-site selection and relative inter-specific differences in two poorly studied Rallidae species, the Little Crake and Water Rail. Methods Habitat variables describing water depth, water cover, as well as vegetation type and structure were measured within 3-m radius plots around birds' nests and random points, located in small ponds scattered within a largely cultivated landscape in north-eastern Poland. Descriptive statistics and multi-adaptive regression splines were used to describe nesting habitat and to model nest-site selection in the study species. Results Little Crake nested in sites with deeper water and lower percentage of vegetation cover than Water Rail. Both species chose nest sites according to water depth (probability of Little Crake nests occurrence was the highest around 40 cm and of Water Rail below 12 cm) and vegetation stage in which nests were build (old vegetation was preferred). Little Crake nests were also associated with vegetation height lower than 1.5 m and high percentage cover of old vegetation within a 3-m radius around nests, whereas Water Rail preferred Carex spp. and Juncus effusus for nesting. Conclusion For both species, water depth was the main driver of nest-site selection, followed by vegetation traits. Water depth was also the variable most important in discriminating between the nesting sites of the two species. The different patterns of habitat selection showed by the two species are likely to be due to different morphology and nest characteristics, and are probably driven by the need to maximize both nest and adult safety.