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

Figures & data

Figure 1. Frequency of the most common vegetation types at nesting sites of Little Crake and Water Rail.

Figure 2. Comparison of habitat variables at nesting sites and nesting plots (area within 3-m radius around nest) of Little Crake (white box plots) and Water Rail (grey box plots): (a) water depth at nest site and mean water depth at nest plot; (b) vegetation height at nest site and mean vegetation height at nest plot; (c) percentage cover of old and new vegetation, and open water cover at nest plot; (d) distances from the nest to the nearest shore, open water and Grey Willow shrub. The band inside the box represents the median and whisker plots represent range of values without outliers.

Figure 3. Graphical summary of the MARS model for nesting-site selection in Little Crake. Species–habitat relationships show the probability of nest occurrence in relation to mean water depth (cm), vegetation height (cm), vegetation stage (old, new, mixed) and percentage cover of old vegetation (% cover in the 3-m radius around the nest). Sunflower plots (upper row: nests; lower row: random points) are also shown in each graph.

Table 1. Summary of selected MARS models (see text and Supplementary online material) for the two studied species.

Variable	Coefficient	No. of subsets	RSS	GCV
Little Crake
(R^2 = 0.28, cv R^2 ± sd = 0.17 ± 0.01, AUC ± sd = 0.74 ± 0.00)
Intercept	2.66
Mean water depth (below 42 cm)	0.05	100.0	5	5
Mean water depth (above 42 cm)	−0.08
Vegetation height (above146 cm)	−0.12	1	11.7	29.8
Old vegetation cover (below 40%)	0.04	1	11.7	29.8
Vegetation stage mixed	−1.18	3	45.1	60.0
Water Rail
(R^2 = 0.52, cv R^2 ± sd = 0.36 ± 0.09, AUC ± sd = 0.86 ± 0.07)
Intercept	2.54
Mean water depth (above 12 cm)	−0.12	4	100.0	100.0
Vegetation type Carex spp.	2.67	2	37.9	46.1
Vegetation type J. effusus	17.68	1	25.5	31.4
Vegetation stage mixed	−2.73	3	48.7	58.9

For complex hinge functions, the effect for the specified range of values is reported; see Figs 3 & 4 for a detailed description of the species–habitat variables. Abbreviation: no. of subset, number of model subsets generated by the pruning pass, which include a given variable; RSS, decrease in the residual sum of squares; GCV, generalized cross-validation of the model; R², model's R²; cv R² ± sd: model's R² calculated over the cross-replicated models and the relative standard deviation; AUC ± sd: the area under the curve (and its standard deviation) calculated over the cross-replicated models. See text for explanation.

Figure 4. Graphical summary of the MARS model for nest-site selection in Water Rail. Species–habitat relationships show the probability of nest occurrence in relation to mean water depth (cm), vegetation stage (old, new and mixed) and vegetation category (in the order: Typha spp., P. australis, Carex spp., J. effusus, P. arundinacea, A. calamus and S. sylvaticus). Sunflower plots (upper row: nests; lower row: random points) are also shown in each graph.