Habitat variables affecting nest predation rates at small ponds: a case study of the Little Crake Porzana parva and Water Rail Rallus aquaticus

Figures & data

Figure 1. Distribution of midfield ponds in the study area of the Mazurian Lakeland, NE Poland. White circles – ponds with the Little Crake nests, black circles – ponds with the Water Rail nests, grey circles – ponds with artificial nest. Circles with more than one colour represent ponds where nests of both species were found or where artificial nests were placed.

Figure 2. Marks left on eggshells by known predators (based on nests monitored by camera traps). Egg of the Little Crake (A) depredated by the Marsh Harrier, with thin, horizontal V-shaped groove. Egg of the Water Rail (B) with a deep V-shaped grooves, and specific small hole on the adverse side of the egg made by the Marsh Harrier's tip of the upper mandible (arrows). Egg of the Water Rail depredated by the Raccoon Dog (C) and by the Least Weasel (D), with rounded holes and heavily cut edges (photos by J.J.).

Table 1. Habitat variables measured at successful and depredated Little Crake and Water Rail nests (mean ± se).

Variable	Little Crake		Water Rail
	Successful nests (n = 41)	Depredated nests (n = 29)	Successful nests (n = 21)	Depredated nests (n = 41)
Distance to the land-water edge (m)	10.09 ± 1.25	11.93 ± 2.08	5.55 ± 0.96	9.29 ± 1.54
Distance to open water (m)	3.38 ± 0.48	5.84 ± 1.23	25.52 ± 6.03	29.04 ± 5.42
Water depth (cm)	45.69 ± 3.11	41.96 ± 3.02	16.37 ± 2.12	18.11 ± 1.63
Vegetation density (%)	45.89 ± 1.49	47.07 ± 2.74	67.38 ± 3.06	58.90 ± 2.38
Vegetation height (cm)	61.05 ± 4.60	66.76 ± 5.69	71.24 ± 7.70	79.73 ± 5.67

Figure 3. The causes of Water Rail (white bars, n = 41) and Little Crake (grey bars, n = 29) nest failures. Presented data include both predators identified by camera traps and by marks left on eggshells.

Table 2. Maximum likelihood estimates of nest survival (NS) during standardized incubation and nesting periods for natural Little Crake and Water Rail nests.

	n	Incubation period			Nesting period
		NS	se	95% CI	NS	se	95% CI
Little Crake	63	0.629	0.070	0.506–0.782	0.557	0.079	0.423–0.733
Water Rail	52	0.373	0.029	0.320–0.433	0.210	0.030	0.159–0.279

n, Number of nests; se, standard error; 95% CI, 95% confidence intervals (lower and upper).

Figure 4. DSR (solid line) and 95% confidence intervals (dashed lines) in relation to: (A) the distance of Little Crake nests to the land-water edge, (B) vegetation density in the vicinity of Water Rail nests and (C) vegetation density in the vicinity of artificial nests. All survival rates were predicted according to the top models; curves in (B) and (C) were generated from model averaged DSR values.

Table 3. Summary of model selection results of the DSR for natural nests of the Little Crake, Water Rail and artificial nests. Models are compared according to Akaike's information criterion corrected for small sample size (AIC_c), the difference in AIC_c from the best supported model (ΔAIC_c), model weight (w_i), number of parameters (k) and model deviance. Only models with ΔAIC_c < 2 are presented.

Model	AICc	ΔAICc	wi	k	Deviance
Little Crake nests
S(.) + year + dist_land + dist_water	136.11	0.00	0.13	5	126.03
S(.) + year + dist_land	136.53	0.42	0.10	4	128.48
S(.) + year + dist_land + veg_density	137.18	1.07	0.07	5	127.11
S(.) + year + dist_land + dist_water + veg_height	137.25	1.14	0.07	6	125.14
S(.) + year + dist_land + veg_height	137.27	1.16	0.07	5	127.19
S(.) + year + dist_land + veg_height + veg_density	137.57	1.46	0.06	6	125.46
S(.) + year + dist_land + dist_water + veg_density	137.58	1.47	0.06	6	125.47
S(.) + year + dist_land + dist_water + wat_depth	137.89	1.78	0.05	6	125.78
Water Rail nests
STT + year + veg_density	134.71	0.00	0.21	6	122.56
STT + year + veg_density + dist_water	136.02	1.31	0.11	7	121.81
STT + year + veg_density + dist_land	136.30	1.59	0.10	7	122.10
STT + year + veg_density + wat_depth	136.33	1.62	0.10	7	122.12
Artificial nests
ST + year + veg_density	223.07	0.00	0.20	4	215.04
ST + year + veg_density + veg_height	223.24	0.17	0.18	5	213.19
ST + year + veg_density + dist_water	225.03	1.96	0.07	5	214.97
ST + year + veg_density + dist_land	225.05	1.98	0.07	5	215.00

Covariates included in the models: constant survival rate (.), linear time trend (T), quadratic time trend (TT), year, distance to open water (dist_water), distance to the land-water edge (dist_land), vegetation density (veg_density), vegetation height (veg_height) and water depth (wat_depth).

Table 4. Model averaged parameter (based on models with ΔAIC_c < 2) and cumulative weights (from all possible models) of predictors from models of nest survival of the Little Crake, Water Rail and artificial nests. Covariates ranked according to cumulative weights.

Variable	β	se	∑wi
Little Crake nests
intercept	5.029	1.006
year(2012)	1.252	0.767
year(2013)	−0.556	0.629
dist_land	−0.066	0.027	0.83
dist_water	−0.031	0.020	0.49
veg_height	−0.003	0.003	0.39
veg_density	−0.009	0.008	0.37
wat_depth	0.001	0.002	0.28
Water Rail nests
intercept	−1.450	1.094
T	0.064	0.032
TT	−0.001	< 0.001
year(2012)	−0.103	0.557
year(2013)	1.618	0.571
veg_density	0.042	0.016	0.96
dist_water	0.001	0.001	0.37
wat_depth	0.003	0.004	0.33
dist_land	−0.006	0.009	0.30
veg_height	0.000	0.000	0.27
Artificial nests
intercept	2.243	0.527
T	−0.104	0.021
year(2012)	0.095	0.316
veg_density	0.036	0.010	1.00
veg_height	−0.005	0.004	0.49
dist_water	< 0.001	0.002	0.28
dist_land	0.001	0.006	0.27
wat_depth	0.000	0.000	0.27

Covariates are: constant survival rate (intercept), linear time trend (T), quadratic time trend (TT), year (reference categories are year 2014 for natural nests and year 2013 for artificial nests), distance to open water (dist_water), distance to the land-water edge (dist_land), vegetation density (veg_density), vegetation height (veg_height) and water depth (wat_depth).