Large-scale spatial distribution of breeding Barn Swallows Hirundo rustica in relation to cattle farming

Figures & data

Figure 1. The study area: (a) Lombardy (black) in Italy (grey) and Europe; (b) Lombardy with the 489 municipalities in the low Po plain (grey), the Parco Regionale Adda Sud (white border) and the 168 point counts (dots) used for map validation; (c) the Park with the 160 censused farms (open circles, farms with no breeding Barn Swallows; grey circles, 1–9 breeding pairs; full circles, 10–76 breeding pairs); (d) predicted colony presence and size at all the 682 farms in the municipalities of the Park based on both logistic and linear models (open circles, farms where the logistic model predicted absence of swallows and the quasi-Poisson model predicted presence of 2 breeding pairs[se =± 0.66, 95% CI = 1–4]; grey circles, farms with 7 predicted breeding pairs [se =± 3.16, 95% CI = 3–17]; full circles, farms with 14 predicted breeding pairs [se =± 1.41, 95% CI = 12–17]).

Table 1. Binomial, zero-truncated Poisson and Poisson glm (the latter fitted by quasi-likelihood estimation) of the presence, colony size and number of breeding pairs per farm of Barn Swallows.

Predictor variable	B	se	z/t	df	P
Presence of Barn Swallow (160 farms)
Intercept	–0.669	0.268	–2.493		0.013
Cattle farming ceased in the last four years	1.922	0.846	2.273		0.023
Cattle farming present	2.984	0.458	6.522		<0.001
Model χ^2 2 = 57.574, P < 0.001, Cohen's kappa = 0.578, overall predictive success = 80.63%
Colony size (109 colonies)
Intercept	1.881	0.085	21.999	106	<0.001
Cattle farming ceased in the last four years	0.362	0.150	2.417	106	0.017
Cattle farming present	0.880	0.090	9.786	106	<0.001
logL ratio test: χ^2 2 = 132.601, P < 0.001, pseudo-R ^2 = 0.149, R ^2 cv= 0.143
Number of breeding pairs per farm (160 farms)
Intercept	0.800	0.298	2.648	157	0.008
Cattle farming ceased in the last four years	1.193	0.524	2.275	157	0.024
Cattle farming present	1.867	0.314	5.952	157	<0.001
F 1,157 = 28.53, P < 0.001, pseudo-R ^2 = 0.304, R ^2 cv= 0.216, φ = 12.265
z-values refer to coefficients in the binomial glm; t-values and df refer to coefficients of the zero-truncated Poisson and quasi-Poisson glm; φ is the dispersion parameter.

Table 2. Poisson glm of the estimated number of Barn Swallow colonies per municipality in the Park.

Predictor variable	B	se	z	P
Intercept	–0.363	0.398	–0.912	0.362
log-number of farms with cattle farming	1.088	0.132	8.222	<0.001
Model: χ^2 1 = 60.401, P < 0.001, pseudo-R ^2 = 0.679, R ^2 cv= 0.686

Figure 2. Estimated number of Barn Swallow colonies in relation to the number of farms with cattle farming at each municipality of the Park. The fitted Poisson glm curve from Table 2 (solid line) and its 95% CLs (dashed lines) are shown.

Figure 3. Predicted Barn Swallow distribution at regional scale: (a) predicted density of Barn Swallow colonies; (b) predicted number of Barn Swallow colonies in all the 489 municipalities of the low Po plain. The municipalities with the predicted maximum density (4.16 ± 0.42 se colonies km⁻²) and the predicted maximum number of colonies (208 ± 67 se) are indicated. Highlighted areas in (a) include 11.7% of colonies in 3.7% of the study area; highlighted area in (b) includes 10.2% of colonies in seven municipalities; small maps represent standard errors of estimates at each municipality.

Figure 4. Correlation coefficients between estimated density of colonies and mean number of Barn Swallow pairs per point count in municipalities at different distance belts (<20 km; 20–40 km; 40–60 km; 60–80 km; and >80 km) from the Park boundary. Bars represent one-sided 95% CLs; numbers represent sample sizes; n.s., P > 0.05; *P ≤ 0.05; **P < 0.01; ***P < 0.001 (one-tailed tests).