Table 2. Model of WNV seropositivity in birds weighing less than 50 grams, based on species ecological traits.

Ecological trait	Odds-ratio (95% confidence intervala)	p-value
Intercept	0.007 (0.001–0.02)	<0.001
Body massb:
<12 grams	Reference
12–18 grams	2.3 (1.0–5.4)	0.02
18–28 grams	3.6 (1.5–8.9)	0.004
>28 grams	3.1 (1.4–8.1)	0.006
Nest height:
Above ground and <4 meters	Reference
On the ground	2.7 (1.4–4.9)	0.003
>4 meters	0.4 (0.1–1.2)	NSc
Exposure of nestlings:
Precocial species	Reference
Altricial species	2.1 (1.2–3.8)	0.01
Use of urban/suburban habitats:
Absence or rare	Reference
Other cases	1.0 (0.4–2.2)	NS
Nocturnal gregariousness:
No	Reference
Yes	0.8 (0.5–1.5)	NS
Breeding sociality:
No	Reference
Yes	0.6 (0.2–1.9)	NS
Migratory status:
No	Reference
Yesd	0.7 (0.4–1.7)	NS

^aParametric bootstrap confidence intervals.

^bClasses based on quartiles of body mass in the 150 European species weighing less than 50 grams.

^cNot significant.

^dMost of the birds spent the European winter south to the Sahara.