Day and night use of habitats by northern pintails during winter in a primary rice-growing region of Iberia

Abstract

Loss of natural wetlands is a global phenomenon that has severe consequences for waterbird populations and their associated ecosystem services. Although agroecosystems can reduce the impact of natural habitat loss, drivers of use of such artificial habitats by waterbirds remain poorly understood. Using the cosmopolitan northern pintail Anas acuta as a model species, we monitored home-range and fine-scale resource selection across the agricultural landscape. Individuals were tracked using GPS-GSM transmitters, and a suite of environmental and landscape features were measured throughout the winter seasons. Spatial patterns of habitat use were analysed using generalized linear mixed effect models by integrating field-observations with GPS telemetry. All birds used rice fields as foraging grounds at night and commuted to an adjacent reservoir to roost during daylight. Home-ranges and maximum foraging distances of nocturnally foraging birds increased with decreasing availability of flooded fields, and were positively correlated with moonlight levels. Birds selected flooded rice paddies (water depth range: 9-21 cm) with standing stubble and substrate with pebbles smaller than 0.5 cm in diameter. Density of rice seeds, rice paddy size, and other environmental and landscape features did not emerge as significant predictors. Our findings indicate that nocturnal foraging of northern pintails within rice fields is driven primarily by straw manipulation, water level and substrate pebble size. Thus, the presence of standing stubble in flooded paddies with soft bottoms should be prioritized to improve foraging areas for dabbling ducks. These management procedures in themselves would not increase economic costs or affect rice production and could be applied for dabbling-duck conservation throughout the world.

Fig 1. Study area (Extremadura rice fields; SW Spain) showing reservoirs, rice fields, main infrastructures and urban areas.

The asterisk indicate the reservoir (Gargáligas) used by northern pintails as roosting site during daylight.

Fig 2

(A) Farmer ploughing a rice paddy after fall rains. Standing stubble is depicted in background, whereas in the foreground the paddy is ploughed and partly flooded with stubble incorporated into soil. (B) Flooded rice paddy with standing stubble; the stubble-free furrows created by the harvester are visible.

Fig 3. Variation in the proportion of flooded area (rice paddies with and without standing stubble) through the winter season in a 20 km radius from the diurnal roosting area.

The total flooded area in late winter was a 75.1% lower than in early winter.

Fig 4. Examples of nocturnal home-ranges of northern pintails overwintering at Extremadura rice fields.

Panels A and B: the Pint 06’s MCPs during the second week of December 2012 (1,312 ha; 96% of rice paddies were flooded and moonlight brightness 6%) and during the second week of January 2013 (4,082 ha; 39% of rice paddies were flooded and moonlight brightness 10%). Panels C and D: the Pint 05’s MCPs during the fourth week of January 2013 (4,765 ha; 45% of rice fields flooded and moonlight brightness 88%), and during the second week of January 2013 (2,637 ha; 39% of rice fields flooded and moonlight brightness 10%).