Aerial counts of waterbirds in Doñana marshes

Aerial surveys were conducted over an area of 91890 ha (Rendon et al. 2008), including four main flooding areas: Doñana National Park, Northern part of Doñana Natural Park together with adjacent, unprotected areas, Veta La Palma (Main Island), and Bonanza salt pans. The three first placed on the right hand of the Guadalquivir River and the last one on the left hand. Flights were conducted on a small twin engine plane under good flying conditions (wind and visibility), avoiding rainy, windy or foggy days. The plane normally departs from Jerez de la Frontera International Airport (XRY), although 7 different airports or airfields were sometimes used. For this reasons, different tracks were followed to cover the main flooding areas above described. Flight speed was between 150 and 200 km/h depending on wind speed and direction and flight height was around 100m range: 50m - 300 m. Depending on the number of birds, the track was straight (few birds) or circling over the area to ensure all birds fly, thus being easier to be counted. Aerial surveys were normally conducted in the morning, and lasted 1.5?3 h, depending on the extent of the study area inundated at that time. Three experienced and especially trained counters were in charge of counts. Before a counter was replaced, the new and the old one worked together for a year, ensuring a common procedure to minimise observer effects. Flock counts were conducted following Tamisier and Dehorter (1999). Counts were attributed to any of the 58 localities that could be distinguished from the air, although some localities were combined (especially in the natural marshes) when water levels were so high they obscured the boundaries. Only those species that fly when the plane approaches and/or are big enough to be distinguished from the air are considered.