low-quality ponds occupied also inc

low-quality ponds occupied also increases; (5)
high-quality ponds hold more breeding pairs than
low-quality ones; and (6) mean reproductive
success of Little Grebes is the same across all
individuals independent of the quality of the pond
they occupy due to the interplay between pond
quality and competition for resources.
METHODS
Study area and species
This study was carried out in the Vega Baja Valley
(centred on 38°04¢N, 0°51¢W) in southeast Spain
where over 2600 ponds have been built since the
1980s to store water for agricultural purposes
(Sánchez-Zapata et al. 2005). These irrigation
ponds are distributed over an area of 95 840 ha.
The climate is of a semi-arid Mediterranean type
with low mean annual rainfall (300 mm) and a
warm mean annual temperature (18 °C).
The Little Grebe is a small, socially monogamous
diving bird whose pair-bond is maintained for the
whole breeding cycle and beyond, and which
displays territorial behaviour in the breeding season
(Cramp 1998). Its nest is a floating platform
anchored to substrate that normally consists of
submerged vegetation. This species therefore offers
several possibilities for studying reproductive biology
in relation to habitat quality. Little Grebe pairs
stay on the same pond throughout the breeding
period and do not abandon it (R. Sempere unpubl.
data). Consequently, a close relationship between
patch quality and reproductive performance might
exist. Furthermore, these wetlands are relatively
simple to census owing to their reduced emergent
vegetation and their small size (Gutiérrez & Figuerola
1997). This also facilitates the discovery of nests
and the subsequent monitoring of chick growth.
Field procedures
Censuses were carried out between 2002 and 2006
in May and June. In total, 156 irrigation ponds
were monitored during the 5-year study period.
The ponds were randomly selected from those
available and a census of each pond was conducted
annually. The number of Little Grebes was determined
visually using binoculars and telescopes
(Koskimies & Väisänen 1991).
The reproductive performance and occupation
date of ponds by Little Grebes was analysed in a