Habitat use of little egrets in Dorset
Wednesday 16th July 2014
With their ability to fly great distances to find suitable habitat, birds have arguably the greatest ability of all animals to move into and exploit new areas outside of their native range in response to environmental change. In particular, many bird species have responded to climate change by shifting their range northwards. One example of a bird species shifting its distribution is the little egret (Egretta garzetta), a piscivorous bird which began to colonise southern England during the 1980s. The little egret has spread rapidly, and has now been spotted as far north as Shetland. However, despite this well-documented spread, habitat use by little egrets within the UK, and how such patterns of habitat exploitation compare with native piscivores, remains unknown.
As part of our work in the Individual-based Ecology group at Bournemouth University, we wanted to examine how little egrets were using habitat within areas that had been recently colonised. In our recently published article in Hydrobiologia we examined the overlap in habitat preferences within a river catchment (the River Frome in Dorset) between the little egret and two native species, the grey heron (Ardea cinerea) and great cormorant (Phalacrocorax carbo). These three species are the main piscivorous bird species found in lowland river catchments in the UK. The River Frome catchment is a mosaic of different habitat types, with the key available habitats for a waterbird being river, lake, ditch, pasture field, arable field and estuary. We carried out repeated surveys of the River Frome catchment in 2009 and 2010 between Maiden Newton and Poole Harbour, recording where individuals of each species were observed. For each of the habitat types we calculated an electivity value, taking into account the number of birds of each species using that habitat, and the total available area of that habitat. A positive value indicated that a species had a preference for that habitat, whilst a negative value indicated that the species was avoiding that habitat.
We found that all three bird species showed strong preferences for river habitat in all seasons, with the other available habitat types used as auxiliary feeding areas. This seasonal use of multiple habitat types was consistent with little egret habitat use within its native range further south in continental Europe. Within the River Frome catchment we found strong egret preference for aquatic habitats, in particular freshwater habitats, compared with pasture and arable agricultural habitat. Little egrets showed greater shared habitat preferences with grey herons, the native species to which little egrets are most morphologically and functionally similar. Our research was the first study to quantify little egret habitat preferences within a recently colonised area of the UK.
Our study showed how a simple electivity index, informed by the types of data routinely collected for bird populations, could be used to quantify and compare the habitat preferences of different species. For more information on this research please see the published article in Hydrobiologia:
Wood, K.A. & Stillman, R.A. (2014). Do birds of a feather flock together? Comparing habitat preferences of piscivorous waterbirds in a lowland river catchment. Hydrobiologia. DOI: 10.1007/s10750-014-1921-6