The Crossness Constant Effort Site

The group started a new Constant Effort Site at Crossness Nature Reserve near Thamesmead, Greater London in 2003. The site is compact and bordered to the south and east by the some of the last remaining Thames-side grazing marshes, to the west by a sewage works and golf course and the river Thames to the north. Beyond these fringe habitats is mostly industrial and residential developments, making Crossness something of an oasis in a predominately urban environment. The habitat within the reserve is mainly reed bed and emerging scrub, with elder, hawthorn and bramble the main constituents.

The CES visits during 2005 comprised some 70 hours of fieldwork or 33,360 net/foot hours. Although the visits are for a standard time period and with a fixed footage of mist net, the catches do vary more than is accountable for by the progressive increase in juvenile birds added to the population during the breeding season. During the 2005 CES season the weather was generally conducive to mist netting with few very breezy days and no rain-shortened visits. However, there were a number of sessions when the skies were clear and temperatures rose quickly during the morning to the mid eighties Fahrenheit, which made the nets more visible and caused the birds to retreated to the cool shade of the bramble bushes and phragmites beds and reducing the catches to some extent as the nets were closed early.

Unfortunately, due to other ringing commitments, we were not able to complete all twelve visits this year. Despite this, the overall number of birds captured during CES sessions was 11% higher than in 2004.

The more commonly caught resident species showed mixed fortunes, with poor adult survival rates for Wren, Dunnock, Robin, Greenfinch and Goldfinch but Blackbird, Blue Tit, Great Tit and Reed Bunting all did well. On the other-hand productivity was generally good with substantial increases in the number of juvenile Wren, Dunnock and Blackbird. Adult tit numbers on the reserve in summer are low compared with a typical woodland site, and although both Blue and Great Tit nestling survival was high, the number of juvenile Blue Tit recorded after fledging was low. It may be that these birds had immediately moved to more productive feeding areas, whereas the Great Tit juveniles remained on the reserve. Most finch species showed low adult survival but productivity was high for Chaffinch and Greenfinch, and even the Linnet, which is sadly a scarce species locally these days, seems to have had a successful breeding season.

Of the migrant passerines using the reserve, the two principle breeding species Reed Warbler and Whitethroat, had a disappointing season with juvenile numbers down 27% and 29% respectively. Blackcap numbers were up for the second year running with adults up from 7 to 29 and juveniles up 114%. Other warbler species had a poor year, with the exception of Lesser Whitethroat, whose small breeding population appeared to do well.

The British Trust for Ornithology has recently published provisional results from the national CES database. This shows that in general, adult survival increased amongst the resident species but with the exception of Blackcap, this was reduced in those migrants that are common at Crossness. Productivity was generally poor, with juveniles of most species recorded in reduced numbers, with the notable exception of Dunnock and Greenfinch.