James Chubb

Brent geese on the Exe Estuary

It is a misconception that all ‘serious’ science is carried out by people in white coats, Bunsen burner in hand, wearing Farah slacks and creating dangerous levels of static electricity whenever they walk over synthetic carpets. While this stereotype may hold out for the vast majority of experimental chemists (I joke) a great deal of valuable data is gleaned from non-professional sources. I hesitate from using the word ‘amateur’ as it has a certain quality stigma, but volunteers can, and do, input considerably into our understanding of the natural world; here’s a local example.

The annual record of bird numbers on the Estuary has been compiled once again by David Price, co-ordinator of the Wetland Birds Survey - or WeBS counts - on the Exe Estuary. This invaluable, volunteer-led survey uses a monthly count at locations throughout the estuary to piece together a picture of bird populations from one year to the next. It has been occurring here on the Exe since the late seventies, so we have a really important historical data source of population trends over the last 30 years.

Once again 2008 would seem to have been a bad breeding year for the dark-bellied brent geese, the small dark geese which flock to the Exe estuary every September. This environmental observation may have a local impact, but as with any migrating species, the ramifications and reasons may have a distinctly international flavour. With lemming numbers reportedly down in Siberia, Arctic predators turn to brent geese chicks as an alternative food  source and this has been linked to lowered success of population recruitment. Although it sounds like an employment process, ecologists use the phrase ‘recruitment’ to describe how effectively the overall population has grown from the previous breeding year.

It is adjudged that when the brent goose population fails to successfully fledge 15% of its adult population, then the overall population will decline. Apart from a particularly good year in 2005, when breeding success was reported as 30% for the Exe Estuary birds, the percentage of first-year birds recorded has kept stubbornly below the magic 15% level since 1999. This may not necessarily mean that global brent goose numbers fall below this threshold, perhaps birds wintering on other estuaries have better figures? But using the Exe birds as a sample community of the species as a whole, this is a worrying observation.

It is now probably timely to explain how this figure is calculated in the first place. When brent geese first reach the Exe in September and October, it is still possible to identify the offspring from that year’s breeding season, as they lack the distinct white collar of the full adult birds. By recording the numbers of adult and young, we can calculate the percentage and hence work out the breeding success from fledgling birds successfully reaching the winter destination.

This recruitment finding reflects a worrying decline of brent goose numbers on the Exe, from a population spike in the mid eighties of over 3,500 birds, to its current level just below 1,500. 

While in isolation a fall in overall bird numbers could be explained away as an accidental blip 20 years ago, taken in combination with the evidence of poor reproductive success, this indicates a genuinely declining species.

In 2008 the first winter period count of brent geese peaked in February with 1,807 birds recorded throughout the estuary. The estuary then falls silent of their grunting between April and August for breeding, with birds returning in the second winter period from the middle of August. These first returning birds tend to clamour to feed in the eelgrass beds at the southern end of the estuary, refuelling on this rich food source before feeding in the wider estuary as the winter continues.

The second winter period numbers peaked at 1,302 birds in November, somewhat down on the earlier records. Bearing in mind the cold spells we experienced in early 2009, it will be interesting to see if 2009 records manage to generate counts higher than recent years?

The casual observer can be forgiven for looking on the winter estuary and thinking that all is well. The sight of thousands of waders and waterfowl is indeed impressive. But with numbers falling year-on-year, it is imperative that we continue to act to ensure this spectacle is preserved to the best of our ability into the future. There is very little any of us can do to augment Siberian lemming numbers, but we can all help ensure that the Exe Estuary is a seasonal sanctuary for this winter visitor.

A massive vote of thanks must go to David and his volunteers who continue to provide such a valuable records for the Estuary. The Exe may well be one of the most highly designated and protected sites in the South West of England, but without these findings conservationists would have no starting point from which to focus their work.