Observation on Brent Goose disturbance ............................ Mark Nowers

Observation on Brent Goose disturbance

On Saturday 5th May 2018, I was out in Holbrook Bay on the Stour Estuary counting Dark-bellied Brent Geese as part of a coordinated venture across the Stour and the Orwell. It has long been known that Brents can be found on the Stour in significant numbers at this time of year. The combined total on the day was 1,733 birds (1,595 on the Stour). Standing on a seawall in May, listening to Reed Warblers in the borrow dykes fresh in from sub-Saharan Africa whilst scoping Brents that are still 6000km from home, leaves me in awe of the natural world. It is well known that Brent Geese are particularly sensitive to disturbance. Their intolerance of low-flying aircraft, with helicopters in particular, being well researched. An excellent summary of the issues they face was produced by Allan Drewitt in his Birds Network Information Note - Disturbance Effects of aircrafts on Birds (1999). Within this he references Owens (1977) which stated that “Brent geese on the Essex coast were put to flight by any aircraft up to 1.5km away when at altitudes below 500m”.

Owens,N.W. (1977). Responses of wintering Brent Geese to human disturbance. Wildfowl 28:5-14

Whilst counting a small group (c.50) Brents close in to Holbrook Creek, my attention was drawn to a fastmoving speed-boat on the south shore of the estuary in the main channel, just off the Wrabness beach huts. Given the significant wake it caused, the 8 knot speed limit was clearly being breached and the engine noise was enough to cause a proportion of the flock I was watching to become alert (heads up/calling). It is a common misconception that disturbance in ornithological terms is the Flight Initiation Distance (FID). That is the last resort. What I witnessed was the Alert Distance (AD) - the distance at which a bird(s) changes behaviour in response to a disturbance source. Checking the OS map and GIS later on, I estimated the boat was over 2km from the flock! Whilst I recognise that this is just an anecdotal observation, it does serve to reemphasise this species sensitivity and I felt it warranted wider attention.