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Avian welfare
Mass mortality of starlings roosting by a roadside
  1. J. P. Duff
  1. AHVLA Diseases of Wildlife Scheme, AHVLA – Penrith, Merrythought, Cumbria CA11 9HS
  1. e-mail: paul.duff{at}ahvla.gsi.gov.uk

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IN recent winters mass deaths of flocking wild birds, popularly termed ‘aflockalypses’, usually caused by adverse weather or in-flight trauma, have made the news. With winter commencing, the time when passerine flocking and communal roosting usually occurs, it seems appropriate to mention a flock mass mortality incident in the UK, which caused some public concern and was potentially dangerous to drivers. This was investigated by the AHVLA Diseases of Wildlife Scheme, a national programme set up by Defra in 1998 to investigate disease and mortality in wildlife.

In the incident, in February 2011, 88 starlings (Sturnus vulgaris) were submitted from a busy trunk road joining the A75 and the A74(M) near Gretna, Scotland. The carcases were found on the edge of the carriageway along a short stretch of the road. A driver on the road reported the birds at 07:00 and said that starlings were flocking normally overhead at the time. Only a few birds were crushed and the impression was that the incident had occurred earlier that morning, which was damp and clear (no fog) although there were strong winds.

Postmortem examination was undertaken on a sample of 20 birds; all were in good body condition, and clear evidence of trauma was found in 17 of the 20 with ruptured livers, haemorrhages in lungs, blood clot in tracheas and mouths. Gizzards were empty but intestines in all contained ingesta. There were no obvious disease processes and avian influenza virus was not isolated from tissues.

The cause of death was trauma and the distribution of carcases indicated that the birds had collided with vehicles, although this was not reported and it is possible (although less likely) that the birds flew vertically downward and impacted on the road surface. A site visit was undertaken two days later to assess why the incident had occurred and a further 20 carcases were found (Fig 1). On both sides of the carriageway and at intervals very close to the road edge were mixed species tree plantations. It was clear from the amount of faecal material under the trees (Fig 2) on both sides of the road, that these were very large avian roost sites. The roosts were so close to the edge of the road (within 2 metres) that there was an obvious risk that if birds did not fly with precise orientation then they were likely to cross traffic streams on entry, exit or if frightened at the roost, and collide in-flock with traffic.

FIG 1

Carcases of starlings at the edge of the road two days after the incident occurred

FIG 2

Bird faeces on foliage indicating a communal roost in the trees beside the road

Although apparently very rare in occurrence, flocks of birds flying across traffic streams pose a risk to motorists, particularly if the motorists alter course to avoid collisions with oncoming birds. Motorists may also be concerned when driving past groups of dead birds and when surviving, traumatised birds are seen on the road. The findings in the present incident suggested that it was the proximity of the large roost to the road edge that increased the birds’ susceptibility to fly into traffic.

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