Pollution in a river near Bromsgrove has resulted in the death of a kingfisher.The incident, which affected the River Salwarpe to the south of the town, also left two geese covered in oil.
Officials from the Environment Agency (EA) received information about the pollution earlier this week and are appealing for people to come forwards with information.
After visiting the site, the EA found evidence of a "large quantity" of an unknown substance - believed to be some sort of oil - affecting more than three miles of the river.
Floating barriers and oil-absorbent pads have since been placed across a tributary stream in a bid to protect against further contamination.
Environment officer Grant Milne described the incident at the River Salwarpe - which is a tributary of the River Severn - as "thoughtless" and "nasty".
He added: "We have alerted water companies to increase their monitoring of drinking water intakes downstream of the incident to help prevent damage to their water treatment works."
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