YREKA (CBS / AP) — The U.S. Fish and Wildlife Service is blaming a lack of water for a massive die-off of migrating birds at a wildlife refuge on the Northern California-Oregon border.

Up to 20,000 birds have died at the Lower Klamath National Wildlife Refuge since February — the worst die-off in the region in about a decade.

Fish and Wildlife spokesman Matthew Baun says only about half the refuge was flooded this year. That contributed to a higher concentration of the birds, increasing disease transmission.

The dead have been found to be suffering from avian cholera.

Environmentalists say the die-off shows water resources are not being properly managed.

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