The decline in approximately 70% of the world’s bird species is particularly alarming to scientists because .

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Explanation:

The approximately 70% decline in the world's bird species is alarming to scientists because some species of birds have been found to be good indicators of the health status of the environment. Birds are generally easy to detect in an environment and the ability to fly away due to terrible conditions of habitat makes them a good indicator of the health of habitats or environment in general.

A 70% reduction in the species of birds worldwide is thus alarming due to the direct correlation that exists between the species diversity of birds and the health of the environment. It could presumably be an indication that about 70% of the world's habitat has been destroyed or impacted negatively.