BBC - Earth News - Poisoning drives vulture decline in Masai Mara, Kenya
It looks like these Kenyan vultures are not being poisoned on purpose, but accidentally as poisoned carcasses are left to eliminate predators. But the effect is the same. As the article points out, vultures are critical for the ecosystem.
When I lived in Burkina Faso we used to watch vultures circling the village. They flew incredibly high came down at the right time to feed. They always seemed to know what was going on. On days when the market was running they were especially numerous. It was cheap entertainment on a hot afternoon.
I hope the poisoning does stop so that the vultures can return to a normal number.