IF YOU want to save a species you need to know how many individuals are left. But it turns out that simply counting animals at known hangouts is no good: rare species may be there, even when you can't see them.
Todd Katzner of West Virginia University in Morgantown and his team monitored eastern imperial eagles, Aquila heliaca, living on a nature reserve in Kazakhstan. Only a few thousand of these birds remain.
The number of breeding birds is relatively easy to establish because they stay at their nests. But Katzner focused his study on young non-breeders. These can step in if the breeders die for any reason, and stabilise the population.
The team used two methods to estimate numbers: counting the birds as they left their roosts, and collecting shed feathers for genetic analysis. In 2004, the genetic analysis identified 308 non-breeders, while visual surveys found only 21 (Animal Conservation, DOI: 10.1111/j.1469-795.2011.00444.x).
"What you see isn't what you get," Katzner says. "Most conservation plans are centred around nests and breeding pairs. In reality there's a cryptic population that you don't see but is really important."
Monika Böhm of the Zoological Society of London says genetic studies like these are crucial for monitoring endangered species, especially those that are tricky to observe conventionally.
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