Since Wells Fargo & Co. launched its hostile offer for First Interstate Bancorp last October, a wave of small-to-midsize banks have implemented shareholder rights plans.

At least 10 banks and thrifts have announced the plans, known as "poison pills" because they make it harder for a would-be acquirer to devour a target. And more are expected to follow as industry watchers predict a new age of aggressiveness among banks.

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