Shake Shack unveils its mystery chicken item

'ChickenShack' debuts in Brooklyn
'ChickenShack' debuts in Brooklyn   

The secret's out.

Known for his hamburgers and custard, Shake Shack on Tuesday debuted the ChickenShack, a crispy all-natural and antibiotic-free chicken breast with lettuce, pickles and buttermilk herb mayo. Starting July 7, the sandwich will be available exclusively in its Brooklyn locations for a limited time.

Shake Shack whacked
Shake Shack whacked   

The new item is the chain's first ever chicken sandwich. Previously, the only chicken dish available had been the chicken dog, a hot dog made with chicken, apple and sage sausage.

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The move follows a trademark filing of the term "chicken shack" in April by a subsidiary of the chain. This news sent the restaurant's stock surging, hitting an all-time high of $96.75 on May 22. Since then, its shares have sunk. Shares received a further pounding Tuesday after Morgan Stanley analysts downgraded the stock, sending it down nearly 10 percent.

Shake Shack's foray into a chicken sandwich comes as chicken chains Bojangles' and privately held Chick-fil-A posted strong growth with poultry-packed menus.