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Tinamus major
A Great Tinamou. A medium sized, greyish/brown bird with a plump body, a thin neck, biggish eyes and pointed beak. Very well camouflaged amongst the undergrowth. Spotted in Manuel Antonio National Park, Costa Rica.
Thick woodland undergrowth.
I agree and Merlin is also pretty sure it is a Great Tinamou. Little Tinamou has a shorter, thicker neck. the other Tinamou don't share the range or look the same, except for possibly the Slaty-breasted Tinamou, Crypturellus boucardi,
https://ebird.org/species/slbtin1/
At the glance, I would agree on Great Tinamou... colours fit (and its barred back), and I would say the size as well (but it's difficult to estimate on pics) - Great Tinamou is the size of hen, or small turkey. Little Tinamou is also accounted on Bird Checklist (http://www.exoticbirding.com/costaric.........) for the park; - but this one would have uniformly coloured back, and it's a half the size of Great, roughly the size of grouse... Other three species of Tinamou present in Costa Rica are not registered in the park.
Hope this helps,
Cheers