<ol>
<li>Among birds [that forage] in tree trunks, nuthatches are the only ones that [regularly feed] with their heads facing downward, [when they often find] food bank in crevices [overlooked in] their upward search. [No error]</li>
</ol>
<p>I wonder if [when they often find] is wrong, since their is no specific time mentioned. But how do we fix it?</p>
<ol>
<li>The Bretons who began emigrating to the U.S from the Brittany region of France in the early 20th century were distinguished from other French citizens by their Celtic origion, [but about 40 percent of those who emigrated spoke] a Celtic language closely related to Welsh.
a) the same
b) about 40 percent of whom spoke
c) about 40 percent of which were speaking
d) with about 40 percent of those who emigrated speaking
e) where 40 percent of them were speaking</li>
</ol>
<p>D is fine, but rather long. Is there a right yet more concise answer?</p>
<p>Thanks a lot.</p>