It seems like the accent over the "e" is an older spelling, though still used. Look at the bottom of the page here: http://www.studyspanish.com/lessons/demonstratives.htm
I wrote "these birds eat much rice." Got it wrong. Reported it. ("a lot" is where you park a car. It gets used too often on duolingo, imho. Every instance of "Me gusta mucho..." or "J'aime beaucoup..." requires "I like ... a lot" and it often won't accept "I like very much..." or "I really like...")
Not that I'd count "a lot" wrong here, but "much" is a superior translation.
The short answer is that the plural of the demonstrative adjective "este - this", is is "estos - these". There is no demonstrative adjective: "estes". (Seems like there should be.) You raise the issue of knowledge that these are birds. The issue of an unknown object bears on whether the neuter form of a demonstrative pronoun should be used, but these are clearly adjectives modifying a specific object, the birds.- not pronouns. At first I was asking myself, "What if they were pets?!" After a while "the light went on" and I realized that had to do with the Personal-A rule for direct objects, not this at all. They don't make this easy, do they? (I hope this is helpful.)