esto is neuter and is used with nouns of unknown gender, like the pronoun "it" if you don't know what "it" means. por ejemplo: to say "that is scary" and you don't know what it is, you would use the neuter "eso" for that. if you knew that, say whatever it is, is masculine, you would use "ese" hope this helps
homefire- with estos pájaros, it's because it's the plural form of este / esta and it's a demonstrative adjective, it goes with the noun. But It couldn't be éstos pájaros, because éstos is a demonstrative pronoun, and is neutral, and take place as a subject or an object. EX : estos hombres son abogados (demonstrative adjective) / these men are lawyers. These men are police officers but éstos are lawyers.