I would have expected the above sentence to use てい form (与えていた) as it seems like a repeated action in the past. The speaker is talking about their childhood chores, so I thought that it translated roughly as "At that time, I used to feed the animals myself with food that my mother put in a container."
I'm assuming the reason 与えた is used instead is the word 当時 (at that time). Is this correct? Are there any another common adverbs of time I should watch out for?