An ornithologist at Queen's University is studying the development time of recently hatched black-capped chickadees on Wolfe Island. He randomly samples 20 nests from across the island and measures the weight of each new hatchling in the nest. He repeats this sampling after 1 week, and then again after 2 weeks.

What term best describes each nest included in this study?

sampling unit

observation unit

both sampling and observation unit

measurement unit