What is the topic sentence in the following paragraph from "Bird Counts" by Mike Weinstein?
Bird watchers must keep track of different types of birds. Often, birds are hidden in trees and bushes. Birders
must count them without even seeing them. They identify the birds by their sounds.

The topic in:

Bird watchers must keep track of different types of birds. Often, birds are hidden in trees and bushes. Birders  must count them without even seeing them. They identify the birds by their sounds.

is: Bird watchers' abilities

Explanation:

There are two reasons for backing this answer. In the first place, all of the sentences build and present a profile of abilities a "bird keeper" has. The ext also describes how all of them have them establishing a generalization that means all "bird keepers" develop these abilities to carry on their tasks as 2bird keepers". Therefore, the abilities of "bird keepers" is the correct topic of this paragraph.

invention of the washing machine

in the past, people used sticks and stones or scrub boards to clean their clothing. as it was sweaty, backbreaking work, we owe a world of to james king because he created the model for the modern washing machine in 1851. the most important issue was that this machine cleaned clothes while a person cranked the handle. doing this made people who used it develop strong muscles in their arms. later, in 1908, alva j. fisher created an electric washing machine named thor. this one was not the first automatic washing machine. however, it was one of the most popular models at the time.