。
Flying and building nests are in a bird's nature.
Literal
Fly [listing-たり] nest [object-を] build [listing-たり] do [nominalizer-の] [topic-は] bird [possessive-の] nature [copula-です].
~たり~たりする lists non-exhaustive examples of actions: 'things like flying and building nests.' 本性 (ほんせい/ほんしょう) means 'true nature, inherent character.' The nominalized ~たり~たりするの becomes the topic of the sentence.