The chicken's impressive red comb is thought to be useful for distinguishing sex and breed.
Literal
Chicken [possessive-の] red splendid comb [topic-は] sex [and-や] type [object-を] distinguish time [at-に] is-useful [quotative-と] is-thought.
ニワトリ and トサカ (comb) are in katakana as standard for animal/biological terms. ~と考えられている is the passive 'is thought to be' — standard academic hedging. 見分ける (distinguish/tell apart) is a compound verb. 立派な (impressive/splendid) adds descriptive color.