1, the influence of the environment.
The environment is one of the reasons why bullfrogs call, because bullfrogs like relatively humid environment, when the environment is suitable, bullfrogs will call happily.
2, air humidity
The bullfrogs can feel the rainy day that will be brought soon, so when the humidity in the air increases, the bullfrogs will also express their mood by calling.
3, Food Abundance
Bullfrogs will also call happily when they have enough food to eat because of the abundance of food, thus producing the call of bullfrogs.
4, Courtship
If it is the mating season for bullfrogs, bullfrogs will also attract the attention of bullfrogs of the opposite sex by chirping loudly in order to show themselves.