It's raining.

Swallows don't get wet on purpose. Just because they choose to stand on the wire doesn't mean they are not afraid of the rain. In fact, swallows' feathers are waterproof, and raindrops easily slide off their feathers, so they generally don't get wet. In addition, swallows will maintain a certain posture when flying and standing to reduce the possibility of getting wet by rain.

Swallows choose to stand on the wire not because they are not afraid of rain, but because they need to stay dry to avoid hypothermia. Swallow feathers are waterproof, but not completely waterproof. If swallows stay in the rain for a long time, their feathers may become too wet, leading to hypothermia, which is fatal to them. Therefore, swallows will stay in dry places in rainy days, such as wires and branches, to keep their body temperature and feathers dry.

In a word, the swallow did not deliberately get wet, but chose to stand on the wire in rainy days in order to keep dry and warm.