Context-dependent; can refer to a person who watches birds or a derogatory term.
Como pajero, disfruto de pasar horas observando aves en el parque.
As a birdwatcher, I enjoy spending hours observing birds in the park.
No seas pajero; la vida es demasiado corta para no disfrutarla.
Don't be a fool; life is too short not to enjoy it.