Recently, research of formal models and specification languages of mobile agents is growing in order to cope with problems in using mobile agents such as security and control of complicated behaviors including migration. Such models and languages require the following features to be widely used in practical applications. The first is customizability to conform to a variety of agent platforms whose features may include various migration and communication modes and various agent architectures. The second is sufficient descriptive power to handle various requirements such as security and safety. However, existing models and languages are not satisfactory in these aspects. In this paper, we propose Pigeon, a specification language for mobile agent applications. We can customize not only the syntax of Pigeon but also its semantics by representing the semantics in a logical framework called RL/R (reflective rewriting logic) and making the representation changeable. The high descriptive power of Pigeon is realized by the reflection feature of RL/R. We illustrate the practical advantage of Pigeon by applying it to an example of a B2B EC electronic catalog system and comparing other models and languages.