Black-throated Green Warbler
1) Black-throated Green Warbler = 2 distinct song types: 1 attracts females 1 repels males. But 100 different messages for Marsh Wrens? A lot of redundancy?
2) A great number of song types may be a trick to fool intruders--intruders think the territory if full of males! But birds do not call more variably at beginning of season and once intruders caught on, they would be favored since they would leave more offspring in a relatively empty habitat.
3) Sexual selection and competition for mates--FEMALES use # of song types as an index of males genetic fitness --functions like color.
4) A genetic marker--the more closely birds are related, the more similar the song type and make up.
a) Philopatric birds are aided in knowing birthplace.
b) Recognition of kin, so aggression reduced against kin = "kin selection"