Communication is a substitute for fighting in contests over resources. Communication functions to manage the behavior of other animals without risking direct encounters. Females use communication to assess male qualities as mates, thus sexual selection has influenced communication greatly.
The origin of vocal communication is still exhibited in the first land animals, the amphibians. Frogs and toads differ from birds and mammals in that they continue to grow larger after reaching sexual maturity. Larger individuals are able to produce lower pitched calls and they are better fighters than smaller ones. In amphibians, lower pitched calls are both more threatening to other males and attractive to females. Importantly, the physical structure of vocalizations is directly related to the function of these sounds. The relationship between function and a vocalization's physical form is not arbitrary, as is the case with human words.
How is the relation between body size and pitch of calls manifested in birds? The body size/sound pitch relationship is more symbolic and best describes the motivation of the calling bird. Birds use low, harsh, vocalizations when aggressive and high, tonal ones when appeasing or fearful. This relationship is described by the motivation-structural rules model, which relates size symbolism to motivation and is derived from the primitive relation between body size and fighting ability. The motivation-structural rules model is useful in producing hypotheses to test the relation between the physical form of vocalizations and their function.
Vocalizations used for long distance communication, such as most bird song, is a different matter. Here, motivation is usually not as important as sounding close to others. I develop ranging theory to describe how birds assess their distance from one another. The avian ability to resolve very short time intervals between sounds is used to perceive degradation. Degradation is any change in the song as it travels from the singer, such as from reverberation, changes in frequency or amplitude components above those due to spherical spreading of the sound. By comparing a song in their own memory with one that they hear, they are able to judge the distance over which the sound traveled. Ranging theory helps to explain why song learning evolved in some groups as well as the function of dialects, song repertoires, and song complexity.

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The present review discusses whether sexual selection for complex songs modified brain structures in songbirds. If producing complex song is costly in males, song complexity should be correlated with some indexes of brain structures. At the same time, if selecting a potent mate based on song is costly in females, they also should develop some brain specializations correlated with the cognitive effort. Most existing studies focused on the volume of song control nuclei and song behavior. There are two main pathways for song control in the songbird brain: the posterior pathway that directly controls song production and the anterior pathway related with song perception. For the posterior pathway, between-species comparisons found positive correlations between the volume and behavior in males. However, most within-species comparisons failed to find such correlations. Several studies examined the relationship between song perception and the anterior pathway and lesion studies showed reduced selectivity for song in both males and females. Song selectivity and the volume of anterior pathway were positively correlated in females. Taken together, data are equivocal for the production part, but more consistent for the perception part. To study further the relationship between brain evolution and sexual selection in birdsong, methods for measuring song complexity need to be refined and other anatomical indexes besides volumetric methods should be examined.

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To maximize their reproductive success, male birds have to acquire females, ensure their paternity, and increase the survival of their offspring. Males therefore allot their reproductive efforts to several activities during the nesting cycle, because there are trade-offs among the reproductive behaviours. In the Great Reed Warbler, Acrocephalus arundinaceus, there is a negative correlation between singing frequency and intensity of mate guarding, and between singing frequency and male feeding rate. Furthermore, male Great Reed Warblers adjust the time of singing by estimating the risk of paternity loss and abundance of potential secondary mates. In the Bush Warbler, Cettia diphone, males vigorously sing throughout the breeding season, which may be due to frequent female remating due to nest predation, resulting in a continuous source of potential mates for males. Thus, the timing of male singing is influenced by the costs and benefits of singing, which are related to other reproductive activities. Therefore, to understand singing activity, the social and ecological circumstances of the species should be considered, especially the occurrence of extra-pair fertilization, the pattern of parental care, the pattern of female arrival at the breeding area, and successiveness of pairbonds.

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