Specific Distinctiveness in the Communication Signals of Birds

Abstract

When using releasers in phylogenetic study, it is essential to consider whether the compared species are sympatric. If they are allopatric, it is often possible to find relationships between all releasers. If they are sympatric it is essential to know something of the function of the releasers, before their taxonomic value can be assessed. Signals that are in some way involved in reproductive isolation are likely to be highly divergent between closely allied sympatric species. They will therefore be useful as characters for specific diagnosis, but of limited value at higher levels of classification. In birds this group will include the male colours of most sexually dimorphic species, especially those that rely on visual recognition, have a short pair bond, and whose reproductive isolation is not yet complete (SIBLEY in press) : also advertisement, pair formation, courtship and some appeasement displays: songs, when they are loud and play an important part in pair-formation: some courtship and perhaps food and nest calls. Releasers of sympatric species whose function discourages specific distinctiveness, will often converge on common types, for the value either of mutually similar signals, or of signals that are for some extrinsic reason most efficient for the context. They are likely to be of limited taxonomic value, even at the specific level. This group includes colours of Batesian and Mullerian mimics; distraction, alarming and aggressive displays used against predators (LACK 1941), alarm calls and displays (HUXLEY 1938) and some nestling and fledgling calls. Releasers selected for moderate specific distinctiveness, with both intra- and inter-specific functions, diverge at a relatively slow rate. They are therefore of little use in specific diagnosis, but are valuable for classifying genera and families. This group includes cryptic colours, especially of female and young; mobbing, pre-flight and aggressive displays; flight and aggressive calls, and some owl-mobbing calls which have secondary functions. A similar moderate specific distinctiveness is found in close-range signals, again valuable in discerning relationships. This group includes colours of eggs and the nestling palate, and perhaps the eye, beak and face colours of adults; some copulatory, submissive and begging displays; soft calls, such as certain alarm cries, and songs which have no function in reproductive isolation, perhaps especially the songs and calls of densely colonial birds.