A novel song performance constraint on buzzes produced through pulsatile expiration differs from constraints on trills in a songbird (Setophaga cerulea)

Song performance constraints for songbirds should reflect physical or neurological challenges of producing sounds, resulting in upper limits to certain measured song features for individuals or populations. These concepts are well-studied for trills where birds repeat syllables or notes punctuated by ‘mini-breaths’ where air is replenished. Tradeoffs can quantify the challenge of simultaneously reaching the upper limit of performance for two song features in a trill (e.g. frequency bandwidth and trill rate). However, many species also produce rapid trill-like vocalisations known as ‘buzzes’, with a a single breath, through ‘pulsatile expiration’. Buzzes are common, but specific performance tradeoffs are not well studied. To test for constraints specific to buzzes and compare with those of trills, we studied songs of Cerulean Warblers (Setophaga cerulea) which include both vocalisations. We describe a novel song performance tradeoff between frequency bandwidth and duration for buzzes, reflecting a constraint of respiratory volume. The tradeoff between frequency bandwidth and trill rate (i.e. vocal deviation) for trills was supported, consistent with previous studies of New World Warblers (Parulidae). This is the first study to demonstrate a constraint specific to buzzes and suggests these vocalisations may be an important component of song performance and quality for some species.