Original Article

Behavioral Ecology and Sociobiology

, Volume 54, Issue 5, pp 465-471

First online:

Do male barn swallows (Hirundo rustica) experience a trade-off between the expression of multiple sexual signals?

  • Nicola SainoAffiliated withDipartimento di Biologia, Sez. Ecologia, Università degli Studi di Milano Email author 
  • , Maria RomanoAffiliated withDipartimento di Biologia, Sez. Ecologia, Università degli Studi di Milano
  • , Roberto SacchiAffiliated withDipartimento di Biologia Animale, Università degli Studi di Pavia
  • , Paola NinniAffiliated withLaboratoire de Parasitologie Evolutive, CNRS UMR 7103, Université Pierre et Marie Curie
  • , Paolo GaleottiAffiliated withDipartimento di Biologia Animale, Università degli Studi di Pavia
  • , Anders Pape MøllerAffiliated withLaboratoire de Parasitologie Evolutive, CNRS UMR 7103, Université Pierre et Marie Curie

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Abstract

Some sexual selection models envisage exaggerated male secondary sexual characters to be costly and therefore reliable indicators of the quality of potential mates to choosy females. If male secondary sexual characters have a natural selection cost, they may be linked to each other by reciprocally constraining relationships that would prevent individual males from increasing their level of multiple signaling. Barn swallow (Hirundo rustica) males have at least two costly signals relevant to socio-sexual interactions: tail length and song. Under the hypothesis that a trade-off exists between male signals, we manipulated the maintenance cost of tail ornaments to test whether this reduced the quantity and quality of song, a condition-dependent, phenotypically plastic signal. Contrary to our expectation, tail elongation had no effect on singing activity and song complexity. However, tail-elongated males produced songs with longer terminal parts ('rattles'). Long rattles are associated with highly competitive social contexts and high circulating levels of testosterone, suggesting that tail-elongated males were more frequently involved in either male-male aggressive or inter-sexual interactions. Therefore, this study shows that males are not displaying multiple signals at the maximum possible level, implying that this system is open to unreliable communication. However, long-term trade-offs between signal expression and viability may prevent males from displaying both signals at higher rates.

Keywords

Condition-dependent signals Hirundo rustica Multiple signals Reliable communication Sexual selection