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

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 tailelongated
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.