Animal signals, such as the complex songs of birds, tend to be costly.
A bird, by singing, may forfeit time that could otherwise be spent on
other important behaviors such as foraging or resting. Singing may also
advertise an individual’s location to rivals or predators and impair the
ability to detect their approach. Although these types of cost may be
important, discussions of the cost of singing have generally focused on
energy costs. Overall the evidence is equivocal: for instance, while
Eberhardt found increases in energy consumption during singing for
Carolina wrens, Chappell found no effect of crowing on energy
consumption in roosters.
To obtain empirical data regarding the energy costs of singing,
Thomas examined the relationship between song rate and overnight
changes in body mass of male nightingales. Birds store energy as
subcutaneous fat deposits or “body reserves”; changes in these reserves
can be reliably estimated by measuring changes in body mass. If
singing has important energy costs, nightingales should lose more body
mass on nights when their song rate is high. Thomas found that
nightingales reached a significantly higher body mass at dusk and lost
more mass overnight on nights when their song rate was high.
These results suggest that there may be several costs of singing at
night associated with body reserves. The increased metabolic cost of
possessing higher body mass contributes to the increased overnight
mass loss. The strategic regulation of evening body reserves is also likely
to incur additional costs, as nightingales must spend more time foraging
in order to build up larger body reserves. The metabolic cost of
singing itself may also contribute to increased loss of reserves.
This metabolic cost may arise from the muscular and neural activity involved
in singing or from behaviors associated with singing. For example, birds
may expend more of their reserves on thermoregulation if they spend
the night exposed to the wind on a song post than if they are in a
sheltered roost site. Thomas’s data therefore show that whether or not
singing per se has an important metabolic cost, metabolic costs
associated with singing can have an important measurable effect on a
bird’s daily energy budget, at least in birds with high song rates such as
nightingales.