Relationship between Channel Island Melospiza melodia (Song Sparrow) bill size and vegetation, seed extraction time, bite force, and climate (2014-2016)

Inferring the environmental selection pressures responsible for phenotypic
variation is a challenge in adaptation studies as traits often have
multiple functions and are shaped by complex selection regimes. We provide
indirect evidence that morphology of the multifunctional avian bill is
primarily shaped by climate and thermoregulatory ability in Melospiza
melodia (Song Sparrows) on the California Channel Islands. Our research
builds on a study in Song Sparrow museum specimens that demonstrated a
positive correlation between bill surface area and maximum temperature,
suggesting a greater demand for dry heat dissipation in hotter, xeric
environments. We sampled contemporary sparrow populations across three
climatically distinct islands to test the hypotheses that bill morphology
is influenced by habitat differences with functional consequences for
foraging efficiency and is related to maximum temperature and,
consequently, important for thermoregulation. Measurements of >500
live individuals indicated a significant, positive relationship between
maximum temperature and bill surface area when correcting for body size.
In contrast, maximum bite force, seed extraction time, and vegetation on
breeding territories (a proxy for food resources) were not significantly
associated with bill dimensions. While we cannot exclude the influence of
foraging ability and diet on bill morphology, our results are consistent
with the hypothesis that variation in Song Sparrows’ need for
thermoregulatory capacity across the northern Channel Islands selects for
divergence in bill surface area.