Color of artificial light at night affects incubation behavior in the great tit, Parus major

Artificial light at night (ALAN) has been recognized as a biodiversity
threat due to the drastic effects it can have on many organisms. In wild
birds, artificial illumination alters many natural behaviors that are
important for fitness, including chick provisioning. Although incubation
is a key determinant of the early developmental environment, studies into
the effects of ALAN on bird incubation behavior are lacking. We measured
nest temperature in nest boxes of great tits during the incubation period
in two consecutive years. Nest boxes were located in eight previously dark
field sites that have been experimentally illuminated since 2012 with
white, green, or red light, or were left dark. We tested if light
treatment affected mean nest temperature, number of times birds leave the
nest (off-bout frequency), and off-bout duration during the incubation
period. Subsequently, we investigated if incubation behavior is related to
fitness. We found that birds incubating in the white light during a cold,
early spring had lower mean nest temperatures at the end of incubation,
both during the day and during the night, compared to birds in the green
light. Moreover, birds incubating in white light took fewer off-bouts, but
off-bouts were on average longer. The opposite was true for birds breeding
in the green light. Low incubation temperatures and few but long off-bouts
can have severe consequences for developing embryos. In our study, eggs
from birds that took on average few off-bouts needed more incubation days
to hatch compared to eggs from birds that took many off-bouts.
Nevertheless, we found no clear fitness effects of light treatment or
incubation behavior on the number of hatchlings or hatchling weight. Our
results add to the growing body of literature that shows that effects of
ALAN can be subtle, can differ due to the spectral composition of light,
and can be year-dependent. These subtle alterations of natural behaviors
might not have severe fitness consequences in the short-term. However, in
the long term they could add up, negatively affecting parent condition and
survival as well as offspring recruitment, especially in urban
environments where more environmental pollutants are present.