Data from: Parental control: Ecology drives plasticity in parental response to offspring signals

Bird species differ in their parent-offspring interactions, and these
differences may be caused by environmental variation. When food is
plentiful, the chicks that are begging the most are fed the most. When
food is scarce, bird species instead feed the largest offspring. While
this variation could be due to parents responding to signalling
differently based on food availability, it could equally be due to
offspring adjusting their behaviour, to variation in information
availability, or to confounding factors not related to the environment. We
tested between these competing explanations experimentally, by
manipulating food availability in a population of wild great tits, Parus
major, while standardising offspring size and behaviour by creating mixed
cross-fostered broods just before filming. To simulate variation in
ecological conditions, we experimentally manipulated food availability in
an alternating pattern: half of the broods received supplemental food
(mealworms and waxworms) in a feeding tray given to parents, while the
other half experienced natural conditions. We wanted all parents to be
exposed to equivalent information from their broods during filming, so
that we could rule out the possibility that offspring are driving any
differences in parental provisioning preferences. We therefore
standardized brood size and offspring supplementation history across all
broods immediately before filming on day 8. We measured chick body size by
weighing chicks during the cross-fostering, and measured begging intensity
from our videos. We painted all chicks with a dot of red, non-toxic
acrylic paint on the head just before filming, so that we could
individually identify chicks in the videos. All videos were coded by the
same observer, blind to the experimental treatment and to chick weight
ranks. The order in which the observer coded the videos was random
concerning whether parents were supplemented and unsupplemented. Adult
identity was determined by the difference in crown feather glossiness of
males and females. This isolated the effect of parental strategies while
holding offspring begging and size constant across treatments, but with
sufficient variation within broods to generate usable information for
parents. We found that when food was more plentiful, parents were: (1)
more likely to preferentially feed the chicks that were begging the most;
and (2) less likely to preferentially feed larger chicks. Overall, our
results suggest that parents have more control over food distribution than
suggested by scramble competition models, and that they flexibly adjust
how they respond to both offspring signals and cues of offspring quality
in response to food availability. Consequently, depending upon
environmental conditions, parental plasticity and predictably different
signalling systems are favoured.