Plastic use as nesting material can alter incubation temperature and behaviour but does not affect yellow-legged gull chicks

Optimal incubation temperature is crucial for embryos' development and survival. With the increasing use of plastics in gulls' nests, it is essential to understand how their incorporation affects incubation temperature, parental behaviour, and hatching success. Considering this, we conducted an experiment where plastic was introduced into yellow-legged gulls (Larus michahellis) nests. The experiment comprised three groups: a control group, a group with low amount of plastic, and a third with a high amount of plastic. This design allowed us to investigate the effects of plastic on 1) the heart rate of incubating adults, 2) the number and duration of adults' absences from their nest, 3) how the presence or absence of the adult influenced egg temperature, and 4) chick hatching success, physiological parameters, and bill colour phenotype. We observed that incubation temperature was consistently higher in nests with plastic. The number of absences was higher in the low plastic group at increased temperatures, though the duration was significantly lower in both plastic groups than in the control, possibly to mitigate the effects of heat stress. During higher environmental temperatures, heart rate was higher for the high plastic group. The increase in heart rate in the low plastic group was less pronounced with increasing environmental temperatures. No significant effects were observed on hatching success or in the health condition of young chicks, except for high values of haemoglobin in both plastic groups, which might indicate dehydration.