Heavy metal pollution exposure affects egg coloration but not male provisioning effort in the pied flycatcher Ficedula hypoleuca

Heavy metal pollution is known to negatively affect numerous traits in
birds, including foraging, metabolism, immunity, and reproductive success.
In this study, our primary aim was to assess the impact of metal pollution
exposure on the visual appearance of the pied flycatcher (Ficedula
hypoleuca) eggs. Specifically, we focused on blue-green biliverdin-based
coloration, a trait expected to function as a signal of female quality to
males. In line with the sexually selected egg coloration (SSEC)
hypothesis, which posits that males respond to more intensely colored eggs
by increasing their provisioning effort, our second objective was to
investigate whether metal pollution exposure affects this specific
signaling mechanism and subsequent male behavior. Our results showed that
although coloration did not correlate with female quality or male
provisioning effort, egg blue-green coloration decreased in polluted areas
compared to non-polluted control areas. Our analysis of reflectance data
revealed that this difference was due to an increased ultraviolet
reflectance of eggs from polluted areas, likely caused by changes in
eggshell microstructure (e.g. porosity). We therefore propose that metal
pollution exposure may compromise crucial color signals of bird eggs.
Avian visual modeling indicated that eggs laid by different flycatcher
females are generally very similar, making discrimination by males
challenging and perhaps impossible especially in dark cavities. Overall,
our results suggest that the SSEC hypothesis may lack adaptive relevance
for the pied flycatcher in Northern Europe, even in environments
influenced by anthropogenic activities.