Heavy metal pollution and the sexually selected egg coloration hypothesis

Submitted by Michi on 20 June 2024.

A clutch of six pied flycatcher eggs (photo by Charli Davies) 

This blogpost features the article 'Heavy metal pollution exposure affects egg coloration but not male provisioning effort in the pied flycatcher Ficedula hypoleuca' by Lisandrina Mari et al.  

Text by Lisandrina Mari

Avian eggshell coloration has long been a topic of fascination for naturalists and researchers because it serves a variety of functions, ranging from camouflage to mimicry or even eggshell strengthening. Changes in coloration due to anthropogenic impacts could be an indication of concern with respect to the persistence of populations that breed in man-affected environments.

In the pied flycatcher (Ficedula hypoleuca), the blue-green coloration of eggs has been posited to serve a sexual signaling role. Biliverdin, the pigment responsible for the coloration of eggs, possesses antioxidant properties and should be costly for females who invest its deposition into their eggs. As such, it must represent an honest signal of their quality and that of their offspring. In return, males perceiving this signal should adjust their investment into the brood by increasing their provisioning effort towards nestlings hatched from bluer eggs. Because of the strong implications of this hypothesis for pied flycatcher reproduction, we aimed to investigate whether anthropogenic pollution could affect egg coloration, male investment, or both.

                                                  A male pied flycatcher bringing food to its nestbox, as recorded by our video-cameras. Photo by Lisandrina Mari.

We conducted our study in Harjavalta, Finland, where nestboxes have been installed along a gradient of metal pollution around a smelter. We monitored 71 flycatcher nestboxes during the 2023 breeding season, half of them found in polluted localities within a 5km radius from the smelter. We photographed all eggs and used avian visual modeling to quantify eggshell coloration as perceived by the flycatchers. Four days after hatching, we video-recorded the provisioning behavior of flycatcher parents, quantifying the number of visits made to their nestbox. We expected flycatchers breeding in contaminated areas to lay less intensely colored eggs and males to exhibit less nest attendance in response, compared to those breeding in non-polluted areas.

                                         Differences between the habitats found in the polluted (left) and control, non-polluted areas (right) of our field area in Harjavalta, South-Western Finland. Notice the lack of vegetation on the forest floor in polluted areas, though both pictures were taken mid-May. Photo by Lisandrina Mari.

Our results showed that in polluted areas, the intensity of eggshell blue-green coloration was indeed lower than in control areas. However, when decomposing reflectance into red, blue, green, and UV channels, we found strong differences between areas in the UV-part of the spectrum only. Since UV coloration is structural (i.e, not due to a pigment), this result indicates that metal pollution does interfere with the eggshell structure, and this affects the overall perceived color of eggs. However, we did not find a relationship between color and male feeding effort.

We argue that the lack of relationship between egg colour and male offspring provisioning may be explained by the fact that from an avian vision perspective, the eggs laid by different females were very similar, making discrimination challenging for males (emphasized by the fact that flycatchers are cavity-nesters, and visual discrimination is even harder in dark environments).

While our study suggests that egg coloration might not serve a sexual signaling role in our flycatcher population, it does provide important insight on how anthropogenic changes such as heavy metal pollution can affect eggshell microstructure and coloration, which could still compromise egg color signals.

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