Why do birds lay eggs that vary in colours and patterns? This question has been asked time and time again with no clear evolutionary explanation. Many hypotheses have been suggested, mainly focussing on the role of these attributes working towards camouflage, personal signatures to recognise each egg from brood parasites, or as a means to strengthen the shell itself. Within Australia, some iconic species such as the Emu (Dromaius novaehollandiae) make us speculate that dark green eggs must be coloured to protect the embryos from cooking in the harsh desert, but this is just one theory amongst many.
The science around bird eggs (oölogy) has become more popularised in recent years, resulting in a small flock of scientists dedicated to discovering the nature and purpose of this captivating ancient trait. Something widely understood is that traits associated with egg colouring tend to be similar within closely related groups of species, with little variation within the same species. However, variation within a species is observed in some cases, and this is the interest of my own research.
We made use of eggs collected by early naturalists and placed in museum collections to ask the eternal question of what is driving variation. We focused on the iconic Australian Magpie (Cracticus tibicen), a species whose range covers almost seven million square km of Australia, half of which is parasitised by a cuckoo. The range also includes the extremes of environmental parameters, ranging from the Tasmanian forests to the arid outback to the humid tropics. These variables allowed us to ask questions about why the same species of bird would lay such variable eggs. Is it due to the pressure and arms race with the cuckoo? Is it to do with climate? Or perhaps a combination of multiple factors?
In this study, we found considerable variation in background colour, with eggs varying from blue to white to brown to red; an unusual trait within a bird species. We also found extensive variation in patterning, some eggs being fully clear of marks, others were completely covered in patterns. Our results were mixed, and inconclusive about the cuckoo’s influence on the appearance of these eggs. We showed that environmental measures contributed to this variation, however it was not even nearly as simple as we were hoping. We were eager for results to indicate that all subspecies would be similar, or perhaps all environment types would house similar eggs, and it was not the case.
While not fully shedding light on our question, we have partly explained the reasoning for such amazing variation in our renowned Australian magpies. Climatic factors explain a small variation; the principal driver behind such unusual variability, however, remains unaccounted for. These birds are common around the land, and yet no one knows about this mysterious laying habit; hopefully this will allow people to see our swooping friends in a new light- they are protecting their precious and enigmatic eggs.