Birds Eggs: The Biology of the Beauty

Wednesday, July 10, 2019

This time of year is mating season for so many wild animals, birds included. A plethora of species have been pairing off, making nests and laying eggs. If you have had the pleasure of birds nesting in your yard, you may have noticed that depending on the species of the bird the eggs they produce can vary greatly in appearance. There are all sorts of different colours, shapes and patterns that make up the array of beautiful eggs you may have encountered.

Although not everything is known about how or why certain eggs become coloured the way they are, several patterns and colours are known accommodations to certain species life history traits. For example, ground nesting birds such as Killdeer tend to have speckled eggs that act as camouflage to hide them from predators. In general, eggs of shorebirds tend to be speckled which helps them to blend with the ground substrate such as rocks, pebbles and sand.

Killdeer eggs, well camouflaged in a ground nest

The opposite of ground-nesting birds whose eggs are constantly exposed are those of cavity-nesting species, whose eggs remain hidden out of sight. Because these eggs are not often exposed to predators, they do not have the same camouflage appearance and are white in colour. Without predator pressure, there is no advantage to cavity-nesting birds having eggs that are not pure white. In fact, any bird species that typically nests in locations with little predator pressure tend to have white eggs. This includes birds that nest at heights such as ospreys and larger birds who are better able to defend their nests such as Canada Geese. 

The shape of the eggs also depends on where the nest may be located. Cliff-nesting birds tend to have long, pointed eggs that will roll in tight circles, and therefore not roll off a cliff. Ground nesting birds tend to have pyriform shaped eggs to maximize the number of eggs that will fit in a nest. 

                                                 Pictured: A Black Tern and its eggs 

Obviously, there is a lot more to explore when it comes to the variance in egg colour, shape and size of bird species. Scientists do not even know all the answers! However, it is something interesting to think about when you come across different birds nests with eggs. Observe the size, the colour and think about how those birds behave. Then try to imagine how these particular eggs would aid the species in flourishing. Are they shaped to avoid falling from heights? Camouflaged to evade predators? It is so amazing to realize the adaptations that are present in each individual species and how birds have evolved eggs that give their young the best chance to make it in this world. 

Happy watching 



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