Since we are about midway through the month of May, a topic of interest among birders right now is the nesting and breeding season which is now underway here in our neck of the woods and most other regions in Canada and the United States.

While we are busy identifying nests and hope that they are soon brimming with little, hungry mouths, there is undoubtedly a lot that goes into those eggs before they hatch.  Building a nest, attracting a mate and laying and incubating eggs is a dutiful process for mother birds and with that in mind, these birds will do almost anything to ensure the survival of their brood.

 

The unfortunate reality is that many of the eggs laid by birds each and every year do not make it to the hatching phase.  Factors such as weather, accidents, and predators play a vital role in the mortality rate of chicks both inside and outside of the egg.

In an effort to increase the chances that their offspring will survive, some female birds will partake in what is referred to as Egg Dumping, also known as Intraspecific Brood Parasitism.  Egg dumping occurs when female birds lay their eggs in the nests of a variety of other species of nests, often choosing a species that is relatively similar to their own.

They will do this in an attempt to further protect their eggs and young.  As “host” mothers often can have the ability to notice and reject foreign eggs, those birds dumping their eggs will do their very best to camouflage them among the natural eggs.  To take it even further, oftentimes the dumping female will actually go as far as to remove one of the host eggs, in an effort to conceal the dumped egg.

Where this hurts birds is how the host mother reacts and depends greatly on the number of dumped eggs in any given nest.  This activity is very common among waterfowl species and wood ducks, as an example, will simply abandon a nest that has too many eggs.  Tree sparrows, on the other hand, have been found to have an innate ability to detect foreign eggs, but will not always reject them.

Another way hosts pay for this type of behavior is when their own hatchlings are born and eventually killed by parasitic chicks that were deposited by another species of bird.

Common Egg Dumping Species

  • Wood ducks
  • Canvasbacks
  • Redheads
  • Common eiders
  • Hooded mergansers
  • Tree Sparrows
  • Starlings