This bluebird nest had six eggs, but one was significantly larger and slightly lighter in shade than the others. Sometimes an extra large egg is a double-yolked egg. In this case, when it hatched (one day before the other eggs), the result was identified as a starling. Starlings had been attempting to nest in nearby gutters, and the entrance hole of this box had been enlarged by woodpeckers, permtting starlings (which are larger than bluebirds) access. A hole reducer will be placed on the box to prevent future invasions. Photo provided by L. Henke of NC.
Egg dumping is when a female bird lays her egg(s) in the nest of another bird, sometimes creating very large clutches. Egg dumping is not uncommon with chickadees and wood ducks. The normal clutch size for wood ducks is 10 to 15, but nests have been found with 30+ eggs, which results in low hatch rates.
When egg dumping occurs between members of the same species, it is called conspecific or intraspecific brood parasitism. (Note: conspecific brood parasitism has been documented in at least 236 species of birds (Yom-Tov 2001), and also occurs in insects, fishes and amphibians.)
Also see obligate brood parasitism, which occurs with cowbirds. Cowbirds do not build their own nest, but instead parasitize the nests of other birds, taking out one egg and replacing it with their own.
In conspecific brood parasitism, the dumping female may have been unable to obtain a nest site, lost a nest site, or had their own nesting attempt disrupted (e.g., through predation.) Sometimes there is a floating population of females unable to find a nest site, so they dump their eggs. Floaters are usually younger and smaller than breeding birds. In one study (1999, Sandell and Diemer), female starlings without a known nest were trapped and then tracked. 47% either laid an egg in a fake nest, or carried a fully developed egg in their reproductive tract.
The advantage to the dumping bird is that they get to reproduce, and it may increase the likelihood that some of their eggs escape predation. The disadvantage to the dumpee is that they may end up spending precious energy rearing offspring that do not carry their own genes.
The reaction of birds to dumped eggs varies. Some birds (e.g., Wood Ducks) may abandon a nest with too many eggs. Some birds (like Tree Sparrows) do recognize dumped eggs, but do not always reject them (Polacek et al, 2013). Nesting starling females will remove eggs deposited before they start to lay. Females who are laying parasitic eggs often take out one of the host eggs at the same time.
Sometimes the eggs from different females will have a slightly different shape or shading.
If you are not sure what species of bird laid an egg, remember that it is illegal in the U.S. to disturb or destroy the eggs of native birds without a permit from the U.S. Fish & Wildlife Service. There are exceptions for non-native, invasive birds: e.g., the eggs (and nests, young and adults) of House Sparrows and Starlings can be removed and destroyed.
REFERENCES AND MORE INFORMATION
Sandell MI and Diemer M., Intraspecific brood parasitism: a strategy for floating females in the European starling, Anim Behav. 1999 Jan;57(1):197-202
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