100 Years of the Migratory Bird Act

This year marks 100 years since the passage of the Migratory Birds Treaty Act, one of the earliest pieces of conservation legislation passed by the US Government. This law made it illegal to kill native birds or remove or disturb their nest and eggs. (16 US Code Section 704 subsection A) This act served to protect many endangered species of birds which were under threat from over hunting like waterfowl whose feathers were highly prized. This act has seen many successes and has helped replenish and protect many bird species like the Snowy Egret and other waterfowl.

On side effect of this law was that egg collecting became an illegal hobby. While this act was extremely good protecting birds, it also means most of the eggs we have in Museum collections are now a 100 years old. Bird eggs can be valuable sources of data regarding pollution and environmental damages, and this data can be invaluable in crafting governmental responses to these threats. In a very real sense birds are, for the lack of a better phrase, a canary in the coal mine. For example the impact that DDT and similar pesticides were having on bird eggs were noticed far earlier than its impacts were seen or felt on humans, as illustrated in the work of Rachel Carson in Silent Spring. As such bird eggs can serve as a vital tool for scientists when considering the impact of contaminants in the ecosystem and their potential impacts on human life.

The Migratory Bird Treaties Act has presented a challenge and an opportunity for scientists interested in studying birds for contaminants. The challenge is that because much of the avian life in the United States is composed of native bird species, which are protected under the Migratory Bird Treaties Act, scientists cannot easily collect bird eggs from these birds. However, nnon-native birds, such as the House Sparrows, are exempt from the law. House Sparrows are native to western Eurasia and Northern Africa. these birds were introduced to NY in the 1800’s and have since spread across the US. There non-native status allows Sparrow Swap to revive the egg collecting so that House Sparrow eggs can be used to research pollutants, and more importantly in terms of helping conserve native species, swapping out the eggs allows us to study a method for controlling this invasive species.

However, because only house sparrow eggs and a handful of other species are legal to collect makes it very important that the eggs collected are indeed House Sparrow eggs. The color, speckling, and shape of an egg can change a lot not only between species, but also within a single species. This can make it hard sometimes to confirm just by looking at an egg, what species it is from. Where the nest is (in a cavity or in a tree), what the nest is made of (pine needles or moss), the structure of the nest (neat or messy) and if you see an adult bird fly in and out of the nest are all important clues that can confirm the identity of the eggs. For example, House Sparrow nests are made of coarse dried vegetation, often stuffed into the hole until it’s nearly filled. The birds then use finer material, including feathers, string, and paper, for the lining.

Below are a few examples of other species with similar eggs to house sparrows:

House Sparrow Eggs. Photo from Sialis.org


Cowbirds are indigenous birds which will sometimes parasitize a sparrows nest by laying an egg in the nest. As such they are protected by the Migratory Bird Treaties act and their eggs cannot be removed from a nest without permission from your States wildlife management agency. Please remember that while the two eggs look similar, that one is a house sparrows egg, and one is a cowbirds. An easy way to tell the difference is to look at the size (if they are in the same nest) and the degree to which the eggs are round. The larger and more round one of these eggs are the more likely they are cowbird eggs. In addition if there are other intact eggs of another bird species in the nest or adult birds brooding, it is likely that the eggs are cowbird eggs unless the brooding birds are House Sparrows.

Cowbird egg in a bluebird nest. Photo from Nestwatch.

Tufted Titmouse
Tufted titmouse eggs and those are also protected species and you can tell the difference between the two since House Sparrow eggs are generally glossier and a bit longer on average than a Tufted Titmouse egg. Titmice build cup-shaped nests inside the nest cavity using damp leaves, moss and grasses, and bark strips. They line this cup with soft materials such as hair, fur, wool, and cotton, sometimes plucking hairs directly from living mammals.

Tufted Titmouse Eggs. From Sialis.org

House Finch
We have also received House Finch eggs, these birds are also protected species. Compared to a House Sparrow the Eggs of a House Finch are smaller, lighter colored, generally less speckled, and the speckles are generally black to purple in color. A House Finch’s nest is a cup made of fine stems, leaves, rootlets, thin twigs, string, wool, and feathers, with similar, but finer materials for the lining. This what House Finch Eggs look like.

House Finch Eggs. Photo from Nestwatch.

We hope this helps if you come across an unusual egg! Don’t hesitate to contact us hear at Sparrow Swap if you have any questions.

This post was written by Rohan Krishnamoorthy. To learn more about Rohan and the rest of the Sparrow Swap Team, check out our Team Page.