Has the decrease in bird populations affected the rise in ticks and other invasive bug species?

Nancy Eaton, Director/Librarian, Waterford Library Association

Hopefully everyone is well aware of the devastating declines to our bird populations – highlighted in the article published in Science magazine in October 2019 – in which lead author Ken Rosenberg documented a “net loss of 29% of the breeding bird population over the last half-century.” These numbers should be worrisome to everyone, not only because of our accelerating loss of biodiversity, but because birds are our “canaries in the coal mine” and show that nature is not resilient to the impacts we are having. So, to the question at hand, does this decline in birds result in more insects and ticks? (Fun fact 1: ticks are arachnids, not insects).

We can break this into a couple of groups to better understand the relationships at play. Along with the decline in birds, there is also a documented decline in insects, dubbed the “insect apocalypse.” These two things are correlated, as some of the most impacted families of birds are aerial insectivores. On the flip side of insect population changes, we are seeing increases in the numbers of invasive species. These non-native invaders are able to flourish because they do not have the natural controls that were present in the areas where they evolved.

The browntail moths is a great example of a species spreading here, defoliating oaks and apple trees (fun fact 2: apple trees are not native), and causing human-health concerns along the way. In Europe, the browntail’s native range, there are birds, parasites, and even diseases that kill them (although they are considered a pest there too). We do have a few species that can eat them in Maine. Blue jays are known for eating the larva, especially in winter, and our uncommon black-billed cuckoo can eat the hairy caterpillars, thanks to their ability to shed and regurgitate the inner lining of their stomach when too much indigestible material builds up. So perhaps if we had more cuckoos, we would have less browntails, but I don’t think they were ever abundant enough for us to have considered them a formidable opponent to an introduced invasive species.

Ticks are not commonly preyed upon by birds, so it is hard to make connections between bird declines and tick abundance. Wild turkeys are a good example of a species in Maine that eats ticks, although studies have shown that they do little to reduce the populations of ticks in an area. Counter to almost all other bird populations, turkeys are increasing in Maine, and quite dramatically since their reintroduction in the 1950 and ’60s. It is much easier to draw parallels between the increase in ticks and the increase in deer and mice populations. Suburban sprawl is creating plenty of habitat and safe places for our deer and mice, and we see an increase in ticks related to that.

We can all help stop these massive declines in our bird populations by taking six simple steps:

• Make windows safe by reducing reflectivity

• Keep cats indoors (they kill 2.6 billion birds annually in the U.S. and Canada)

• Plant native plants

• Avoid pesticides

• Drink shade grown coffee

• Reduce plastic waste

Native species of birds – like barn swallow, house wren, and chimney swift – presumably predate barns, houses, and chimneys, which is where we typically find them these days. If so, what were they using instead of barns, houses, and chimneys?
—Eric T., Portland

I do love a great philosophical bird question! Which came first, the barn or the barn swallow? We can look at a few examples of different species that use man-made nesting sites and see how that has affected them over time.

The house wren is a good one to start with. They are an example of a typical secondary cavity nester, which does not excavate its own cavity, but relies on other species – like woodpeckers, which we call primary cavity nesters – to create a cavity. Those primary cavity nesters typically only use the cavity once, then will create a new one each year. This is when species like the house wren, or eastern bluebird, tree swallow, and many more, will move in.

For these birds, the emergence of more man-made dwellings probably didn’t have much impact on their populations. If anything, we had a very negative impact by doing things like cutting down dead trees, which would contain cavities that could be used over and over. (People tend not to like dead trees on their property, so if you must take it down, you can supplement by putting up a birdhouse, or several, to replace where a cavity would have been.) It is worth mentioning that we have a couple of non-native cavity nesting species in Maine, like starlings, that outcompete the native cavity nesters, so we are definitely net negative on this one.

Barn swallows, on the other hand, have definitely benefited from new man-made places to nest. They originally nested in caves, which in a place like Maine are fairly uncommon. What I find interesting is that their use of man-made structures goes back over 2,000 years, and their increasingly close association with man-made places has occurred independently all across the globe. Chimney swifts could also be in this category; they used hollow trees and tree cavities before chimneys, and other man-made structures they now nest in, appeared in the New World.

I’d be remiss to not mention purple martins, which now nest almost exclusively in man-made birdhouses in the eastern United States, and this connection predates European colonization. There are records from early naturalists, as far back as 1712, showing that Native Americans would hang gourds for purple martins to nest in.

Time will tell if these species will become increasingly, or even wholly, reliant on man-made dwellings, or just benefit from them.

Do you have a nature question for Doug? Email questions to ask@maineaudubon.org and visit maineaudubon.org to learn more about virtual and backyard birding, online classes and other programs about wildlife and habitat.

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