Bluebird nest. Photo by cbgrfx123, Flickr
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Over the past five years Deborah and I have gotten very familiar with many types of birds’ nests. Working with the Cavity Nesting Team at Harrison Hills Park we have found and described a variety of grass and twig spun nests, cup nests, moss nests and random stick pile nests inside of our nesting boxes and have been able to work out the identities of the birds who have constructed them.
Tree swallow nests. Photo by T. Schweitzer, Flickr
Bluebirds, for example, use fine grasses when they weave their tall, cup nests, while tree swallows tend to use coarser grasses and almost always include a large number of feathers (either their own or feathers from other bird) in their nests. Chickadees build their nests with a predominance of mosses, while house wrens simply fill the nesting box with a seemingly chaotic array of small sticks that, somehow, has enough room for eggs, nestlings and the incubating adult. House sparrows make relatively shapeless nests in part out of natural materials with a large amount of added, human-made trash (candy wrappers, string, ribbon and even cigarette butts (I’ll come back to a further discussion of cigarette butts in a nest later)).
Chickadee nest. Mrgfan, Wikimedia Commons
We started our Cavity Nesting Project in 2015, and none of us on the team had a great deal of familiarity with birds’ nests. Also, we were initially uncertain about whether we should leave previously used nests in the boxes through the season or remove them when their individual nesting cycle was completed. Several scientific studies had found that the presence of old nests actually encouraged subsequent nest formation in natural nest cavities and nesting boxes. Other papers, though, stressed that old nest removal was important to help control the proliferation of nest parasites.
After our first, early spring round of bluebird nesting in 2015 we did remove several of the nests and put them in sealed plastic bags so that we could take them away from the nesting sites (we did not want to attract nest predators) and then dispose of them. One bag, though, ended up on top of the garbage can in my garage rather than in the trash itself. After three or four days I heard a buzzing noise in the garage and found that the bag was now full of trapped, adult blowflies.
This inadvertent experiment indicated to us that blowflies were present in our nesting areas. Blowflies can be significant nest parasites and can cause not only debilitation but even death of nestlings. From then on we removed all old nesting materials and disposed of them in sealed plastic bags. The positive impacts of reducing these parasites, we assumed, more than offset the loss of the possible stimulus that the old nests could impart to the nesting birds.
Nests are, of course, the place where birds lay and incubate their eggs and nurture their nestlings. Many bird species make iconic, woven, cup-shaped nests. Many other bird species, though, make very different sorts of nests.
KIlldeer nest. R. Cameron, Flickr
Killdeer, for example, put almost no work at all into the construction of their nests. They may push a few stones or some sticks or vegetation around to make a small clear spot (called a “scrape”) in which they lay their eggs. The eggs are well camouflaged by their colors and patterns and are remarkably hidden even when out in the open. Many years ago Deborah and I and our children rented a summer cottage on Chincoteague Island and were noisily greeted by a female killdeer every time we stepped out of our front door. We knew that the female was protecting a nest somewhere in the front yard (a weedy, sand and pebble habitat that was maybe 1600 feet square). Over the week we were on the island, we searched carefully through the weed cover of the yard but never did find the nest.
Killdeer also make scrape nests up on the gravel roofs of the buildings. At Penn State New Kensington it is a loud Sign of Spring and Summer to be dive bombed by some of these roof nesters whenever you go out or in the doors of the Engineering Building!
There are other birds who make even less of a nest than the killdeer. Cliff nesting murres and guillemots simply lay their eggs on bare rock ledges. They rely on the shape of their eggs (pointy at one end and rounded on the other) to make sure that any rolling of the egg will simply take it in a circle (and not straight off the edge of the cliff).
Birds like cowbirds and old world cuckoos don’t make nests at all but instead deposit their eggs in the nests of other bird species. Cowbirds originally were birds of the prairies that followed the great herds of bison. They fed on the insects that were attracted to the animal herds or stirred up by their activity. The unpredictable timing of the herd movements did not allow the cowbirds sufficient weeks to nest and raise their young. Using the nests of other birds and letting those birds rear their young was a logical evolutionary solution to their nesting crisis. The alteration of the North American forests by European settlement, though, opened ecological corridors for the prairie cowbirds and allowed them to move out into a wide variety of habitats. Today cowbirds are found all across North America and are responsible for a significant proportion of the declining populations of many native song birds.
Short-eared owls hunting storm petrals on Genovesa Island
A few birds make nests in underground burrows. Storm petrels, for example, on the Galapagos Island of Genovesa make their nests down in the volcanic rock tunnels and are hunted both inside the tunnels and at their surface entrances by diurnally active short-eared owls. The lava tunnels provide the only protective cover on this rocky, treeless island and the petrels (and their predators!) have quickly adapted to using them.
Birds’ nests range in size from delicate, spider webbing and thistle, bottle-cap-sized nests of hummingbirds to great platforms of thousands of pounds of sticks piled high up in trees by bald headed eagles. Some nests are meant to be used just once, while other nests may serve a number of generations of nestlings.
What plant materials a bird uses to build its nest also has an impact on the health and success of the nestlings. In a paper published this past summer (Proceedings of the Royal Society B, June 6, 2018), researchers at the Max Planck Institute for Ornithology found that starlings reared in nests to which aromatic herbs (including hogweed, cow parsley and goutweed) have been added by the parents (or by the researchers) had higher red blood cell counts, more robust immune system functions and fewer bacteria than starlings reared in non-herb infused nests. Further, the parental starlings incubated the eggs and nestlings longer in nests that had the added herbs suggesting that the added herbs acted as a tranquilizer for the parent causing it to linger longer on the nest.
Nest made with human trash. K. Stuedel, Flickr
Also, a study published in the Journal of Avian Biology (June 20, 2017) looked at house finch nests in Mexico City. These Mexican house finches, like the house sparrows I mentioned previously in Harrison Hills Park here in Western Pennsylvania, add cigarette butts to their nests. These cigarette butts add nicotine (a powerful, “natural” pesticide) and other chemicals to the nests that reduce the numbers of nestling parasites (like ticks). Further, the study suggests that parental house finches may actually add cigarette butts in direct proportion to their perception of the parasite load in the nest!
So, it’s spring and there are nests everywhere doing the job in some obvious and some subtle ways to help rear the next generation of birds!