How urban birds decide where to feed and nest in our neighborhoods
By Jeffrey Ling
“Bugs are here, bugs are there,
Everywhere you find ’em,
For little fleas have lesser fleas,
. . . and so on, ad infinitum.”
THE GREAT WEB
I memorized that version of a nursery rhyme a long time ago and have seen its truth in my work as an arborist ever since. Insects are common in modern forests and landscapes and so are vertebrates. Urban and suburban landscapes have been shaped by human needs. Yet, every landscape, every collection of plants, every cluster of trees, is a focus point for animals, no matter its complexity, context, character, age or location. We must remember that whatever other purposes it serves for us, a landscape is also a home for animals. Even a “bad” landscape — one with incorrect assumptions or mistaken choices in the site’s design and plant selection — is still a habitat. Within this ‘web of life’ — from the debris on the ground, to understory plants, to the tops of trees — animals of all sizes can and do grow and thrive.
Arborists have always acknowledged this reality. We meet many animals as we climb and work in trees. However, the tree owners are often unaware of the interlocking connections of one animal to another or the relationships between animals and the landscape features around them.
For example, it has been reported that upwards of 20 different species of mites can reside and interact in a single spruce. Some feed on the tree’s tissues and some on each other as they all become food for insects and birds.
In the urban forest, some of the most significant insects are aphids and adelgid species that inhabit and feed on trees. From an ornamental horticulture perspective, they are universally ruled unacceptable and routinely treated to stop damages and to deter the mess of dripping trees. Yet, these bugs offer significant food sourcing for both other insects and hummingbirds. In this way, the tree is the base on which all insects, living to their own ends, create a foundation for the hierarchy of species found in the trees. Squirrels bound in and around the trees. Deer nibble on foliage, fruit and twigs beneath, while raccoons and opossums claim cavities as their winter dens. The urban and suburban forest supports wildlife large and small.
BIRDS: PLAYING BY THE RULES
Nowhere in the urban forest is the relationship between animals and woody perennials more frequently seen — or more unique to each species — than in the case of birds. Urban and suburban trees directly impact the types and numbers of birds on a site, in a neighborhood or throughout a city. All trees, whether specifically planted or volunteer trees — those that spring up on residential property on their own — will create avian habitats, food sources and rearing sites.
But why do some trees fill certain roles for some birds and not others? Examining birds’ interactions with our landscapes, it becomes clear that there are rules that apply to the bird-tree relationship. Three prime directives rule birds’ selection of a particular property and even an individual tree. Familiarity with these rules can help homeowners wishing to create a welcoming habitat for bird populations.
- All birds have tolerances and preference ranges that lead them to accept or reject a site.
Every bird makes “value judgments.” They respond to their surroundings or react to changes therein. It doesn’t matter if the change is a radical alteration like tree removal or an incremental change like that of trees growing or being pruned over time. The avian community fostered in every habitat is a result of the positive and negative influences of the landscape’s character at that time.
- All birds learn over time.
Birds are intelligent and learn from new experiences. Many relate what they learn to offspring as well. Any bird hatched on a site or in a landscape feature will consider it home, but sites that can foster and support more individuals will do so over time as birds learn about the spot. As an environment changes, birds will learn that it has become more or less hospitable, and areas that attract birds once are more likely to see future generations as well.
- Feeders bring visitors; harbors bring residents.
Across America, bird feeding supplies are sold to homeowners desiring to watch birds in their yards. The installation of feeders — whether seeds, suet or sugar water — is primarily for the entertainment of people; it seldom impacts the local bird population over time. Most birds that visit feeders are either locals or migrants passing through. Consistency over the years — coupled with environmental support — is needed to have any real impact on any bird species. Feeding alone will not significantly increase bird populations; birds need a place to live, too.
To see these rules in practice, let’s take a look at some of the ways tree features influence birds’ decisions and therefore affect the numbers and types of birds in a given area. The height at which birds fly, roost and nest is species-specific. So too is their preferred tree structure and texture. Some species of birds demand a specific height for their nest. Cardinals and bluebirds, for example, will not nest above 15 feet while others, like orioles, demand high, exposed branches. Most birds require a specific density of cover and others seek a specific branch angle. Bird species that nest in rookeries need a tree or grove that can hold dozens or even hundreds of nests. If a bird’s exacting requirements are not met, it moves on.
A number of bird species need cavities to nest; some may be manufactured by woodpeckers for self-use, then appropriated by other species. Larger cavities and tree hollows are possessed by mammals and larger birds, like owls and wood ducks. Landscapes with these harbors will naturally draw more birds to them.
Food sources provided by trees are also important. Just like you and me, birds want food sources that are filling and reliable. A great example of this is the new cultivars of crabapple (Malus spp.) and their “persistent fruiting habit.” Almost universally, these trees are planted by homeowners for their multiseason traits, but it is their habit of growing small, colored fruits that hang on the twigs all winter that makes them interesting to birds. These fruits are not palatable for northern birds in November, but they turn to softened pulp after freezing, and become a primary source of food later in winter.
In America, probably no tree is more influential to several bird species than the ornamental pear (Pyrus calleryana). Though exotic, the species is not invasive and is a preferred urban specimen, planted extensively over the last 30 years, and would certainly appear on any list of the most predominant trees in landscapes and along streets or parking lots in much of America. Their density draws many birds, and their abundant and persistent small fruits are a welcomed food supply in the very early spring when it is difficult for many birds to find enough sustenance.
It’s not just ornamentals at work. The creation of “urban scrub” — shrubs and small trees — has also enlarged the populations of birds like cardinals (Cardinalis virginianus) and robins (Turdus migratorius). For both of these birds, as well as goldfinches, (Spinus tristis), purple finches, (Carpodacus purpureus) and many sparrows (Passeridae), the scrub provides the necessary type of roosting and nesting site.
Recently, many people in the Midwest have reported seeing robins in the snow, arriving to northern climates much earlier than expected. This may or may not be a function of global climate change, but it is certainly a reflection of micro-environment change — specifically, localized heat islands and persistent fruiting. All three rules are at work here: While urban buildings and concrete create warm harbors for the robins and other birds, it is the abundance of persistently fruiting trees that feed them in February and March, changing their calendar. Also, the lower canopy of ornamental trees has proven to be their preferred nesting habitat. Once birds have hatched in a location, they learn that “this is my home, my neighborhood” and teach that to their offspring as well.
The robins have certainly benefitted from our warmer cities and abundant food sources, but one invasive species seems to have benefitted even more, judging by the explosion of its population. Hundreds of millions of European starlings (Sturnus vulgaris) now flock in the winter, almost darkening the skies in some areas, leaving trouble and messes in their wakes. These birds, like the robin, rely on persistent fruiting trees. Unlike the robin, they are omnivores, subsisting on any and all food sources.
What the passenger pigeon (Ectopistes migratorius), a native species, was to 19th century America, the starling is for much of the country today in terms of numbers. In winter, the large black clouds of these birds are the new urban wildlife reality for many sites, particularly in the Mississippi, Missouri and Ohio River regions. Not only are their nighttime roosts loud, but their huge nightly deposits of manure — 1,000 birds can produce more than 70 pounds of manure nightly — leave homeowners and city managers discouraged. In this, the behavior of starlings mirrors 19th century reports of the now-extinct passenger pigeon that routinely killed forest trees with its high-ammoniate manures. Part of the passenger pigeons’ demise was uncontrolled hunting, but a greater influence was the loss of the forest habitat that supported the pigeons. The starling has a broader tolerance and has found urban America and its urban trees and landscapes to its liking.
Another change in America’s bird population is the population growth of crows and ravens (Corvus) in urban settings. Among the smartest birds on the continent, they adapt readily to manmade landscapes. But these birds are gregarious in both their roosts (winter nighttime resting places) and rookeries (nesting/rearing sites). They demand tall trees in groupings to handle their numbers and branches stout enough to hold their collective weight. As urban forests mature, trees planted in groupings can be selected by these birds.
These birds also find natural food sources in urban environments in the form of small acorns. Pin oak (Quercus palustris), northern pin oak (Quercus ellipsoidalis), shingle oak (Quercus imbricaria) and willow oak (Quercus phellos) all set acorns that can be swallowed by these birds. These oaks are preferred in urban plantings, and it is no surprise that, as these trees proliferate and grow larger, Corvus populations increase in response to both the food source and the availability of high platforms for roosting and nesting.
Perhaps the ultimate success story of wildlife adapting to urban, man-directed sites is the non- migratory Canada goose (Branta canadensis maxima). Nearly extinct in the 1950s, this bird is now found in almost every state in the continental U.S. and across Canada. The total population is estimated at over 100 million individuals. This impressive increase is due in part to one landscape feature: the retention pond. This, blended with the installation of short-blade, high-fertility turf like golf courses, as well as protection by international treaties and the ability for the female goose to re-ovulate if the first clutch of eggs are lost, has created a nearly miraculous population explosion in half a century. These birds, interestingly enough, are inhibited by trees, not encouraged!
All birds differ — by size, by physiology, by need — yet all are attracted to or discouraged from a potential habitat site by how it allows or restricts species-specific behaviors. The success of micro- and macro-populations is governed by these same principles, whether a substantial woodlot, a homestead garden or a street tree planting.
By knowing and exercising these rules, tree owners can offer stimuli for more desired birds or control numbers of “undesirable” birds. Most importantly, if there is a bird issue, don’t blame the birds. They are almost always responding to what humans have done within the micro-environment that we share.
Registered Consulting Arborist Jeffrey Ling is the founder of Arborwise, Ltd. with his wife, Victoria Ling. He writes from Fort Wayne, Ind.