Avian generations in the making – part 2A: nesting and man-made construction

Hi folks — today I’d like to share with you some of my observations on the second part of avian reproduction — the process in which birds make a nest and lay eggs. (By the way, if you look up “bird nesting” on the Internet, the search will lead you to a human activity: a shared custody arrangement where children reside in one house and the parents take turns living there with them. In the bird world, some avian parents will actually share a home, such as this very old and enormous sociable weaver nest (Philetairus socius) that I saw in Namibia.)

Females of a few species will deposit their eggs in the nests of another, such as some cuckoos and brown-headed cowbirds (Molothrus ater). Some people dislike cowbirds intensely because of their nest parasitism (the cowbird baby will hatch first and eat all the food or perhaps get rid of the other eggs or babies). I figure that this is how the cowbird evolved – it didn’t make a choice to wipe out other species so I can’t blame or hate the bird for it. But the cowbirds do appear to pick on particular species as involuntary “foster parents” and this may be affecting the success of the other species’ reproduction.

There are two general broad categories of bird nests – those located in or on man-made objects such as nest boxes, atop downspouts, in vehicles, in plant pots and other places and those constructed by birds in trees, shrubs and on the ground (i.e., the natural environment which I will discuss in the next blog so this doesn’t get too long!). When birds make a nest in a “human area”, people will often try to accommodate them, not using the object or vehicle or making the space safer. For example, when American robins (Turdus migratorius) put a nest on one of my downspouts, I covered the rainwater container underneath it so the fledglings wouldn’t drown when they leapt to freedom. (I did it just in time, too!)

Swallows and phoebes will also use human constructions as places to situate their homes. The cliff swallows (Petrochelidon pyrrhonota) gather up mud, making countless trips to wad up small balls of the material to carry back to a place like this pier at Cane Creek Reservoir where they line up their nests in a row.

   

Barn swallows (Hirundo rustica) locate their nests inside, using rafters to place a nest; they may end up sharing space with paper wasps and organ pipe mud dauber wasps (Trypoxylon politum), which doesn’t seem to bother them.

The Eastern phoebe (Sayornis phoebe) may use an inside corner of a patio overhang as a site that is conveniently accessible from outside.

Purple martins (Progne subis) living east of the Rocky Mountains almost exclusively nest in homes provided by people. The hanging gourd nests can be seen in fields but nowadays other types of plastic constructions are also used.

    

In order to attract barn owls (Tyto alba) back to areas where many traditional barns have been razed, people are also placing special boxes in fields with plenty of open space in front of them so that the owls will have a hunting territory adjacent to their front door. Made of heavy plastic, these boxes may be monitored by organized groups in an effort to document their use.

   

At the Mason Farm Biological Reserve, an eagle scout project involved bringing in cranes to attach very large nest boxes to the tops of trees for barred owls (Strix varia) – so far, I have only seen Eastern gray squirrels making use of these nests.

And then there are the nest boxes that people put up to attract Eastern bluebirds (Sialia sialis), brown-headed nuthatches (Sitta pusilla) and screech owls. In my yard, other birds use these boxes, too, including the Carolina chickadees (Poecile carolinensis) such as my one-legged,  banded friend Chantal, Carolina wrens (Thryothorus ludovicianus) who may puzzle about how to get a long twig into the box and house wrens (Troglodytes aedon), who pile up twigs in a rather untidy stack inside the box.

   

   

Chickadees construct lovely mossy nests lined with hair, fur or plant fibers.

      

The nuthatches have nests with bark strips.

    

Sometimes people will intervene when a nest is in danger. A Carolina wren put her nest into a boat at Cane Creek Reservoir that was rented out to people and keeping the nest there was not a good option as she would be missing her nest for hours when the boat was gone. The land manager was so kind as to relocate the nest into a tree right in front of the boat’s resting place; unfortunately, inclement weather caused the nest to dislodge that evening.

   

Another danger may also threaten the birds and their nests – predators. A friend and I found it unusual to see a tree swallow (Tachycineta bicolor) putting a nest in a bluebird box on the edges of a farm. Several days later, we saw a black rat snake (Pantherophis obsoletus) peering out of the box and the swallows were nowhere to be seen.

   

If you can manage to mount your boxes on poles rather than trees, put both squirrel and raccoon baffles on them and also place them away from overhanging branches, they should stay relatively safe from the squirrels, snakes and raccoons. Next up: birds’ nests with no human connections.

 

 

Owls – raptors with fantastically functional feathers

Barred owl IMG_9617© Maria de Bruyn resThe owl is the wisest of all birds because the more it sees, the less it talks. – African Proverb

The world likes to have night-owls, that it may have matter for wonder. – German proverb

Sayings about wise owls are well known and may account for the names given to a group of owls – a wisdom, study or parliament. On the other hand, in some cultures, owls are associated with bad luck, death and the stealing of souls. Owl fossils date back as far as 58 million years ago! These appealing birds have been depicted in in cave paintings in France, in Egyptian hieroglyphics and in Mayan art. There are about 200 species of owls around the world and they are found on all continents except Antarctica and a few remote islands.

Owl anatomy

Barred owl eye feathers IMG_9802©Maria de BruynresOwls have interesting anatomical features. Like humans, they have binocular vision as a result of their upright posture and forward-facing eyes. A circle of feathers around each eye, as seen in the barred owl  (Strix varia) here, can be adjusted to focus and channel sound to their ears (which are asymmetrical in many species!). These feathers can magnify sound as much as 10 times, giving them acute hearing.

Barred owl IMG_9832©Maria de Bruyn signed res

Like bald eagles, owls have three eyelids: one is used to blink, another for sleeping and the third – the nictitating membrane – helps them keep their eyes clean and healthy. Because they cannot turn their eyes within their bony eye sockets, owls must rotate their heads up to 270 degrees to look around. They are far-sighted and cannot see clearly right in front of their eyes, but their distance vision is very good, especially in low light.

Barred owl IMG_9117©Maria de Bruyn resOwl feathers have fringes with different degrees of softness, which helps muffle sound when they fly.

Screech owl IMG_3069©Maria de Bruyn resSome owls, like the Eastern screech owl (Megascops asio) have tufts of feathers on their heads; the function of these might be to indicate a bird’s mood, to show aggression or to help keep the bird camouflaged.

Screech owl rehabilitated by CLAWS that serves as an educational bird

Behaviors

Different owl species make different sounds for finding mates or announcing their presence to potential competitors. These sounds help ornithologists and birders locate and identify species.  For example, the barred owl is said to have a call that sounds like “who cooks for you?”

These carnivorous birds usually swallow their prey whole (depending on the size of the animals such as fish, mice, rats, hares). Scientists who study their diets use owl pellets as an aid; the owls regurgitate bones, scales and fur in these pellets. The larger owls will also detach body parts they don’t wish to eat. For example, colleagues speculated that it was a great horned owl (Bubo virginianus) that took down a red-tailed hawk (Buteo jamaicensis) at a local nature reserve recently, leaving behind an entire wing that wouldn’t have provided much nutrition.

great horned owl IMG_3065©Maria de Bruyn resred-tailed hawk IMG_2745©Maria de Bruyn res

Great horned owl rehabilitated by CLAWS in Chapel Hill, NC. This bird was shot in the eye and cannot be released again; it serves as an educational bird.

The main threats to owl survival today include habitat loss, pesticides and human predation due to negative superstitions. There are fortunately efforts to preserve owls where they may be seen as beneficial. For example, a barn owl may eat up to 1000 mice each year, so that farmers try to attract them to help control rodents in agricultural fields. Projects to re-introduce barn owls (Tyto alba) are taking place in the Czech Republic and the United States. The local Audubon Society to which I belong has a barn owl project, in which they have placed barn owl nest boxes in various places in an effort to attract these birds as more permanent residents. It would be very cool to see the boxes occupied one day!

Barn owl project IMG_6412© Maria de BruynBarn owl box IMG_2501©Maria de Bruyn

Check out CLAWS, a wildlife rescue and rehabilitation center in North Carolina! http://www.nc-claws.org/