Avian Reproduction: Nests

An updated version of these notes can be accessed from a new "Avian Biology' page
(http://people.eku.edu/ritchisong/avian_biology.html).

Birds use nests to protect eggs and nestlings from predators and adverse weather. To minimize predation, birds may use or build nests that are inaccessible, hidden, or camouflaged. Nests may also help keep eggs and nestlings warm.

Orientation and microclimate of Horned Lark nests -- Across their range, Horned Larks typically construct nests adjacent to & north of objects such as a tuft of grass or a rock. Hartman and Oring (2003) studied the importance of nest orientation to nest microclimate in Horned Larks in California. Nests showed a significant northern bias in orientation angle and were 49% shaded in the early afternoon, the hottest part of the day. Artificial nests of eastern, western, and southern orientations exhibited little to no shade during this time. A northern nest orientation ensures maximal shading by the grass tuft to the south, may protect nests from cool evening winds, and provides increased daytime ventilation of the nest through exposure to prevailing winds. In addition, shade may also help conceal nests from predators.  Photo credit: www.epa.gov/gmpo/education/photo/birds-animals.html

The position of vegetation (curved black bars) around Horned Lark nests relative to the direction of prevailing winds. The dashed circle represents the outline of a nest. Each individual curved black bar represents the vegetation around one nest. Each solid circle represents one entrance orientation. The dashed line designates the mean entrance orientation angle of 345° for all nests (n = 10; Hartman and Oring 2003).

Types of nests:

• Scrape nests are simple depressions in the ground (sometimes with a few stones added) or in the leaf litter. Such nests are used by some penguins, shorebirds, gulls, terns, nighthawks, vultures (e.g., Black Vulture nest below), and other species.

Source: http://www.ctbirding.org/ct_black_vulture_nest.htm

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Are wader nest scrapes adaptively designed to minimize clutch cooling rate? - Arboreal avian nests likely function partly to insulate clutches.  However, reasons for the construction of nest scrapes are poorly understood.  Working on Pectoral Sandpipers, Dr. Jane Reid tested the hypothesis that using a lined scrape reduces the rate of clutch heat loss & investigated whether scrapes are effectively designed to minimize heat loss rates. The use of both an unlined scrape and of lining material reduced the rate at which a test object lost heat.  Constructing a lined scrape is therefore likely to serve to insulate a clutch. The rate of conductive heat loss from within a scrape increased with scrape depth and decreased with lining depth.  Convective heat loss increased with wind speed in shallow scrapes but not in deep scrapes. Mean observed scrape depth approximately equaled that which minimized convective cooling while minimizing conductive heat loss.  Further, on average, Pectoral Sandpipers used approximately the lining depth that minimized conductive heat loss while minimizing material useage.  Scrape dimensions therefore approximated those likely to minimize overall heat loss, given the conflicting thermal pressures of the environment. Available lining materials differed in insulative quality both when wet and dry.  Pectoral Sandpipers did not use lining materials in proportions that reflected local availability.  Instead, relative use was correlated with a material's insulative quality when wet.  Pectoral Sandpipers therefore used lining materials appropriate to minimizing heat loss given their damp breeding environment. - Jane M. Reid, University of Glasgow

Pectoral Sandpipers nest on the arctic tundra, often near water. The nest is lined with grass, moss and lichens. Typical clutch size is 4 eggs. Photo source: http://picturethis.pnl.gov/picturet.nsf/f/uv?open&SMAA-497TFH

• Burrow nests are very effective at protecting eggs and young from predators & maintaining an appropriate microclimate for eggs & young. Some birds, like Bank Swallows and Belted Kingfishers (pictured below), usually construct their own burrows, while others, such as Burrowing Owls, may use burrows contructed by other species.

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Spacing of Bank Swallow tunnel entrances (Ghent 2001) -- Tunnel entrance distributions at six Bank Swallow sand-pit colonies showed consistently nonrandom, too-regular patterns, supporting the hypothesis that the distance between tunnel entrances is determined by territorial disputes at the tunnel mouths. Average nearest neighbor distances, and numbers of burrows per unit area of pit face, were both consistently greater than their random expectations, a paradox that is explained algebraically. Evidence of tunnel coalescence and communal nesting was found in a completely evacuated colony of 30 tunnels. Bank Swallow colonial behaviors may have evolved to maximize populations on small exposed bank faces along streams and rivers, and that they are still behaving as if only small fractions of large sand pit banks are available. On small areas, regular spacings help to avoid too close tunneling and concomitant bank collapse. It is argued that a tolerance for communal nesting, once coalescence has occurred, involves less tunneling than the forced evacuations of entirely new tunnels by ousted pairs. Excavation of 30 tunnels. Three of the five coalescing groups led to double-length nest chambers with two grass-mat nests. Vertical separations (e.g., 8 cm between tunnels 8 and 9)  indicate where neighboring tunnels crossed w/out coalescing.

Area A = 4.26 sq. m determined by visual framing of a too  regularly spaced burrows of a bank swallow colony. A' = 7.37 sq. m is the area on which 27 (solid circle)  burrows leading to a nest would have to be distributed  to equate their observed average nearest neighbor distance  r to its random expectation E[r]. Open circles represent  6 tunnels not leading to a nest (not included in the calculations)

• Cavity nests (e.g., in trees or cacti) are used by numerous passerines, woodpeckers, owls, parrots, and some waterfowl. Some birds, such as woodpeckers (like the Gila Woodpecker below), construct their own cavity nests and are referred to as primary cavity nesters. Species that use natural cavities or cavities constructed by primary cavity nesters are called secondary cavity nesters.

• Platform nests are relatively flat nests that may be located on the ground, in a tree, or on the tops of rooted vegetation or or debris in shallow water (like the Western Grebe nest below).

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Bird nests vary from a simple accumulation of materials on the ground to elaborate refuges in or on secluded & elevated substrates. Dial (2003) observed that nest construction and placement are correlated with other features such as flight ability. For example, basal avian taxa (ratites & many Galliformes) create a simple depression in the ground to harbor their incubating eggs, like those of nonavian dinosaurs. The progression of nest complexity moves from cryptic ground nests of some galliforms to simple elevated nests (e.g., Columbiformes, Cuculiformes, & Ciconiiformes). Taxa that construct elevated nests in a bush or tree or on a cliff or rock ledge tend to be better  fliers than simple ground nesters.

Young raised in elevated & cavity nests, including primary (Psittaciformes, Piciformes, & Coraciformes) and secondary (many Passeriformes) cavity nesters, have a robust forelimb flight apparatus, and less hindlimb mass, which is consistent with increasing flight capacity. As nest placement (e.g. invisibility, inaccessibility), construction (e.g. impregnability, camouflage), and attendance (e.g. feeding, protection, incubation) increase in complexity, a concomitant enhancement of flight styles is observed, including maneuverability and acceleration. The most complex nests are associated with some Passeriformes, particularly swallows, oropendolas, and weaver finches. Weaver finches (Ploceidae, Passeridae) and oropendolas (Icteridae) build intricately woven chambered, pendant nests hung from the resilient thin branches of bushes and trees in predator-rich environments. Perhaps the most predator-proof nests are those of swallows (Hirundinidae) and swifts (Apodidae) that often construct mud encasements secured to the most remote overhanging feature within their habitat (e.g. cliffs and human-made structures) (Dial 2003).

• Cupped nests are, of course, cup-shaped. Such nests may be constructed of various materials and in a variety of locations. Pettingill (1985) categorized cup nests as follows:
• statant cupped nests - nests located in the crotches and branches of trees and shrubs and supported mainly from below. Many passerines and hummingbirds build such nests.

Source: http://www.sandiegozoo.org/wildideas/animal/hummingbird.html

• suspended cupped nests - nests not supported from below but from the rims, sides, or both:
• pensile - nests suspended from the rims and sides; rather stiff, e.g., those of kinglets and vireos (like the Black-capped Vireo pictured below)

• pendulous - nests suspended from the rims and sides; rather flexible and deep, like those of orioles

• adherent nests - cupped nests whose sides are attached by an adhesive substance (e.g., mud or saliva) to a vertical surface, like those of swifts and some swallows (see Barn Swallow nest below)

• ground nests - cupped nests on the ground; sides are sometimes extended upward and arched over the top making a domed structure. Several passerines, particularly those that occupy open habitats like grasslands and tundra, build ground nests.

Tits weave fragrant nests. - Birds weave aromatic plants into their nests, apparently to keep their home clean and bug-free for raising chicks. Blue Tits on the fragrant Mediterranean island of Corsica can even smell when it's time to refresh fading fragments, ecologists have shown (Petit et al. 2002). Female blue tits gather lavender, yarrow, curry, mint and other scented plants for their nests shortly after laying eggs, and continue to do so until the chicks leave home. "They are real botanists and do a great job exploiting their environment to protect their chicks," says Marcel Lambrechts of the Centre for Functional Ecology and Evolution in Montpellier, France. The birds make a pot-pourri of 10 aromatic plants from the 250 species in their habitat. Many of the chemicals in these plants ward off bacteria, viruses, parasites, fungi and insects. Lambrechts's team removed the aromatic plants from 64 nests and then placed a hidden box containing lavender and yarrow underneath half of the nests. In the first 24 hours, only the birds with empty boxes replenished their herb supply. After 48 hours, the other half of the birds began to restock too, as the scent from the hidden herbs waned. "This field test directly shows that birds are attending to odour cues," says Larry Clark, who studies similar behaviour  in European Starlings at the National Wildlife Research Center in Fort Collins, Colorado. The Blue Tits select for chemical diversity as well as high concentrations of chemicals, he points out, underlining the importance of olfaction in avian behavior. -- Kendall Powell, Nature Science Update

Photo by M. Lambrechts

http://www.trekearth.com/gallery/Europe/ Denmark/photo59174.htm

Birds Use Herbs To Protect Their Nests  –  Researchers from Ohio Wesleyan University suggest that some birds may select nesting material with antimicrobial agents to protect their young from harmful bacteria. They presented their findings at the 2004 meeting of the American Society for Microbiology. "If the fresh herbs and plant materials that parent birds bring into the nest have a sufficient concentration of antimicrobial compounds, they could protect the nestlings from harmful bacteria," says researcher Jann Ichida. To find out if plants brought into the nest might prevent disease, Ichida and colleagues tested 12 different volatile plant materials against feather-degrading bacteria. Results showed that several types of plant materials and extracts including usnic acid, ascorbic acid, yarrow, and two oak species inhibited the growth of a number of harmful bacteria. "If the fresh herbs have a sufficient concentration of these chemicals, they could protect the nestlings from harmful bacteria," says Ichida. "By practicing medical botany, parent birds exercise effective home nest security and protect their offspring from select biodegrading microbes that affect the health of their young." - Science Daily

• the structure and composition of nests exhibit little intraspecific variation
• birds raised in isolation tend to build species-specific nests

The time needed to build a nest varies with the complexity of the nest and other factors such as time of year and weather. In temperate areas, construction of the first nest of the breeding season may take longer than nests later in the season (particular in resident species like Eastern Bluebirds and Northern Cardinals). Generally, passerines build nests over a period of a few days. Construction of the nests of some larger birds, such as raptors, may require as long as several weeks.

• A South American ovenbird (Rufous Hornero, Furnarius rufus), which may take months to fashion one nest from clay or mud mixed with bits of straw, hair, and fibers. The tropical sun bakes the walls brick-hard.

• Or Bald Eagles, which use sticks, some two inches thick and several feet long, to make nests sturdy enough to support a human adult. They may look like a jumble of materials, but the sticks are usually placed in layers, beginning with a triangle, followed by more rotated, triangular layers.

Source: http://www.alaska.com/akcom/galleries/eagles/v-5/

Birds are capable of grand engineering feats. But are they engineers? Not in the way you might think. Just as birds know how to fly, they know how to build a nest without instructions or apprenticeship. It's a matter of instinct. "They are 'hard-wired,' " Causey says, "sort of like robots." Birds craft their nests without consciously thinking about it. How then did some species of birds develop such well-engineered, elaborate nests? One possible explanation involves natural variation and evolution. If a particular bird happens to build a nest that is stronger or more predator-safe, that bird's offspring are more likely to survive and pass along this trait to succeeding generations. Another possibility is that when females choose mates based on the quality of the nests they build, this means the best nest builders are more likely to breed. Nest-building, therefore, may still be evolving, but so slowly that no one really detects any change. This makes nest-building one of the most difficult bird behaviors to understand.

Generally, in temperate areas, nest-building coincides with the arrival of spring. That's when birds' internal "clocks" tell them it's time to mate and raise a brood. In the tropics, though, some birds build nests and breed year-round. The nest, of course, is where females lay eggs and brood young. Which bird actually does the building – the male or female – varies by species. In some cases, both collect material to build the nest and join in its construction. One of the more peculiar routines is that of the male Marsh Wren (Cistothorus palustris), which migrates north before the female and builds as many as 10 dummy nests in his territory. These dummy nests are not lined with soft materials; lining is added by the female only to the chosen nest. Dummy nests have been shown to have adaptive importance, with active nests built near larger numbers of dummy nests being more successful than those near fewer dummy nests (Leonard and Picman 1987).

We usually think of nests in tree branches. But some birds build nests on the ground, some in bushes. Others might attach their nests to the sides of cliffs. Barn Swallows have an affinity for barns. Chimney Swifts, as the name implies, favor chimneys and other man-made enclosures.
Carolina Wrens will nest in almost any cavity, from an empty can to a coat pocket. To keep nests together and secured in place, birds need good adhesives. They use a variety of natural substances to do the job, including mud, saliva, spiderwebs, caterpillar silk, leaf mold, and certain plant fibers. Materials that make up the nest are intertwined, and with the weaver bird, are actually woven or thatched together using grass, strips of leaves, and twigs.

Birds can make hundreds of trips to collect materials. And while they seem to prefer natural objects (helpful as camouflage), some use almost anything that works and that they can carry. Candy wrappers, cellophane, shredded money – even barbed wire – have shown up in nests. But however coarse the outside of the nest is, the inside is usually lined with soft materials to make it comfy.

One theory about why birds build open nests is that they had to. Larger species shooed them out of the tree holes. Birds intent on avoiding bullies and predators may build nests that have coverings or that hang from the end of tree branches. Less aggressive birds may even build near more aggressive species, for protection. Although some large birds (eagles, hawks, and sea birds) return to nests they've made, most birds rarely do. Eagles may return annually to the huge platform nests they build in the treetops. Some measure as much as 6 feet in diameter. On the flip side are hummingbirds, which fashion tiny cup-like nests not much bigger than a thimble.

Nest-collecting was popular from about 1870 to 1920, says Douglas Causey, ornithologist at Harvard University. Most of the nests in the museum's collection date to that period. Back then, collectors didn't have the ecological awareness that people have today. They would climb trees and saw off branches to "collect" a nest – with its eggs intact. Once museums had at least one of each, searching for more made no sense. From about 1920 to 1970, nest and egg collections gathered dust. "They just took up space, and no one knew what to do with them," Causey says. Some were thrown away. During the 1970s, though, interest in bird eggs came back. Scientists noticed that eggshells of some birds had become thinner and more breakable. The insecticide DDT was found to be the cause. Researchers compared the thickness of old eggshells with new. Their findings helped persuade Congress to ban the pesticide. Without eggs from 100 years ago, no comparisons could have been made. Egg and nest collections are important time capsules. They tell of environmental conditions at particular times. This, Causey says, is a good reason to collect more today. Researchers are now examining the old plant material in nests to determine how much carbon monoxide was in the atmosphere decades ago.

In the United States, it is unlawful to collect the nests of migratory birds, unless for approved scientific study. Although not strictly enforced, the law exists to protect birds in the wild from disturbance. When ornithologists study a nest, they know that if it has been unattended for several days, it's usually safe to conclude that the birds have gone and won't be back.  -- Ross Atkin (Copyright © 2002 The Christian Science Monitor).

Literature Cited:

Dial, Kenneth P. 2003. Evolution of avian locomotion: correlates of flight style, locomotor modules, nesting biology, body size, development,
and the origin of flapping flight. Auk 120:941-952.

Ghent, A.W. 2001. Regular Spatial Patterns of Bank Swallow (Riparia riparia) Tunnel Entrances, with Some Possible Evolutionary Implications.
American Midland Naturalist 146: 414-423.

Hartman, C.A. and L. W. Oring. 2003. Orientation and microclimate of Horned Lark nests: the importance of shade. Condor 105:158-163.

Leonard, M.L. and J. Picman. 1987. The adaptive significance of multiple nest building by male Marsh Wrens. Animal Behaviour 35:271-277.

Petit, C., M. Hossaert-McKey, P. Perret, J. Blondel & M.M. Lambrechts. 2002. Blue tits use selected plants and olfaction to maintain an aromatic environment for nestlings. Ecology Letters 5: 585 - 589.

Pettingill, O.S., Jr. 1985. Ornithology in Laboratory and Field, Fifth ed. Academic Press, New York, NY.

Welty, J.C. and L. Baptista. 1988. The life of birds, fourth ed. Saunders College Publishing, New York, NY.

More lecture notes:

Avian Reproduction III: Clutch Size, Incubation, & Hatching

Useful Links:

Eggs, Nests, and Predators

Feathered Nests

Hormonal Control of Incubation/Brooding Behavior: Lessons from Wild Birds

Incubation: Heating Egg

Incubation Time

Masterbuilders

Nest Building

Nest Lining

Nest Materials

Nest Sanitation

Parental Care

Precocial and Altricial Young

Who Incubates?

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