(1) NEST LOCATION: Since many species are quite flexible in situating their nest, the symbol often represents only the most likely location. The symbol for the primary site is given at the left-hand margin of the summary line. Secondary locations are indicated by words or abbreviations shown below that symbol. These words are shown in the list beginning on the next page just under their symbols. For a very few unusual sites the location is given by a word only (no symbol). In the summary line, the numbers just under the location information indicate the most likely height in feet above ground, and the numbers in parentheses (if present) indicate the general range of heights at which nests of the species have been found. The heights indicated apply only to those nest locations given above the numbers, not below. A species whose nests are found in trees as high as 50 feet, in shrubs, and on the ground will be shown as O'-50'.

Note that in this key the most abbreviated form of a word is always given, although where space permitted we spelled words out in the summary line.

Locations Defined by Topographic Features:

Bank. Includes river banks, areas of soft soil on steep island slopes, etc., where nest burrows are excavated.

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Ground. Includes nests placed among the roots, or in niches among the roots of fallen trees, among tules and reeds (in marshes), among grasses, on bare rock, or simply scraped in the dirt or sand.

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Cliff. Includes nests situated in natural crevices or on ledges of cliffs typically offering a commanding view of a defensible position, and sometimes chosen when no suitable trees are available.

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Locations Defined by Supporting Plant Structures:

Shrub. Includes nests placed within any multi-stemmed woody plant (i.e., one that does not have a distinct single trunk extending several feet between the ground and the lowest branching point).

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Deciduous tree. Includes nests placed in any broad-leaved tree, whether it sheds all of its leaves in the fall ("deciduous") or not ("live"): oaks, maples, poplars, hickories, magnolias, etc. Also used for species that use broad-leaved and coniferous trees more or less indiscriminately.

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Coniferous tree. Includes nests placed in any tree that bears cones: pines, spruces, junipers, firs, etc.

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Snag. Nests in a standing dead tree. Also used for species that use cavities in dead and live trees more or less indiscriminately.

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Vine tangle. Includes nests in vines, brambles, brush piles, etc.

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Floating on water. Almost always anchored to live emergent or submerged vegetation.

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(2) NEST TYPE: The symbol shows the type of nest most frequently used by that species. Birds of the same species tend to construct similar nests, but the materials available often differ from area to area. (See essays: Masterbuilders; Nest Materials; Nest Lining.) Secondary nest types are listed beneath the symbol and usually discussed in the treatment paragraph.

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Scrape. A simple depression usually with a rim sufficient to prevent eggs from rolling away. Those of many duck species are almost bowl-shaped. Occasionally with lining added.

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Cup. Typical of songbirds, this is the archetypal "bird nest." Hemispherical inside with a rim height several times the diameter of the eggs. In some cases bulky, but always with a deep depression.

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Saucer. A shallow cup with the height of the rim not more than two times the diameter of the eggs. Also a flattened nest of pliable vegetation as in some wetland birds.

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Platform. A structure in a tree, on a cliff, or providing a dry place above marshy ground or water, usually big enough for the bird to land on, with or without a distinct depression to hold the eggs. Typical of many raptors and birds of wetlands.

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Cavity. Either excavated, as is typical of woodpeckers, or natural cavity found in dead or dying limb or tree. Sometimes a cup or other structure is built within.

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Crevice. Eggs placed in a crack in the face of a cliff, between boulders, in a human-made structure, etc.

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Burrow. Eggs placed in a chamber at the end of a tunnel. Tunnels either excavated by the birds (most kingfishers, puffins, storm-petrels) or usurped from small mammals, especially ground squirrels and prairie dogs.

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Pendant. An elongate sac-like nest suspended from a branch.

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Spherical. Globe-shaped or ball-shaped. A roughly round structure, fully enclosed except for a small opening usually on the side or at one end.

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(3) WHO BUILDS THE NEST: The male (M) and/or female (F) code(s) below the nest symbol indicate which sexes participate in nest building. If both sexes participate, but one does much more than the other, there is a minus sign (-) preceding the symbol of the less-involved sex, and that sex is presented second. In cooperative breeders, if birds other than the breeding adults help with construction, there is a plus sign (+) following the codes for the breeding adults.

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(4) EGGS: The mottled egg symbol is used if the eggs have markings, the white egg symbol if it is unmarked.The number(s) just below the symbol indicate the most common clutch size, or the range (there is often individual and/or geographic variation in clutch size). The number(s) in parentheses below that line or to its right indicate more extreme values recorded in the literature. In many species the data on clutch size are limited. Furthermore it is often difficult to determine when clutch sizes at the higher end of the range indicate the production of two or more females laying in the same nest (we have tried to exclude such values here), and when clutch sizes at the lower end of the range indicate incomplete production. Clutch size can also be affected by a female's age, by whether the clutch is produced early or late in the season, by whether it is the female's first clutch or a replacement clutch, as well as by other factors (see essays: Average Clutch Size; Variation in Clutch Sizes; Brood Parasitism; and Cooperative Breeding).

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(5) MATING SYSTEM: The following abbreviations found below the clutch size listed beneath the drawing of the egg indicate which breeding system is typical for the species.

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Monogamy. One male mates with one female (see essay: Monogamy).

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Polygyny. One male mates with two or more females (see essay: Polygyny).

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Polyandry. One female mates with two or more males (see essay: Polyandry).

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Promiscuity. Males and females mate more or less indiscriminately (see essays: Promiscuity; Leks).

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Polygamy. Both polygyny and polyandry occur.

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Cooperative. Two females rear broods in the same nest simultaneously and/or non-breeding birds serve as helpers at the nest of one or more breeding pairs (see essay: Cooperative Breeding).

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Monogamy is, by far, the most common mating system in birds, and unless there is evidence to the contrary, we have assumed a species to be monogamous. Note that very often a small percentage of birds in a population deviate from the mating system of the majority. It is not, for example, unusual in an otherwise monogamous population to find 5 percent of the males polygynous. Conversely, in virtually every population of a polygynous species there will be at least a few monogamous pairs. In our classifications, unless at least 15 percent of the birds use the minority system (given in parentheses), it usually will not be identified in the summary line. Here again, careful observations are needed to see if nonmonogamous systems, especially polygyny, are more widespread than currently thought.

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In cases when the mating system is unknown, a ? is entered.

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(6) INCUBATING SEX: The male (M) and/or female (F) codes at the top of the fourth column of the summary line indicate whether both parents, or only one parent, incubates. As in nest construction, if both sexes are involved but one spends much more time on the nest, there is a minus sign (-) before the symbol of the less-involved sex, and it is listed second; in cooperative breeders, if birds other than the breeding adults are involved, there is a plus sign (+) following the symbols of the pair.

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(7) LENGTH OF INCUBATION: The number(s) following the "I:" are the usual number of days (or recorded range of days) from the start of incubation to hatching. Note that hatching of a clutch is often synchronized by delaying the start of incubation until the last egg is laid. Incubation time is somewhat geographically variable within species, and your accurate observations could add to our knowledge of that variation. Numbers appearing in parentheses represent recorded extreme values.

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(8) DEVELOPMENT AT HATCHING: Birds show great variation in their degree of development at hatching, and we show the maturity of hatchlings for each species on the line under the incubation time. In North America there are no fully developed (PRECOCIAL 1 ) young at hatching. Our most fully developed young at hatching are classified "PRECOCIAL 2," exemplified by ducklings and shorebird chicks. They are downy, open-eyed, mobile at birth, and find their own food while following their parent(s). At the opposite extreme, our songbirds are "ALTRICIAL" -- born naked, immobile, and wholly unable to feed themselves.

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Developmental patterns are explained fully in the essay Precocial and Altricial Young. In North America seven conditions of young at hatching are found:

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PRECOCIAL 2 Mobile, downy, follow parents, find own food.

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PRECOCIAL 3 Mobile, downy, follow parents, are shown food.

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PRECOCIAL 4 Mobile, downy, follow parents, fed.

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SEMIPRECOCIAL Mobile, remain at nest, fed.

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SEMIALTRICIAL 1 Immobile, downy, eyes open, fed.

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SEMIALTRICIAL 2 Immobile, downy, eyes closed, fed.

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ALTRICIAL Immobile, downless, eyes closed, fed.

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Other than in the summary line, the word "precocial" used alone in this guide refers to the first four categories collectively; similarly the word "altricial" refers collectively to the last three categories.

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(9) TIME FROM HATCHING TO FLEDGING: The number(s) under the development pattern, following an "F:", are the usual number of days (or recorded range of days) until precocial young are able to fly competently and the time required before altricial young leave the nest (altricial species may not be able to fly competently when they depart the nest). Numbers in parentheses represent recorded extreme values. Again, fledging times are variable and your accurate observations may be useful. Note that fledging rarely means the end of parental care. Precocial young of some species, such as oystercatchers, stay with and are helped by the parents long after they can fly; after they have left the nest, altricial young may be fed more than twice as long as they were fed in the nest.

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(10) WHO TENDS THE YOUNG: How the parents divide the feeding (and guarding) of the young is shown just under the hatch-to-fledge time. For precocial species whose young are not fed by the parents, this indicates who tends the young while they feed. The letters coding the sex of the tending parent are the same as those used in (3) above.

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(11) DIET DURING BREEDING: The symbol shows the primary type of food eaten during the breeding season. Secondary types of food commonly taken may be shown by words or abbreviations below the symbol. Many of these words are shown in the list below just under their symbols. Additional specific food items taken less frequently are listed in the treatment paragraph and details of primary food types are often given there (since the symbols cover broad categories). Remember, many species have an entirely different winter diet, and if so, this is usually described in the treatment paragraph.

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Animal Foods Include:

Small Mammals. Anything from shrews to ground squirrels and rabbits, but most often rodents.

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Birds. May include their eggs -- if so, that will be mentioned in the treatment paragraph.

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Small Terrestrial Lower Vertebrates. Includes reptiles (lizards, snakes, etc.) and amphibians (salamanders, frogs, etc.).

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Fishes. Sometimes includes fry and eggs, in which case that usually will be mentioned in the treatment paragraph.

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Terrestrial Invertebrates. May include insects, spiders, mites, snails, slugs, worms, millipedes, sowbugs, etc. Usually predominance of insects.

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Aquatic Invertebrates. May include aquatic insects, crayfish, shrimp, snails, bivalves, etc.

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Carrion. Prey found dead.

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Plant Foods Include:

Nectar. The sugary solution found in many flowers.

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Fruits. Includes berries, which are simple fleshy fruits.

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Greens. May include leafy parts of both aquatic and terrestrial plants.

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Nuts. Hard, dry, single-seeded fruits, often acorns and beechnuts.

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Seeds. Includes grains, sunflower seeds, conifer seeds, etc.

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Omnivorous. A variety of plants and animals too diverse to specify here; neither plant nor animal food usually comprises less than one-third of diet.

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Many birds, especially passerines, that eat both seeds and insects take proportionately more insects when seasonally available. Although seed-feeding birds frequently consume gravel to aid in grinding seeds, we have not included grit in our description of diets.

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(12) FORAGING TECHNIQUES: The major method each species uses to obtain food during their breeding season is the last symbol on the right in the summary line. The symbol is often supplemented by a word or abbreviation that indicates less frequently used foraging techniques. These words are shown below just under their symbols. In the case of foraging techniques we again find gaps in the record, offering ample opportunities to provide the missing information. Note both primary and secondary techniques may be used to obtain primary food items.

Techniques for Picking Food from Ground Surface or Plants While Walking or Clinging Include:

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Ground Gleaning. Picking up items from the surface of soil, turf, sand, etc. Includes scavenging dead aquatic organisms from shorelines.

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Gleaning from Foliage and Occasionally from Branches. Takes invertebrates and/or fruit from vegetation, not from the surface of the ground.

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Gleaning from Tree Trunks and Branches. Describes foraging that only rarely includes removal of invertebrates from foliage as well. Includes excavating and drilling into bark.

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Hovering Techniques Include:

Gleaning while Hovering. Takes nectar, insects or berries from plants above the ground while hovering

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Hovering and Pouncing. Hovering before swooping or dropping down on prey.

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