After-Hatch-Year (AHY): Designation given to a bird known to be in at least its second calendar year of life, but
whether it is in its second year or still older cannot be determined.
After-Second-Year (ASY): Designation given to a bird that is known to be at least in its third calendar year of life, or
Age of Effective Homeothermy: Homeothermic animals are able to keep their internal body temperature constant
even when the outside temperature varies. When referring to a brood of young birds, the age of effective
homeothermy is the age in days when the brood as a group becomes homeothermic, as a result of individuals' growth
in mass that reduces their mass to body surface ratios and growth of insulating feathers, aided by clustering together
to further reduce heat loss. Large broods usually have an earlier age of effective homeothermy than small broods,
due to the clustering effect.
Airfoil: A special shape found in wings of birds and some other flying animals and devices, that produces a lifting
effect as it moves forward through air.
Air Sacs: A system of thin-walled membranous sacs found only in birds. They fill parts of birds' body cavities and
penetrate their muscles and bones. They interconnect with each other and with a bird's lungs to form an efficient one-
way path for air movement during breathing, and they also help in removal of waste heat generated during flight.
They are not used for exchanging oxygen and carbon dioxide.
Air Space: A pocket of air between the shell membranes at the large end of a bird egg. As the developing embryo
uses water and additional water evaporates from the shell, air moves into the egg from outside, expanding the air
Albumen: Egg white; albumen is composed primarily of water and protein.
Allantois: A membranous sac that balloons off a bird embryo's gut. Solid wastes produced by the developing
embryo that can't be passed through the egg shell are diverted into the allantois.
Altricial Young: Young birds that hatch underdeveloped and in many cases naked or with sparse down; such
helpless young require complete parental care.
Anthropomorphism: Interpreting behavior of birds or other animals in terms of human feelings, motivations, or
Asynchronous Hatching: Pattern of hatching in which the eggs of a single clutch hatch over a period of several
days, resulting in a brood of young of different ages. This pattern occurs when incubation begins at the time the first
egg is laid. Because eggs are laid one per day, at one- to two-day intervals, the embryos of the earliest-laid eggs
have already started to develop by the time the later eggs are laid, and they hatch sooner.
Banding: The placement of a thin, light-weight metal band bearing a unique number on the lower leg of a bird, that
can be use to trace its movements.
Barb: Parallel rays branching out from each side of a feather's main shaft or rachis.
Barbules: Tiny hooked branches extending from each side of a feather's barb. The hooks act like velcro to fasten
adjacent barbs together.
Begging: Behavior displays, usually a combination of physical and vocal, that young birds make to stimulate other
birds to feed them.
Birdhouse Network: Operated by the Cornell University Laboratory of Ornithology, it collects breeding data on
cavity-nesting birds, and promotes their study and proper management.
Bolus: A mass of prey items held together in the bill and throat of an adult bird for transfer to a young bird as food.
Brood (noun): The combined nestlings in a nest.
Brood (verb): Sitting on hatched young, or sheltering them under the wings, primarily to keep them warm, but also to
protect them from sun, rain, or predators.
Brood Patch: A patch of skin on the breast and belly of birds that has lost feathers, and become swollen through
both the retention of large amounts of water in the tissues and the expansion of blood vessels feeding the skin. It
develops a few days before egg laying in most individual birds (either male or female) that incubate their eggs by
sitting on them, and increases the efficiency of heat transfer to the eggs.
Brood Reduction: Practice carried out by some parent birds whose young hatch asynchronously and thus vary in
size. The later hatching, smaller young can be at a disadvantage. Usually parent birds first feed the most vigorously
begging offspring until it can swallow no more, and then only then move on to another. In years with low food supplies
only the largest and strongest young will survive, but in years with abundant food smaller young may live as well.
Brood reduction ensures that at least some young will survive in years when food is scarce.
CAM: See Chorioallantoic Membrane.
Candling: A process used to view the contents of a bird egg by shining a bright light through the egg from
underneath. Candling can be done to determine whether an egg is fertile or not, and, if it is fertile, to examine the
growth and development of the embryo.
Chalaza: The gelatinous, usually milky white, stringy coils of albumen (egg white) that surround and protect the egg
yolk, and are visible at either end of the yolk as twisted cords. The chalazae attach to the far ends of the eggshell
and form a suspension system for the yolk that allows it to rotate throughout embryonic development.
Chorioallantoic Membrane: A membrane that forms during embryonic development that eventually lines the inner
egg shell surface. It has many capillaries through which oxygen and carbon dioxide gases are exchanged between
the embryo and outside air.
Clutch: A complete set of eggs; those laid in an uninterrupted series, for a single nesting, by one female.
Clutch Size: The number of eggs in a given clutch.
Cloaca: Common opening at the lower end of the avian digestive tract for the digestive, excretory, and reproductive
systems; it receives feces from the large intestine, uric acid from the kidneys, and eggs or sperm from the gonads,
and releases these materials through the vent.
Cloacal protuberance: A swelling, visible externally in hand-held male passerine birds, caused by the enlargement
of structures in the terminal ends of the ducts that carry sperm from the testes to the cloaca. They form temporary
sperm storage receptacles and may help keep sperm cooler. They may assist in the transfer of sperm to the female’s
Cloacal Contact: When a male’s cloaca touches a female’s cloaca during copulation, allowing the male’s sperm to
be transferred to the female.
Communal Roost: A group of birds gathered to spend the night together, sleeping. The place they roost.
Contact Call: A sound produced by a bird that appears to inform a nearby bird (usually a family member) of the
caller’s location. Often uttered by a mated male and female.
Contour Feathers: Feathers that make up the exterior surface of a bird, including the wings and tail; they streamline
and shape the bird.
Copulation: In birds, the coming together of the male’s and female’s cloacas, during which sperm are transferred
from the male to the female.
Coverts: Small feathers that partly cover the flight feathers of a bird's wings and tail. They help streamline the bird's
profile, reducing frictional drag as it flies through air.
Courtship Displays: Displays performed for the opposite sex to acquire a mate of the same species, maintain a pair
bond, and/or stimulate and synchronize breeding behavior.
Delayed Plumage Maturation: A situation in which one sex remains in sub adult plumage longer than the other sex.
Desertion: When one or both parent birds abandons a nest and its eggs or young.
DNA (deoxyribonucleic acid): The genetic material of all cellular organisms and some viruses, forming the
chromosomes of these organisms. DNA consists of long strands of nucleotides. The sequence of the nucleotides is
the “genetic code,“ which contains the instructions for making proteins, which in turn determine the characteristics of
DNA Fingerprinting: A technique by which the nucleotide sequences of selected portions of the DNA of individuals
of the same species are analyzed and compared to determine how closely related the individuals are likely to be.
Down: Soft, fluffy feathers whose barbs do not cling together, so they trap more air and thus provide extra
insulation. Young birds have natal down before molting into their juvenile plumage.
Ectoparasites: Parasites, such as flies, ticks, fleas, lice, and mites, inhabiting the exterior of a host’s body.
Egg: 1. The ovum; the female reproductive cell sometimes called the egg cell, both before and just after it is fertilized
by a sperm cell. 2. The hard-shelled structure laid by birds, containing the embryo, yolk, and white.
Egg Tooth: A short, pointed, calcareous structure on the tip of the upper beak that develops in bird embryos shortly
before hatching; the embryo rubs and pounds the egg tooth against the inner wall of the eggshell to break it open and
hatch. The egg tooth sloughs off or is resorbed within a few days after hatching.
Embryo: A developing young bird that is still inside its egg.
Embryonic Development: The biochemical processes, programmed by DNA, that take place within the egg,
through which a fertilized egg develops the specialized tissues and organs and of the embryonic bird.
Endoparasites: Invertebrate or protozoan parasites that inhabit the interior of a host’s body.
Enzymes: A protein that assists a biochemical reaction without being consumed in the reaction.
Extrapair Paternity: When young in a nest are fathered by a bird other than the resident male.
Feather Sheaths: A thin, cylindrical tube of keratin surrounding and protecting a developing feather. It eventually
breaks open to let the mature feather unfurl.
Feather Tracts: Areas of a bird’s skin where feathers are attached.
Fecal Sac: A tough, flexible bag enclosing the feces of most passerine nestlings; it allows the parents to remove and
dispose of the feces more easily--parents sometimes grab the fecal sacs as they emerge from a nestling’s cloaca.
Many parents carry the sacs some distance from the nest and drop them, but others consume them.
Fertilization/Fertilized Egg: When the nucleus of a sperm cell and the nucleus of an ovum unite to form a single
cell, the fertilized egg, that has genetic instructions in the form of DNA from both parents. Fertilization takes place
before any albumen or membranes are laid down over the ovum (yolk).
Flange (Oral Flange): Brightly colored enlargements around the base of the bill in nestlings of many species in
which the parents feed the young. The flanges extend from the corner of the mouth and taper toward the tip of the
bill, and are well-supplied with tactile nerve endings. Touching a flange causes the mouth to spring open, and the
colors may help parents to place the food properly.
Fledging: The time when nestlings that have been reared in a nest leave the nest. The act of leaving the nest by an
Fledgling: A young bird that has recently fledged.
Flight Feathers: The long feathers of the wing (Remiges) and tail (Rectrices) that are most involved in propelling
and steering birds as they fly.
Floaters: Birds that do not hold territories or form pair bonds, but cruise around areas containing territorial
individuals, waiting for a chance to take over a territory or nest, or sneak a copulation with a paired bird.
Foraging: Behavior performed to obtain food.
Frequent Copulation: An option a male bird has for trying to assure he is the father of at least some of the young in
Gaping: Begging behavior of young birds that begins shortly after hatching in which they open the mouth widely; may
be accompanied by a begging call.
Gene: The sequence of base pairs within a molecule of DNA that codes for one specific protein.
Genus: Level of classification of organisms above “species” and below “family.”
Geographic Range: The geographic area within which a species generally remains at a particular time of year. A
species may have different breeding and nonbreeding ranges.
Germinal Spot: The light-colored site on the egg yolk where the embryo will eventually develop.
Golondrinas de las Americas (Swallows of the Americas): A community of biologists dedicated to studying the nine
swallow species of the genus Tachycineta from Alaska to Argentina using a standard set of uniform procedures to
Gonads: The primary sexual organs, the testes and ovary, which produce, respectively, sperm and eggs.
Habitat: The surroundings in which an organism lives. It consists of physical factors, such as light, temperature, and
moisture, as well as living organisms, such as plants and animals.
Hatching: Emerging from the egg. A clutch may hatch Synchronously (all at about the same time) or
Asynchronously (over a period of days).
Hatch Year (HY): The age designation of a young bird that is still in its first calendar year of life. On December 31
of its first year it is still a HY bird. On January 1 becomes a Second-Year (SY) bird.
Hyperthermia: Condition in which the body temperature rises a few degrees above normal; if the animal cannot
bring its temperature down, it soon dies.
Hypothermia: Condition in which the body temperature drops below normal. If the animal cannot bring its
temperature back to near normal, it soon dies.
Immediate Cause: In bird behavior a particular stimulus in an individual bird's current environment that causes the
bird to release a particular automatic behavioral response. Also called a Proximate Cause.
Incubation: The process by which animals that lay external eggs keep those eggs at the proper temperature for
embryonic development. In most birds incubation is done by sitting on the eggs.
Infanticide: The killing of a young member or members of one’s species; sometimes done to obtain the nest of
another adult or to cause another adult to reproduce with the killer.
Interspecific Competition: Direct competition for limited resources among individuals of different species.
Intraspecific Competition: Competition (for food, territories, mates, etc.) that occurs among members of the same
Intruder: A bird that approaches and encroaches on another’s territory or nest site; often while prospecting for nest
site vacancies or determining if a resident is weak enough to be forced off its resource.
Iridescent Colors: Structural colors (colors that are not due to pigmented substances) that shimmer and glitter
because they change in brightness as the angle of view changes.
Juvenile: A young bird, usually in its Hatch Year, first calendar year of life.
Juvenile Plumage: Feathers worn by juvenile birds after they have molted their natal down; it consists of the first
true contour feathers.
Keratin: A tough protein that forms scales and claws and is the primary structural component of mature feathers.
Kerfs: Shallow grooves cut horizontally below the entrance hole on the interior of the front of a nest box. Kerfs
provide footholds for nestlings and adults as they climb up to the hole.
Lores: The small areas extending from the front of a bird’s eyes and the base of its upper beak.
Mate Guarding: Following of a female bird by its male mate in order to prevent her access to other males and their
access to her. An option a male bird has for assuring he is the father of at least some of the young in his nest.
Membrane: A thin flexible layer of tissue connecting, covering or separating various body parts or organs.
Metabolism: All the chemical processes that take place in the cells and tissues of the body.
Migration: The regular predictable movement of all or part of a population from one location and climate to another
location and climate; usually refers to seasonal journeys between breeding grounds and wintering grounds.
Mobbing: Behavior in which a group of birds swoop and dash at a potential predator. Mobbing birds usually give
alarm calls that are easy to locate and thus attract additional birds. Mobbing behavior may distract the predator, drive
it away, or let it know it has been detected.
Molt: The process of shedding all or part of a bird's feathers and replacing them with new ones.
Mortality: Death; factors causing death of an organism.
Nest: A structure built, excavated, or taken over by a bird, in which the eggs are laid and remain until they hatch. In
many species, the young remain in the nest until they are able to fly.
Nest Cup: A depression in the nest to hold the eggs.
Nesting (Breeding) Season: The period of time during the year when a particular species may breed.
Nestling: A young bird still living in the nest; a bird that has not fledged yet.
Obligate Secondary Cavity Nesters: Bird species that nest in cavities but do not or cannot excavate their own,
instead obtaining cavities that were created by other species or by physical processes such as decay or erosion.
Oil Gland (Uropygial Gland): A gland, located at the base of the tail on the dorsal side of a bird’s body, that
secretes oils that birds spread over their feathers during preening. The oils keep the skin supple and the feathers
and scales from becoming brittle, but they do not appear to waterproof the feathers.
Ovary: The female gonad; it matures and releases egg cells periodically throughout the breeding season in a
process called ovulation. In most birds only the left ovary is functional, and it enlarges greatly during the breeding
Oviduct: The tube that transports the egg from the ovary to (in birds) the cloaca. After the egg enters the oviduct as
an egg cell (ovum) and is fertilized, different sections of the ovary add different substances to the ovum to produce
the hard-shelled egg. In most birds only the left oviduct is functional.
Ovum: The female reproductive cell sometimes called the egg cell, both before and just after it is fertilized by a
Parabronchi: Tiny tubes in bird lungs through which air moves in a one-way flow. Oxygen and carbon dioxide are
exchanged in microscopic capillaries in spaces within parabronchi walls.
Parasite: An organism that feeds upon the tissues of another, host organism. The parasite lives either on the
outside of the host (an ectoparasite) or within the host's body (an endoparasite).
Passerines: All birds in the Order Passeriformes; also called Perching Birds. Nearly one-half of all the world’s bird
species are passerines, all of which have a foot adapted for perching on branches or stems.
Pectoralis Muscles: The largest and most powerful muscles in a flying bird's body. Their contractions power the
Pipped: Describes an egg about to hatch, in which the embryo has punctured a small hole.
Plumage: 1. A bird’s entire feather coat. 2. The set of feathers produced by a particular molt.
Predator: An animal that preys on other animals.
Predator Guard: A device mounted on a nest box pole or on a box itself that prevents or discourages predators
from reaching the box’s contents.
Preen: Feather maintenance behavior in which a bird grasps a feather near its base, then nibbles along the shaft
toward the tip with a quivering motion; this cleans and smoothes the feather. Many birds gather on their bill oily
secretions from the oil gland, and then spread them on their feathers as they preen.
Precocial Young: Describes young birds that hatch in a relatively developed state--downy and with their eyes
open. Many are soon able to walk or swim and even eat on their own.
Premature Fledging: A young bird leaving its nest before it is developmentally ready to leave. Death often results.
Primaries: The long flight feathers of the outer wing.
Proteins: Complex molecules composed of strings of amino acids; proteins are the main building blocks of all living
organisms and also act as enzymes, assisting chemical reactions.
Proximate Cause: See Immediate Cause.
Rachis: That part of a feather's central shaft that has rows of parallel barbs on each side.
Range: see Geographic Range.
Raptors: Members of the orders Falconiformes (hawks, eagles, harriers, accipiters, falcons, etc.) and Strigiformes
(owls); the diurnal and nocturnal birds of prey.
Rectrices: The technical name for the long flight feathers of a bird's tail.
Remiges: The technical name for the long flight feathers of a bird's wing; the primary and secondary feathers
together make up the remiges.
Response: In bird behavior an individual bird's specific automatic reaction to a specific stimulus or cue in the bird's
Roost: To sleep overnight.
Scientific Name: The “scientific name” of a species is formed of the genus name (capitalized) and the species name
(lower case); for instance, Tachycineta bicolor is the scientific name of the Tree Swallow.
Secondaries: The long flight feathers of a bird's inner wing.
Single-Brooded: A species that normally nests only once per nesting season.
Songs: Loud vocalizations, often given from an exposed perch, that are presumed to attract mates or repel territorial
Songbirds: Members of Suborder Passeri, which is one of the two large suborders of Order Passeriformes (perching
birds); songbirds have particularly complex voice boxes, which allow them to sing more complex songs than other
Species: Level of classification below “genus”; similar species are placed within the same genus. A species is
usually defined as a group of potentially interbreeding individuals that share distinctive characteristics and are unlikely
to breed with other such groups of individuals.
Sperm: Male reproductive cells; each sperm cell is composed of a DNA-containing head and a propulsive tail.
Sperm Competition: Occurs when females mate with more than male and the sperm "compete" to fertilize the same
Sperm Storage Tubules: Specialized structures within a female bird's reproductive tract where sperm cells from
males can be stored for as much as many days before they are released to fertilize an egg.
Stimulus: In bird behavior a specific cue in an individual bird's environment that automatically triggers a specific
behavioral reaction or response.
Subcutaneous Parasite: A parasite that penetrates outer body tissues of its host lives at least part of its life cycle
Supracoracoideus Muscles: The muscles primarily responsible for raising a bird's wing on the upstroke during
Swallows: Birds of the passerine family Hirundinidae that are adapted for hunting insects on the wing. Swallows all
have slender, short-necked, streamlined bodies; long, pointed wings; and small bills but with very wide gapes; short
legs and small feet.
Synchronous Hatching: Pattern of hatching in which all the eggs of a single clutch hatch at about the same time
(on the same day), resulting in a brood of young all the same size and age. This hatching pattern occurs when
incubation is delayed until the last egg is laid. Because development of laid eggs does not begin until they are
warmed, all the embryos begin to develop at the same time and are ready to hatch at the same time.
Syrinx: The organ, located at the base of the windpipe, used by birds to produce calls and songs.
Tachycineta: The genus of the Tree Swallow. There are a total of nine species in the genus Tachycineta. All live in
the New World. All are rather short-tailed, with blue, green, violet, or coppery upperparts and solid white underparts.
Takeover: When one bird drives out and displaces another from a nest site or territory.
Testes: The male gonads. Testes produce sperm.
Thermoregulation: Process by which an animal regulates its body's heat. Birds and mammals are able to generate
and maintain body heat through their own metabolic processes.
Tree Swallow: Tachycineta bicolor. One of nine species of the New World genus Tachycineta.
Ultimate Cause: In bird behavior the reason, found in the bird species' long-term history, why a particular behavior
has become one of the genetically-fixed response patterns of every member of the species.
Uropygial Gland: see Oil Gland.
Vanes: The two flat surfaces, composed of parallel rows of interlocking barbs, that protrude from flight and contour
Vent: The opening of the cloaca to the exterior of the body. In birds the vent is the only posterior opening, and thus
it releases feces from the digestive system, uric acid from the excretory system, and sperm or eggs from the
Vitelline Membrane: The transparent membrane surrounding and holding together the yolk of an egg.
Yolk: The yellow portion of an egg; the yolk contains nearly all of the lipids (fat) and most of the protein needed by
the developing embryo. The yolk is surrounded and held together by the transparent vitelline membrane.
Learn About Birds at Tree Swallow Nest Box Projects