Bird Reproduction

Definition: Bird reproduction is a key process, which allows birds to create new individuals, transmit their genes, and perpetuate the species. Bird reproduction differs from other types of animals in various aspects, ranging from anatomical (absence of external genital organs in most males, operation of a single ovary in females, which is usually the left, etc.) and physiological (production of hormones, increase of the size of the sexual organs), up to the ethological (exhibitions of pairs, territorial behavior, mating, and parental).

Pair Formation

To find a mate, birds resort to a series of signals, which can be simple or complex, depending on the species. The signs can be auditory:

Songs and calls
Feather vibrations
Drumming displays

Signals can also be visual:

Extravagant crests
Air sacks
Bright plumages

And Behavioral signs such as:

Dances
Spectacular acrobatic flights
Movement of wings and tails
The offering of food items and nests and nest sites

courtship_bird_reproduction — *Silver Grebes(Podiceps occipitalis) in a nuptial dance.*

In general, it is the males who exhibit these signals, and they do so to attract females and provide them with information about their strength, abilities, and health, to demonstrate that they are the most suitable candidates with which to copulate and have strong and healthy offspring with a greater chance of survival.

The Copulation

Once the pair is formed, and the courtship is over, “copulation” takes place, which also varies according to the species. Because most males do not have penises (except for some waterfowl, such as ducks, swans, geese, and ostriches), copulation occurs through the union and rubbing of their cloaca.

The cloaca is a small opening found in both males and females and has a reproductive and excretory function. Birds use the cloaca to transfer sperm, lay eggs, and excrete urine and feces.

The rubbing of cloacas between males and females is commonly known as a “cloacal kiss” and usually lasts a few seconds. For this to happen, the male must climb on top of the female, and both must move their tails, exposing their cloacas, allowing them to touch and transfer sperm and insemination. Subsequently, the sperm is driven through the oviduct to the infundibulum, where fertilization occurs.

mating_bird_reproduction — *A pair of Scaled Ground-Doves (Columbina* squammata) copulating.

Establishment of the Territory

When the pair has been formed, in most species, their territory is established. A territory is an area defended by a bird against individuals of the same or other species. The territory must provide the breeding pair with sufficient food resources and protection from predators, as it will be the area where they will build the nest and raise their young.

The size of the territory varies according to the species and its needs. For example, raptors have large territories, while some passerines have small territories. In birds that form large colonies, such as seabirds, a pair’s territory is a small area around the nest that is defended by the incubating bird (while their partner is looking for food). The territory is established and defended generally by the males; however, the female defends it in some species.

The Construction of the Nest

When the egg finishes forming inside the female, it is deposited in a nest. This process may take a few seconds or several minutes. Bird nests vary in form, structure, the material used, and location, depending on the species. There are cup-shaped nests, cones, ovals, mounds, and pendants; in cliffs, branches of trees, cavities in trees, vegetation on water, etc. In some cases, the birds do not build a nest but place the eggs in depressions in the ground or only on leaf litter. The material used to build a nest depends largely on the elements available in the area.

There are nests made of plant material, such as twigs, herbs, lichens, mosses, and seeds. Animal material includes feathers, hair, spider web, excrement, and saliva mixed with mud and vegetal material. With the ever-increasing presence of human garbage in natural habitats, plastic, paper, ropes, and other materials are frequently included as nest material by birds.

nests of birds_nest hanging — *A Crested Oropendola (Psaracolius* decumanus) brings nesting material to a nest under construction.

The nest can be built by the pair or by only one of the parents, while the other seeks material and defends the territory or does not participate. Some birds reuse an old nest, either on their own or from another pair or even species. Some birds steal already built nests by instigating the owner, causing them to abandon the nest. There are also birds known as nest parasites, which place their eggs in other species nests. The host species then incubate and raise the parasite’s species young.

The number of eggs a bird lays is variable. Some birds only lay 1 egg, such as penguins and albatrosses, others 2 or 3, as do several passerines, and others can lay more than 10, such as chickens and ducks. The number of eggs depends on several factors, such as the availability of food, the time and effort required to raise chicks, the parents’ age, and the nest’s exposure.

Incubation and Hatching

After egg-laying, incubation follows, which is when the birds transmit heat to the eggs through their brood patch. The brood patch is an area on the belly stripped of feathers and filled with blood vessels that facilitate heat transmission. Some species incubate since they lay the first egg, while others wait until the clutch is complete. The incubation can be done by one or both parents and can last between 10 and 80 days, depending on the species.

When the embryo has finished developing inside the egg, then comes the moment of hatching. To break the eggshell, the chick develops a structure in the shape of a tooth located at the beak’s tip.

The Care of the Young

The chicks at hatching can be nidifugous or nidicolous.

Nidifugous: are nestlings that at hatching are covered in down, have their eyes open, and can walk and follow their parents. Some nidifugous birds can look for their food and feed themselves, while others follow their parents. Some chicks are fed by their parents during the first few days or follow them to places where food is readily found. Although nidifugous chicks can feed themselves, they are still watched and protected by their parents.
Nidicolous: The nestlings are defenseless and are not usually born with down. They have their eyes closed, remain in the nest, and are fed by their parents for a certain period until they fend for themselves.

As it happens during the incubation period, the care and feeding of the offspring can be done by one or both parents, depending on the species.

Systems of Pairing in Bird Reproduction

The mating systems are classified according to the number of sexual partners that an individual has, usually during the reproductive season.

The number of mating partners that an individual may have, whether male or female, is linked to the spatial distribution of resources (food, territory, nesting sites), the availability of the individuals with whom to mate, and the type of parental care (if any). These systems are:

Monogamy: It is the most common mating system in birds composed of a mated male and female pair. A mated pair can last for a single reproductive season or a lifetime. In most monogamous species, both parents provide parental care.
Polygamy: Polygamy is when an individual has more than one reproductive partner. The parental care of the young differs among polygamous birds.

Types of polygamy include:

Polygyny: in which a male mates with several females. In polygyny, the female is in charge of the egg incubation and care of the chicks.
Polyandry: in which a female mates with several males. Each male is responsible for caring for a nest, incubating the eggs, and feeding the chicks, which may or may not be its offspring.
Promiscuity: In this breeding system, each individual can mate with several individuals of the opposite sex without establishing a pair of exclusive relationships or lasting links.

Bird Reproduction