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 size of the sexual organs), up to the ethological (exhibitions of couples, territorial behavior, mating, and parental).
To find a mate, birds resort to a series of signals, which can be simple or complex, depending on the species. The signals 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:
- Spectacular acrobatic flights
- Movement of wings and tails
- The offering of food items and nests and nest sites
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, in order 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.
Once the couple is formed and the courtship is over, “copulation” takes place, which also varies according to the species. Because most males do not have penises (with the exception of some waterfowl, such as ducks, swans and geese, and flightless birds, such as the ostrich), copulation occurs through the union and rubbing of their cloaca.
The cloaca is a small opening that is found in both males and females and has a reproductive and excretory function, ie, it is not only used to transfer the sperm and lay the eggs, but also excrete the urine and feces.
The rubbing of cloacas between a male and a female is commonly known as “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.
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 species or other species of birds and also predators. The territory must provide the breeding pair with sufficient resources of food and protection, 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, the territory of a pair is a small area around the nest that is defended by the bird that is incubating (while their partner is looking for food). The territory is established and defended generally by the males; however, in some species, it is the female who defends it.
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, in branches of trees, in cavities in trees, in vegetation on water, etc. In some cases, the birds do not build a nest, but place the eggs in depressions in the ground or simply 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, 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, pieces of ropes and other material al frequently included as nest material by birds.
A Russet-backed Oropendola (Psaracolius angustifrons) bringing 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 simply 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 the nests of other species to the other species incubates and raise their young.
The number of eggs a bird lays is variable. There are birds that 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 age of the parents, and the exposure of the nest.
Incubation and Hatching
After egg laying, incubation follows, which is a process in which 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 the transmission of heat. 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, which is located at the tip of the beak.
A one-day-old chick Great Thrush (Turdus fuscater) and an egg yet to hatch.
The Care of the Young
The chicks at hatching can be nidifugous or nidicolous.
- Nidifugous: are those that at birth are covered by 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 are fed by their parents or they are the ones who tell them where to eat. 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, they remain in the nest and are fed by the parents for a certain period of time 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, and that usually occurs during the reproductive season. The number of couples 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 care parental (if any). These systems are:
- Monogamy: It is the most common mating system in birds, and refers to the formation of a pair between a male and a female. A pair formation can be for a single reproductive season, or for a lifetime. In most species, both parents provide parental care.
- Polygamy: Polygamy is when an individual has more than one reproductive partner. The parental care of the young also differs among polygamous birds.
Types of polygamy include:
- polygyny: in which a male mates with several females. In polygyny, it is the female that 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 or exclusive relationships or lasting links.