Tag: bird mating systems

Mating Monday: Cooperative Breeding

If you missed the previous Mating Monday posts you can check them out here and here.

Cooperative breeding occurs when a mating pair has other birds of the same species helping raise young from one nest. Many time the “helpers-at-the nest” are previous offspring of the parents, but sometimes non-related individuals provide care. This system is widespread throughout the animal kingdom and practiced in approximately 9% of bird species.

“Helpers” play multiple roles. They assist parents with defending the nest and feeding the offspring. A study of Brown-headed Nuthatches (Cusick et al., 2018) found that helpers also removed fecal sacs, but had no part in nest excavation and had no influence on maternal egg production. Cusick et al. concluded that the offspring raised in cooperative groups had more access to food and therefore weighed more. The cooperative nests were also more likely to fledge at least one offspring. Cooperative breeding seems to usually have positive effects for the breeders. But why would an individual forego their opportunity to breed in order to help a nest in which they are not passing on their own genetic material?

There are many factors that influence why cooperative breeding groups exist. One reason may be that environmental constraints limit the opportunities for young birds to breed. Many species that use this system live in regions with low temperature variations and warm winters. This allows the species to become sedentary and establish year-round breeding territories. There may be limited territories due to high-quality habitats being saturated with mature breeders. Low mortality, small clutch sizes, and longevity of offspring contributes to a more stable population that has lower turnover rates, so more birds are available as helpers. Skewed sex-ratios or a shortage of partners (usually females) may also encourage cooperative groups. There are more cooperative breeding groups in altricial species (incapable of moving around on its own shortly after hatching) than precocial (capable of moving on its own shortly after hatching).

Just because cooperative breeding is practiced within a species does not mean that all individuals of the species participate in the system. For example, studies have found that 20-30% of Brown-headed Nuthatches use cooperative breeding as a strategy, with many of the helpers being second-year birds. A study of Long-tailed Tits by Meade & Hatchwell found that helpers had higher overwinter survival rates than non-helpers. Whether or not an individual tit decided to be a helper was determined by the availability of a relative with an active nest. They found that tits that decided to help a relative had a 61% survival rate and the survival rate of those who did not have any relatives with an active nest was 52%. However, male birds that chose not to help a relative with an active nest had a survival rate of 24%. The authors found individuals among the helpers were in good physical condition, the birds without relatives to help were a mix of good/poor condition, and the birds that chose to not help relatives were in poor condition. Meade & Hatchwell also suggested that helping did not have significantly positive effects on reproductive success in the future.

Long-tailed Tit (Image by Alexis Lours via wikimedia commons)

In addition to higher survival rates, helpers may receive other benefits. Helpers may improve their chances of inheriting a mate, territory, or nest. They also gain experience in foraging and may form alliances. Strengthening the genetic makeup of a species through the survival of offspring may be an indirect fitness benefit to the helpers and breeders.

There are many North American bird species that use cooperative breeding. These include Florida Scrub-Jays, Acorn Woodpeckers, Red-cockaded Woodpeckers, Grove-billed Anis, Pygmy Nuthatches, and Gray-breasted Jays.

Brown-headed Nuthatch (Image by DickDaniels via wikimedia commons)

References

Mating Monday: Are Most Birds Actually Monogamous?

One of my favorite sights of spring is seeing bird pairs around my yard. Throughout April the cute American Robin pair would take evening walks across the lawn looking for food. The Northern Cardinal pair frequents the feeders together and I even saw the male feed the female some seeds. It’s a common sight to see pair-bonded birds in the spring because many bird species are considered monogamous. In his 1968 book Ecological Adaptations for Breeding in Birds, British ornithologist David Lambert Lack estimated more than 90% of bird species are monogamous.

Black-crowned Night-Herons are presumed to be socially monogamous (Image by David Horowitz)

Merriam-Webster defines monogamy in zoology as “the condition or practice of having a single mate during a period of time”. In 2002, a study by Griffith, Owens, and Thurman found that “extra-pair offspring” occurs in 90% of bird species. If 90% of species are monogamous, how can 90% also have offspring sired by another male who is not in the pair-bond?

Well, it turns out monogamy in birds is a little more complex than a Merriam-Webster dictionary definition. From an evolutionary standpoint, the idea of monogamy occurring in animals at all is a bit of a strange idea. If an individual has the opportunity to mate with multiple partners and doing so can increase their fitness (aka: their reproductive success), then why would they choose monogamy? Biologists are learning that when it comes to animals, three types of monogamy come into play: social, sexual, and genetic. Lack’s assertion that 90% of species are monogamous refers to social monogamy. This means that bird pairs work together to raise offspring, but there is no guarantee that they are sexually or genetically monogamous. In fact, the Griffith et al. (2002) study contributes to the idea that genetic monogamy (that the two partners only have offspring with one another) is quite rare in birds. True genetic monogamy does occur in some species, such as Common Loons, Dovekies, Florida Scrub-Jays, and some populations of Red Crossbills. However, at least some females in about 90% of bird species will mate with a male who is outside of her pair bond, a behavior called extra-pair copulation.

Why would a female seek out a mate outside of her social bond? First, her social mate may have genes that are inferior, so mating with a more desirable male can allow a female to pass on better genes to her offspring. Secondly, a female is more likely to get her eggs, which are a limited resource, fertilized just in case her social mate is infertile or incompatible. Lastly, extra-pair copulation allows a female to have genetically diverse offspring by multiple fathers. It’s the “don’t put all your eggs in one basket” idea.

Some Mourning Doves stay together for one breeding season and others may mate for life (Image by Michelle Horowitz)

So yes, a vast majority of bird species are monogamous, but it’s not as clear cut as it seems. Many of our cute little bird pairs have something going on “on the side”. Many birds will only stay together for once breeding season and find a new mate the next. There are some species that do mate for life, but that’s not as common as we once thought.

Infidelity, divorce, and even same-sex pair bonds (although rare) occur in birds. And what about the other 10% of birds that don’t practice monogamy? What mating system do they use? Over the next few weeks we’ll explore some of these interesting avian behaviors on Mating Monday. Stay tuned!

Love Birds…or not?

You’ve probably seen an image like this before, especially around Valentine’s Day:

Mute Swans forming a “heart” with their heads. The perfect image of love. Or is it? It’s not as simple as you might think.

When you see a bird with its mate, you might assume that those birds are just mates with each other. Sometimes that’s true, the bird pair you’re watching may be monogamous. However, there is actually a wide variety of avian mating systems, so things are usually not as they seem on the surface.

Wood Duck pair (Image by BirdNation)

3 main types of avian mating systems include social monogamy, polyandry, and polygyny. These systems can be broken down even further, with multiple categories for each system. We aren’t going to go too far into specifics today, but here’s a quick overview of some of the avian mating systems.

Social Monogamy

In social monogamy, one female will form a pair bond with one male. This doesn’t necessarily mean that the pair is mated for life. The pair bond may last only one or several breeding seasons. In the social monogamy system, females may copulate with other males. This means within a clutch of chicks, some chicks may have different fathers. About 92% of bird species are considered monogamous, but it’s actually quite rare to find species that practice true genetic monogamy. Genetic monogamous species only have offspring that are within the specific male/female couple.

Common birds that practice social monogamy include Northern Cardinals, Eastern Bluebirds, Black Vultures, and Mute Swans. Examples of genetic monogamous birds include Common Loons, Laysan Albatrosses, and Florida Scrub Jays.

Northern Cardinal male (Image by BirdNation)
Common Loon on land (Image by BirdNation)

Polyandry

In polyandry, during a breeding season a female will accept sperm from multiple males. There are many reasons why a female would want to have sperm from multiple male partners. A female bird’s oviduct has a special sperm storage tubules (SSTs) where sperm can be stored for days to weeks depending on the species. Eggs are only able to be fertilized for a certain period of time. Therefore, by storing sperm, a female would be able to fertilize a clutch of eggs and not have to mate at a particular time to be able to do so. Females who practice polyandry can use this system as a sort of “insurance policy”; a female is more likely to be able to fertilize eggs if one of her partner’s sperm are defective if she mated with multiple males. Other reasons for polyandry include securing more parental care/resources/genetic benefits from secondary partners, or being forced to copulate by sexually aggressive “partners”.

This mating type is only found in about 1% of bird species. Examples of polyandry in bird species include Spotted Sandpipers, Sanderlings , and Wilsons Phalaropes.

Spotted Sandpiper (Image by David Horowitz)
Sanderling (Image by BirdNation)

Polygyny

In polygyny, males are able to have sexual access to two or more females, however a female will only mate with one male. Males of this mating systems may defend valuable resources that will attract several mates as well as try to monopolize potential mates against rival males. Males may also defend a specific territory and try to convince females to visit and mate with him. This tactic is called lek polygyny. Some males in a lek may not mate with any females, while only a few males will mate with most of the females.

Many grassland birds practice polygyny. Examples include Red-winged Blackbirds, Great Sage Grouse, Bobolinks, and Marsh Wrens.

Red-winged Blackbird, Taylor’s Wildlife Preserve (Image by BirdNation)


And if this all wasn’t complicated enough…

…sometimes birds get divorced. Yep, you read that right. Sometimes the breeding season is unsuccessful and each member of the pair will choose to mate with someone else. Sometimes a partner dies and the widow finds a new mate. Whatever the reason, it just goes to show that avian lives are much more complex than we previously knew or imagined.

So what about those Mute Swans we were talking about earlier? Divorce can rarely occur between Mute Swans, but they are generally monogamous. They do tend to form strong pair bonds and work together well as a team. Does that mean Mute Swans are an accurate depiction of true love? Maybe. Or maybe not…you be the judge. It does prove though that the avian social life is complex and fascinating.