Category: Mating Monday

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: Why Birds Get Divorced

If you missed last week’s Mating Monday on monogamy you can check it out here.

The more I study avian social behavior, the more I realize that humans and birds are not so different after all. Just like in human relationships, sometimes things don’t work out and birds decide to get divorced.

How common is divorce in birds? It’s estimated that divorce occurs in 95% of bird species. Ornithologists describe divorce as when a pair of socially mated birds choose to re-pair with different partners. This can happen after or during the breeding season. There are multiple reasons a bird may want to stay faithful to the same partner. Fidelity has benefits: it helps save energy since time doesn’t have to be wasted finding a new partner. Plus, a familiar pair bond increases cooperation and coordination when raising young. However, when things go wrong there are also many reason why divorce could make more sense than staying together.

It’s common for breeding attempts to fail. Therefore, a pair bond with poor success may find it more beneficial to seek new partners. Mercier et al. states that divorce could be considered an “adaptive mechanism” against having suboptimal partners (2021). A divorcee has the opportunity to find a better mate and possibly more breeding success. Infidelity can also be a reason for avian divorce. Promiscuity in males, but not in females, coincides with divorce rates.

Splitting up doesn’t always have to do with whether or not the nest failed. Sometimes the timing just isn’t right. A study of Eurasian Blue Tits found that 64% of the breeding pairs divorced before the experiment ended, even if the pair had a history of breeding success. Why? If both birds in a pair returned to the breeding territory around the same time they were more likely to stay together, but if the timing was off there was a higher chance of divorce. Since mortality rates are high for many birds, if a mate doesn’t show up “on time” they could be dead or injured. In cases like this fidelity may not pay off. Waiting around could mean losing the chance to breed, so finding a new partner is imperative. Also, studies have found that long-distance migrants such as Arctic Terns have higher divorce rate since unpredictable factors can affect whether or not they return to the breeding grounds on time.

Albatrosses have some of the lowest divorce rates in the avian world. Pairs usually stay together for decades in these long-lived species. However, divorce can still occur and there are some interesting reasons as to why. In Wandering Albatrosses personality can influence divorce. “Shy” males tend to avoid confrontation with bolder males who “intrude” in their territory seeking the female’s attention. As a result the shy bird may give up and desert the relationship. The shy birds were twice as likely to get divorced compared to the aggressive birds.

Black-browed Albatross (left) and Wandering Albatross (right). (Image by Ed Dunens, wikimedia commons)

Environmental factors, such as climate change, may be influencing divorce rates in some albatross species. Ventura et al. studied 15,000 pairs of Black-browed Albatrosses that bred in the Falkland Islands over 15 years. The typical divorce rate of this species is between 1-3%, but in years with unusually warm water temperatures that percentage went as high as 8%. Warm water temperatures means having to travel farther to find food and can trigger stress hormones that leave birds with less energy to raise their chick. Ventura et al. refer to this as the partner-blaming hypothesis. These hormones can lead females to attribute the higher physiological stress to a poor performance by the male. By returning to the breeding grounds exhausted, stressed, and overworked the female may move on to a new partner.

Just for fun I wanted to wrap up this post with some divorce rates: 40-45% in humans, 9% in Mallards, 67% in Piping Plovers, 85% in Emperor Penguins, 99% in flamingos, and 100% in Great Blue Herons. And even swans, which the media likes to have us believe always mate for life, have a 5% divorce rate.

Sweet Mallard Pair (Image by Michelle Horowitz)

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!