Blog #410 Avian Divorce
Human divorces are much more complicated than avian divorces, courtesy of Flicker.
Many social behaviors of birds and humans are similar because of biparental care. One of the shared behaviors is divorce. Instead of the bewildering upheaval of emotions and financial settlements that drive human divorces, avian divorces are biological. In avian divorce, birds switch partners within the population: ornithologists use this term as a metric of mate fidelity. To appreciate avian divorce, one must understand their reproductive behaviors and their urge to maximize their lifetime reproductive success —the total number of offspring that fledge over a lifetime.
The maxim, “male birds attract, females choose”, is true in the case of avian divorce. The decision to end the pair bond, whether it is a North American or European variant, is typically the female’s choice rather than an unintended consequence such as death or separation during migration. In the case of Ospreys, if a male partner cannot supply adequate food to his mate while courting or incubating, the female will search for a better provider. An important role of intruder Ospreys is to replace missing, ailing, or inadequate mates. In previous years, Orpheus’s mate, Ophelia, always arrived at their nest within a day of Orpheus’ arrival. This year she was five days late, only to find that Orpheus, her mate of nine years, had replaced her with a young female nicknamed Ursula.
The adult female is pushing a new squatter off her nest. Courtesy of Don Martel.
Moskoff (2000) put forth three explanations for why avian divorce is adaptive for some bird species. The incompatibility hypothesis states that even though a pair appear healthy, reproduction is unsuccessful. The better-option hypothesis states that divorce takes place when one mate chooses a mate with superior reproductive fitness, leaving the original partner unmated. The asynchronous-arrival hypothesis or “musical chairs hypothesis,” states that the first partner to arrive on the breeding ground will wait only so long for the other partner to return before choosing a new mate. If a mate arrives at the breeding territory too late from migration, it will find its place occupied by another individual. In the case of the Salt Point Ospreys, Ursula won the musical chairs game.
What are the costs and benefits of avian divorce? It is generally accepted that the principal reason for a bird to divorce is to maximize its lifetime reproductive success. In most species, it is the female seeking to improve her breeding success by mating with an older, more seasoned male who has lost his partner. The female is betting that the older male will be a better hunter with excellent parental skills and a more vigorous nest defense than her previous mate. The divorced male often is unable to find a mate until the next breeding season.
The cost of divorce in Ospreys, as in many birds, is high—usually the loss of a reproductive season— but results in a better outcome. If the search for another mate takes place while the individual is already mated, the success of the first eggs may be jeopardized. Evaluation of recently divorced individuals showed lower reproductive success than continuing pairs: the laying date was later, the clutch size was smaller, and the number of young that fledged was lower in the divorced pairs.
Low divorce rates occur in long-lived species with low reproductive rates.The wandering albatross, a long-lived oceanic species (33 years for males, 25 years for females), has a low divorce rate. Mate fidelity may be adaptive for the albatross because of the length of time required to find a new mate. A widowed or divorced wandering albatross male requires an average of 3.2 years to find a new mate, and a widowed female takes, on average, 2.3 years. Such long delays markedly lower the lifetime chick production rates of 14 percent and 15 percent, respectively. Long-lived Ospreys (up to 25 years) have low divorce rates as well.
Long-lived birds, the Wander Albatross (top) and Osprey (bottom) have low divorce rates.
King and emperor penguins, the largest penguin species, illustrate the are exceptions to these rules. Both are long-lived species but with unusually high divorce rates. Neither species has breeding
Emperor and King Penguins return to their nest asynchronously (courtesy of ebird).
performance issues nor do divorces affect the production of future young. Instead, the penguins' divorce
when females return to the breeding grounds before their mates get there. The greater the asynchrony between the two, the greater the likelihood of divorce. Asynchrony explains only some of the divorces. The “expensive fat-storing hypothesis” may explain other penguin divorces. When the birds arrive on the breeding grounds, both sexes have only about half of their maximum fat reserves, which are close to zero as they fast while caring for their eggs. Mates that arrive late must choose a new partner before they exhaust their fat reserves. While individuals can store enough fat to allow them to wait to copulate with their previous mate, they usually don’t delay, suggesting the energetic costs to store additional fat exceed the benefits derived from retaining mates.
Divorce is predictable in certain circumstances. The better-option hypothesis certainly seems to be common and predictable in humans and birds. Based on the costs and benefits of avian and human divorce, populations in which there is substantial variation in quality between individuals prompts individuals to search for mates that will improve their breeding success. Willingness to wait for the previous partners seems most likely to occur in long-lived species like Osprey and great blue heron (both 25 year lifespans), and American flamingoes (49 years), since waiting takes up only a relatively small proportion of total lifetime productivity that might be lost otherwise.
Long-lived great blue heron (L) and American flamingo (R) wait for their mates to return, courtesy of ebird.
On the other hand, short-lived species cannot afford to miss a breeding season so divorce rates are high.
The rule of thumb is that smaller birds have shorter lifespans. Small songbirds live an average of 2–5 years while large hawks, like Osprey, average 15–20+ years, and bald eagles 20–25 years. However, this “rule” doesn’t hold with warblers, which are usually smaller than typical songbirds but average 3–6 years. Hummingbirds average 9 years. However, the maximum lifespans recorded of many of these species have been much higher: the average life expectancy of a European blue tit is 2.7 years, but the oldest recorded to date is over 21 years old. The oldest Osprey on record in North America was 27 years old.
Ruby-throated hummingbird (L) and blue tit (R), courtesy of Wikipedia.
Short-lived species tend to have high mortality rates with low probabilities that both mates will survive to the next breeding season. Natural selection in these circumstances favors divorce and frequent remating. In species with older females, there appears to be a decrease in the likelihood that they will divorce their mates, possibly because there is a direct correlation between age and reproductive success. The number of unpaired individuals in a population also determines the divorce rate. A decrease in unmated birds from predation or other causes inhibits mate switching and increases the costs of divorce. Mate retention may also be more tied to nest-site fidelity than to a particular individual. If so, one would predict a low divorce rate among species like Ospreys, which have very strong site fidelity. Finally, asynchronous arrival at the breeding grounds and uncertainty about a partner’s return encourages divorce if that breeding season is in danger of being lost.
In birds, divorce is an adaptive strategy to maximize lifetime reproductive success. The large differences in divorce rates between species can be explained by the broad range of life histories and differential costs and benefits associated with divorce. Plausible explanations for divorce include incompatibility, a better option available, and late arrival. Even though the ultimate goal is to maximize lifetime reproductive success, the proximate factors leading to divorce in birds, and humans, remain poorly understood but don’t sound so different.
Human couple divorcing, courtesy of the Las Vegas news.
Eyes to the sky!
Candace
Candace E . Cornell
Friends of Salt Point, Lansing, NY
Cayuga Lake Osprey Network
cec222@gmail.com
HELP PROTECT OSPREYS:
Eggs will be hatching soon. Stay 300 feet away from nesting sites during the breeding season. If the Osprey vocalizes, you are too close! Back off Immediately.
Carry binoculars to view wildlife from afar.
Help keep local waters clean, healthy, and safe.
Recycle used fishing lines, twine, and nets, which can kill Osprey.
Join the Cayuga Osprey Network and volunteer to help monitor Osprey nests: cec222@gmail.com.
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