The spotted sandpiper (Actitis macularius) is a small shorebird that breeds across most of North America. It is known for its unique polyandrous breeding system, in which females mate with multiple males and leave the males to incubate the eggs and raise the young. This unusual strategy sets the spotted sandpiper apart from most other bird species.
What is polyandry?
Polyandry refers to a mating system in which one female mates with multiple males in a single breeding season. This differs from monogamy, in which a female mates with only one male. Polyandry is rare among birds, occurring in less than 1% of species. The spotted sandpiper is one of the best-known polyandrous species.
During the breeding season, female spotted sandpipers will mate with up to four males in their territory. Each male may father some of the female’s clutch of four eggs. The female lays one egg per day, usually in the nest of each male. She provides no parental care after laying her eggs.
Why do spotted sandpipers exhibit polyandry?
There are several hypotheses for why spotted sandpipers evolved a polyandrous breeding strategy:
Deception hypothesis
By mating with multiple males, females may trick each male into thinking he fathered the whole clutch. This ensures each male helps incubate the eggs and raise the chicks, even if some are not his genetic offspring.
Dilution effect
With multiple potential fathers, the costs of predation on eggs or chicks are spread across several males rather than a single pair. If the nest fails, no male loses his complete genetic investment.
Reduced infanticide
Related to the dilution effect, polyandry may have evolved to reduce the risk of males killing unrelated chicks after hatching. With mixed paternity, males may be less likely to commit infanticide of chicks in their own nest.
Increased genetic diversity
The female’s offspring will have greater genetic diversity with multiple fathers. This may increase fitness in changing or unpredictable environments.
Limited mate hypothesis
Quality males may be scarce in spotted sandpiper populations. Females mate multiply to ensure fertilization from at least one high-quality male.
Flexible mate choice
Polyandry allows females to choose the best male for each reproductive event, like copulation or egg laying, rather than being stuck with a single low-quality mate.
How does polyandry work in spotted sandpipers?
The polyandrous breeding system of the spotted sandpiper operates in a predictable sequence across the stages of the breeding season:
Courtship and mating
In spring, females arrive at the breeding grounds first. Upon arriving, the male spotted sandpipers establish and defend territories. When a female enters a male’s territory, he performs a courtship display called the broken-wing act. He flies with noisy wingbeats, then lands and drags his wings as if injured. If receptive, the female will mate with the displaying male. She may mate with up to four males.
Egg laying
Approximately 5-7 days after mating, the female will lay a clutch of four eggs. She lays one egg per day, usually in each male’s nest but sometimes in other locations within the territory. Males may try to entice the female to lay in their nest by creating egg scrapes lined with grasses and sticks.
Incubation
Once the female has laid her full clutch, she provides no further care. The male spotted sandpipers take over incubating the eggs, which hatch after 20-22 days. Each male incubates whichever eggs are laid in his nest, regardless of true paternity.
Chick rearing
The male sandpipers continue their parental duties after the eggs hatch. They brood and defend the precocial chicks for 25-35 days until the chicks fledge and become independent. Although not all chicks in a given male’s brood may be his genetic offspring, males provide care indiscriminately.
How does the male behavior differ from females?
Male and female spotted sandpipers exhibit some important behavioral differences tied to their reproductive strategies:
Territoriality
Males vigorously defend breeding territories, while females do not. Territorial defense is essential for attracting mates. Males chase intruders while flashing their white wing undersides.
Courtship
Elaborate courtship displays are given only by males. The broken-wing display attracts female attention and tests male fitness. Females lack equivalent displays.
Parental care
Males alone incubate eggs and brood chicks once the females start wandering away. Female provide no care after egg laying.
Aggression
Because they compete for mates, males tend to be more aggressive than females toward intruders. Testosterone-related aggression is part of male reproductive strategy.
Mate guarding
Males follow mated females closely during the fertile period before egg laying. They peck at her cloaca to try to induce oviposition (egg laying). This mate guarding helps ensure paternity.
How does the polyandrous behavior benefit females?
The polyandrous mating system provides several key benefits to female spotted sandpipers:
Increased reproductive success
By mating with multiple males, females can lay a larger total clutch. Males are limited to incubating 4 eggs, so extra matings do not increase clutch size per male. But females lay 1 egg per male, so more mates means more eggs.
Access to superior territories
Females are free to wander between male territories, selecting the best nesting sites. They aren’t dependent on a single male’s territory quality.
Increased offspring survival
With male-only care, the female maximizes her own survival. The male’s parental effort helps compensate for the female’s lack of care.
Resource acquisition from males
In some cases, mating induces males to provide food gifts to females before egg laying. More mates means more potential food gifts.
Reduced risk of infertility
If one male is sterile or infertile, multiply mating ensures the female can still gain paternity from other mates.
What are the advantages for males?
Despite providing full parental care for offspring he may not have actually sired, polyandry offers some potential benefits for male spotted sandpipers as well:
Some paternity assurance
Males usually gain at least partial paternity within their brood. Though not guaranteed genetically related, the offspring represent some fitness gain.
Potential paternity confusion
If males can’t recognize their own offspring, they may inadvertently invest in unrelated chicks. Some paternity is better than total loss of breeding.
Increased offspring production
Maximum fertility is limited to 4 eggs with 1 female. But if a male attracts multiple females, he can potentially double or triple his reproductive output by having several clutches to care for.
Mate switching
If female abandons the eggs, the male can try to attract a new female to lay in his nest. This mate switching salvages some breeding investment.
Fewer threats to eggs
With multiple attending males, threats like predators are less likely to take all eggs in a given nest. Some will survive the dilution effect.
How does male behavior change after hatching?
Parental male spotted sandpipers exhibit changing patterns of behavior before versus after their chicks hatch:
Territoriality
Males become less aggressive and territorial after hatching. Their focus shifts to brooding and defending chicks rather than competing for mates.
Mate seeking
Sexual displays like the broken-wing dance stop once parental duties begin. Males no longer advertise for additional mates.
Nest association
Prior to hatching, males repeatedly return to their nests to incubate. After hatching, they accompany mobile chicks rather than staying at the emptied nest.
Brooding
Males faithfully brood chicks during the first week after hatching. Brooding time gradually declines as chicks grow and thermoregulate.
Alarm calls
If threatened, males give alarm calls to warn chicks and may perform distraction displays. Protecting offspring becomes a priority.
Foraging
Males start foraging for small invertebrates to feed dependent chicks. Providing food replaces mate seeking efforts.
How many males will a female mate with?
The exact number of males an individual female spotted sandpiper will mate with can vary:
– Average number of mates: 2-3 males
– Maximum number of mates: Up to 4 males documented
– Typical mating range: 1-4 males per female
The number of mates appears to depend on both external factors and female body condition:
Territory density
Areas with higher densities of male territories provide greater mate choice and polyandry opportunities. Females in sparse populations may mate with just 1 male.
Synchrony of females
If multiple females arrive together, male attention gets divided. A given female may get fewer mates. Staggered arrivals favor higher mating numbers.
Male availability
The operational sex ratio – ratio of fertile males to females – impacts female mating capability. Male-biased ratios favor polyandry.
Female age and experience
Older, more experienced female spotted sandpipers tend to mate with more males than yearling females.
Female body condition
Females in better nutritional health can produce more eggs and obtain more mates. Poorer condition limits reproductive potential.
Does male behavior depend on his mating status?
Male spotted sandpipers exhibit behavioral differences depending on whether they obtain a mate:
Mated males | Unmated males |
---|---|
Defend territories | Intrude into territories seeking extra-pair copulations |
Display to females rarely | Display to females vigorously |
Closely guard mates | Attempt to lure away mated females |
Care for eggs and chicks | Keep trying to find mate |
Less aggressive to neighbors | More aggressive seeking matings |
Mated males focus on mate guarding and parental care behaviors. Unmated males continue displaying and intruding into other territories to gain extra-pair copulations. Gaining some paternity is preferable to going completely unmated.
How many eggs does each male incubate?
The number of eggs each male spotted sandpiper incubates depends on where the female chooses to lay her eggs:
– Total clutch: 3-4 eggs per female
– Eggs per male: 1-4 eggs
Typically, the female lays one egg per day in each male’s nest she mated with. However, egg laying patterns show substantial variation:
All eggs in one nest
Rarely, a female will lay all her eggs in a single male’s nest. This male would incubate all 3-4 eggs.
Equal division of eggs
With 2 males, the female may lay 2 eggs in each male’s nest. With 3 males, each may get 1-2 eggs.
Skewed division
The female may lay unevenly, putting 1-3 eggs in one nest and only 1 in others. One male ends up with greater paternity contribution.
Eggs laid outside nests
Some eggs get laid outside male nest scrapes. These eggs often fail, as they lack an attending incubator.
Dump nest parasitism
Females may lay 1-2 eggs in a neighboring unmated male’s nest. This male gains some paternity from parasitism.
What is the incubated clutch size range?
Here is a summary of the typical range in incubated clutch sizes for male spotted sandpipers:
Incubated Clutch Size | Frequency |
---|---|
1 egg | Uncommon |
2 eggs | Common |
3 eggs | Very common |
4 eggs | Common |
5-8 eggs | Rare |
Most males incubate 2-4 eggs. Single egg clutches are rare. Five or more eggs likely reflect combined clutches from multiple females in a single nest. Four eggs is the physiological limit for one female spotted sandpiper.
Conclusion
The unique polyandrous breeding behavior of the spotted sandpiper provides evolutionary advantages to both males and females. Females maximize reproductive output while minimizing parental investment. Males, despite uncertain paternity, increase breeding opportunities by caring for the eggs and offspring. This strategy is rare among birds, making the flexible mating dynamics and paternal care of the spotted sandpiper truly remarkable in the avian world.