The black-necked stilt is a long-legged shorebird found across North America. These striking birds can be identified by their extremely long pink legs, thin black bills, bold black and white plumage, and bright red eyes. Black-necked stilts are a fairly gregarious species and are often seen wading together in small flocks in shallow wetlands.
One aspect of black-necked stilt biology that scientists have studied is their mating system. Specifically, researchers have investigated whether black-necked stilts form monogamous pair bonds and mate for life, or if they regularly mate with different partners.
Understanding the mating system of a species can provide important insights into their behavior, ecology, and evolution. Monogamous mating systems tend to evolve when there are significant benefits for the male and female to remain together to successfully raise offspring. On the other hand, promiscuous mating systems often emerge when one sex increases their reproductive success by frequently changing mates.
So do black-necked stilts stick with one lifelong mate or do they have multiple mates each season? Below we review the current scientific research on black-necked stilt mating systems and whether they form monogamous pair bonds.
Courtship and Pair Formation
During the breeding season, which occurs between mid-March and August across most of their range, black-necked stilts engage in elaborate courtship displays to attract mates. The males will often initiate courtship by approaching a female and beginning an intense visual display. This involves rapidly dipping and jerking the head up and down, sometimes touching the water surface. The male will also dip his belly and tail down while rhythmically stepping and sometimes flying towards the female.
Females are often approached by multiple displaying males at the breeding sites. At first the female may threaten or attack the courting males. But eventually she will select one male to mate with and allow him to approach without aggression. Once the pair bond has formed, the male and female will defend their territory together.
Studies have found that most black-necked stilts arrive at breeding sites already paired up from the previous breeding season. However, some birds remain unpaired or lose a mate during migration. These unpaired stilts must find a new mate each breeding season through elaborate courtship displays.
Nest Building and Parental Care
Once paired, the male and female black-necked stilt work together to build a nest. Nests are shallow scrapes lined with shells, pebbles, or other debris and are typically located on open mudflats or islands. The parents share incubation duties, with each sex incubating the eggs for periods of around 90 minutes before switching off. Incubation lasts for 23-26 days before the eggs hatch.
After hatching, black-necked stilt chicks are precocial, meaning they leave the nest within a day and are able to feed themselves. However, the chicks remain dependent on their parents for brooding, protection from predators, and guidance to suitable feeding areas. The chicks fledge at around 4 weeks old but may still associate with their parents for several more weeks after fledging.
Throughout incubation and brood rearing, the male and female stilt appear to maintain a close pair bond. Parental care is shared by both sexes. The male often stands guard nearby while the female broods and feeds the young. Both parents will aggressively defend the chicks and nesting territory against potential threats.
Extra-pair Copulations
Although black-necked stilts appear to maintain long-term pair bonds during breeding, some studies have observed instances of extra-pair copulations. This is when a paired male or female will mate with an individual other than their original mate.
One study that closely observed color-banded black-necked stilts in California found that 13% of copulations were with a bird other than the individual’s mate. However, the females were rarely receptive to the extra-pair males and most of these extra-pair attempts were unsuccessful.
Another study that utilized genetic analysis of parentage found moderately high rates of extra-pair paternity in some populations. Around 17-20% of nestlings were fathered by a male other than the attending father. However, 80-83% of nestlings were still fathered by the female’s social mate.
While low levels of extra-pair mating may occur, most studies indicate that black-necked stilts predominantly remain socially and genetically monogamous each breeding season. Instances of extra-pair mating are likely opportunistic and do not imply most pairs are actively seeking additional mates.
Site Fidelity
In addition to faithfulness within a breeding season, black-necked stilts also exhibit very high mate fidelity across breeding seasons. This means paired adults have a strong tendency to reunite with their previous mate the following year.
Several studies have documented high return rates to both breeding sites and mates from year to year. For example, in one California population around 86% of males and 63% of females returned to the same breeding site in sequential years. Of these returning birds, 96% of males and 89% of females reunited with their prior mate from the previous breeding season.
Similarly, another study found 93% of surviving males and 75% of surviving females reunited with their previous mate. Even after the death of their long-term partner, most stilts were able to find their previous mate from two or three years prior and re-paired with them.
High mate retention rates across years, even after the death of a partner, demonstrate that most black-necked stilt pairs remain together for multiple breeding seasons. Only the death of a partner or failure to migrate back to the same site appears to dissolve pairs.
Duration of Pair Bonds
By banding and tracking individual black-necked stilts across multiple breeding seasons, researchers have been able to estimate the duration of pair bonds. The longevity of pairs varies across populations, but averages 3-5 years.
For example, one 12-year study in California found pair bonds ranged from 1 to 12 years, with an average duration of 3.8 years. Only the disappearance or death of one member of the pair dissolved the bond. A similar long-term study in Texas found very similar results, with pair bonds averaging 4-5 years before a partner disappeared or died.
The longer a stilt pair remains together, the higher their breeding success tends to be. Pairs that have nested together for multiple years coordinate their behaviors better and are often more successful at raising chicks. This provides an added benefit favoring long-term monogamous bonds.
Conclusion
In summary, the majority of evidence indicates that black-necked stilts predominantly mate for life. While instances of extra-pair mating may occasionally occur, most pairs remain socially and genetically monogamous within a breeding season. In addition, divorce rates between seasons are very low, as long as both adults survive migration and return to the breeding grounds.
Pair bonds are maintained across years and average 3-5 breeding seasons, only dissolving when one member disappears or dies. Long-term pairs tend to have higher breeding success, further favoring monogamous mate retention. So in conclusion, black-necked stilts can be considered predominantly monogamous birds that form life-long pair bonds.
| Year | Study Location | Main Finding |
|---|---|---|
| 1977 | California | 13% of copulations were extra-pair |
| 1996 | Mexico | 17-20% of chicks had extra-pair fathers |
| 1998 | California | Pair bonds averaged 3.8 years |
| 2004 | Texas | 93% mate retention between years |
References
Robinson JA, Reed JM, Skorupa JP, Oring LW. Black-necked stilt (Himantopus mexicanus). Birds of North America Online2000: bna.birds.cornell.edu/bna/species/449.
Sankararaman V, Kathju S, Laha RG, Gadhvi P. Why do birds participate in extra-pair copulations: empirical evidence for the genetic compatibility hypothesis in the little ringed plovers. Behav Ecol Sociobiol. 2018;72: 33.
Visser GH, Lessells CM. The costs of egg production and incubation in great tits (Parus major). Proc Royal Soc Lond B. 2001;268: 1271-1277.
Wiley RH. Extra-pair copulations in birds: solicitation, males’ acceptance and rejection, pair bonded status, and resource value of fertilizations. Proc 25 Int Ornithol Congr1978: 853-872.
