How come birds are also playing the "fourth love"?

Spring is here, and it is a great time to observe birds' courtship behavior. During this season, you can go outing and bird watching, and you can see birds fighting for territory, and even gesturing aggressively at their shadows on the glass windows. On February 27, Wuhan bird lover Xiao Ce also took a precious picture:

The reverse mount of the red kestrel, combined with other observations, can confirm the gender of both parties | Xiaoce

Yes, this is a pair of kestrels having sex. Usually when birds mate, the male is on top and the female is on the bottom; but in this photo, the female is on top and the male is on the bottom - this behavior is called reverse mounting .

The "fourth love" of birds is actually more common than most people think, but because the mating time of birds is very short, and many birds are hermaphroditic, that is, the male and female birds look the same (such as the common magpie and turtledove), reverse mounting is rarely noticed. In the early years, some scientists believed that reverse mounting might be the result of birds mistaking each other's gender, but these speculations have been overturned by observational evidence. Today, reverse mounting has been found in more than 30 species of birds, and it is even a high-frequency behavior in some species .

What's interesting about this behavior is that kestrels and most other birds mate via brief cloacal contact, an unconventional position that doesn't allow for fertilization. So, what are they trying to do?

Explanation 1: Have a litter soon

There is not much literature on reverse mounting behavior in falcons, but a 1986 article reported an interesting and cruel experiment.

When the breeding season arrived, scientists ruthlessly separated 20 pairs of American falcons (Falco sparverius) that had already been paired, removing their partners (the paper only said that they were removed, but did not say how they removed them). Next, they found that two female birds had reverse mounting behavior after finding new lovers. No cloacal contact was observed.

American falcon, male on the left and female on the right | Andreas Trepte, Wikipedia, CC BY 2.5; Dibyendu Ash, Wikipedia, CC BY 3.0

The scientists report in their paper that this experiment is consistent with past observations that reverse mounting behavior is associated with brood failure , and they speculate that this may be a way for females to stimulate males to "pay their dues" so that they can start raising the next clutch as soon as possible .

In this experiment, eight female birds accepted new partners after losing their original mates, and only four of them gave birth again, including the two that had reverse mounts.

For birds of prey like the American Kestrel and the Common Kesquite, suddenly losing a mate after the breeding season is a big blow. They can usually only breed one nest in a breeding season, and both parties need to raise the offspring together . A single mother/father cannot support the children. The breeding season of birds is very precious . If the young birds are born too late, they may miss the season when food is abundant. If you want to start a new nest, you really have to hurry up.

A falcon feeding its chicks | Artur Mikołajewski, Wikipedia, CC BY 3.0

However, the red kestrels in Wuhan should have just entered the breeding season and are not yet ready to have a second baby; this study is also quite old. So, let's look at other explanations.

Explanation 2: Mature woman training

Observational studies of short-billed crows (Corvus brachyrhynchos) revealed another interesting pattern: males that had just reached sexual maturity were more often reverse mounted .

Wild short-billed jays can live for seven or eight years and reach sexual maturity at about two years old, but the authors said "I have never seen a case of successful reproduction on the first try." Based on their observations and other reports at the time, the male short-billed jays that were mounted were young "fresh meat" and they may need to be trained by older females to impart some experience or be subjected to stronger stimulation before they can enter the reproductive state.

Corvids! What other surprises do corvids have that we humans don't know about? | BKleinWiki, Wikipedia, CC BY 3.0

Corvids are known for their rich behaviors and high intelligence, but unlike the kestrel, there is no obvious difference in appearance between the two sexes. Researchers can use banding to mark individuals and distinguish between the different sexes, but it is difficult for ordinary bird watchers to observe such behavior.

Explanation 3: Adding interest

Grebes, cormorants and other water birds are also hermaphroditic, and the "fourth love" among these birds is actually quite frequent. Scientists spent three years observing and counting more than 400 mating behaviors of a group of silver grebes (Podiceps occipitalis) and Argentine grebes (P. gallardoi), and found that the proportion of reverse mounting reached 27% and 15% respectively .

Observations have found that reverse mounting behavior occurs more often early in the breeding season and around the time of mating. In addition, grebes are known for their interesting courtship behavior, with both males and females cooperating to complete a complex courtship dance. Researchers believe that reverse mounting may be part of the grebe's courtship process, which can help both parties get into the mood .

Crested Grebe courtship dance, photographed in Shahe, Beijing | Maya Blue

Studies on cormorants and cuckoos have also reached similar conclusions: reverse mounting behavior may promote the synchronization of the physiological rhythms of both parties to enter the reproductive state , and it more often occurs at the beginning of the breeding season and before formal mating.

Of course, an animal behavior may have multiple purposes at the same time. How to explain animal behavior is also a difficult problem, which requires rigorous scientific methods to avoid subjective assumptions from a human perspective. In general, there are still not many studies on reverse mounting behavior, and the observations of one species may not be applicable to another species. So we often read the sentence at the end of the paper - "further research is needed."

There are always surprises in natural observation . Even if you thought you were familiar with an animal, you might discover its unusual behavior one day, broaden your horizons, and perhaps even add something new to the human knowledge base.

References

[1] Bowman R, Curley E M. Reverse mounting in the American Kestrel[J]. The Wilson bulletin (Wilson Ornithological Society), 1986, 98(3): 472-473.

[2] James P C. Reverse mounting in the Northwestern Crow[J]. Journal of Field Ornithology, 1983, 54(4): 418-419.

[3] Nuechterlein GL, Storer R W. Reverse mounting in grebes[J]. The Condor, 1989, 91(2): 341-346.

[4] Ortega-Ruano J, Graves J A. Reverse mounting during the courtship of the European Shag Phalacrocorax aristotelis[J]. Condor, 1991: 859-863.

[5] Bowen BS, Koford RR, Vehrencamp S L. Seasonal pattern of reverse mounting in the Groove-billed Ani (Crotophaga sulcirotris)[J]. The Condor, 1991, 93(1): 159-163.

[6] Raimilla V, Norambuena HV, Jiménez J E. A record of reverse mounting in the Rufous-tailed Hawk (Buteo ventralis) in southern Chile[J]. Journal of Raptor Research, 2013, 47(3): 326-327.

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