The fascinating love life of the Dunnock

I watched a Dunnock today, feeding under the garden table, with that characteristic half hopping half walking way Dunnocks have, pecking here and there things too small to be seen at a distance, maybe seeds or small invertebrates. Dunnocks, or Hedge Sparrows (Prunella modularis) are little birds, which live their lives mostly unnoticed amongst the undergrowth and are easily overlooked or taken for House Sparrows. They have a grey chest and head and chestnut backs with dark stripes, a thin beak and orange legs. Both males and females look similar, females just a bit smaller than males. In gardens they often feed on the ground, under bird tables when there is some cover, and they prefer to skulk than to sit out in the open. Only in the spring, where males sing their weak, warbling song from a prominent perch they are somewhat more likely to be noticed (above). Contrasting with their modest attire and retiring habits the Dunnock shows a variable mating system - including a common arrangement of two males and a female, a system called polyandry- , and a courtship behaviour that can only be described as peculiar. I was lucky enough to witness courting Dunnocks a few years back. This is a sketch of what I saw and my description.

Female appears paler, less extensive grey markings than the male. Both individuals are on the ground. The male hops behind the female and she stops a moment with her tail slightly cocked and vibrating her wings, that are dropped.  The male pecks her cloaca repeatedly. Female hops away a little, male follows and the same behaviour starts again. Then the intensity increases, the male pecks her cloaca again and the female stops, then the male jumps on the female and there is a flutter

 What could be the purpose of this bizarre cloaca pecking behaviour? Has it got anything to do with the presence of polyandry? Nick Davies, in a classic paper published almost 30 years ago, provided some answers. He followed a Dunnock population in Cambridge Botanical Gardens. Males outnumbered females due to higher female mortality during winter, and therefore, there was intense male-male competition. He observed several combinations of breeding partners per territory, including male-female pairs, two males and a female (what he called trios), and some more rare cases including two males defending jointly the territories of several females.
Monogamous males guarded the female, following her around closely for a few days before she was due to lay and gaining almost exclusive copulations with her. When there are more than one male in a territory the larger male was dominant to the smaller one and fights were common, with the dominant male trying to chase the subordinate away from the female. Although the dominant male got the best share of copulations, the subordinate also got them as he was usually very persistent and mated with the female unnoticed by the dominant male, or when the male lost track of her - often after a fight. In all events of courtship, be monogamous males or not, the female always exposed her cloaca, often a pumping action was noticed and the male pecked it. After cloaca pecking, the female was seen to eject a droplet of fluid. Davies managed to collect three of these droplets and when he examined them under the microscope he found them to contain bundles of sperm. When he watched the female ejecting the drop of sperm, the male copulated with her immediately after. The purpose of the cloaca pecking behaviour appeared clearer: the male stimulates the female to eject stored sperm from the previous mating, allowing the suitor a shot at paternity. This was confirmed by the fact that the more other male spent near his female, the more a male pecked the female and copulated with her, as the male pursued to increase their chances of paternity. Interestingly, the female played an active role in being part of a trio:  she tries to obtain copulations from the subordinate male, escaping the guarding of the dominant, despite his efforts: obviously ejecting sperm from the previous mating will offer both males a share of paternity. When a female was observed to have mated with two males, the brood raised was fed by both males. In contrast, if the subordinate male failed to mate with the female, he did not contribute subsequently to raise the chicks. As nestlings fed by two males have a better chance of survival, it appears that is in the female interest to mate with both males.

A subsequent study using DNA fingerprinting confirmed what behavioural observations had strongly hinted: monogamous males got 100% paternity, and both dominant and subordinate males fathered chicks (surprisingly more or less equally). The observant reader will notice that the mean paternity of dominant and subordinate do not add to 100% in the polyandry system. This was because an outsider gained access to the female and fathered a chick.

(modified from Table 1 from Burke et al 1989).

Furthermore, this study showed that the chances of males helping the female rear the brood were dependent on them having sired some of the brood. It appears that the male is able to judge if he has had sufficient access to female to gain some paternity and to be worth the effort of helping her rear the chicks.

Although cloaca pecking can be seen as the male bird trying to ensure his paternity, the elaborate courtship of the dunnock also reflects that females are active participants and, that, as they need more than a single male to rear her chicks, this unusual courtship is the way females ensures that both males help her raise her chicks and that she achieves more reproductive success.

A pair of Dunnocks in the garden. What would they be up to?

More information
Davies, N. (1983). Polyandry, cloaca-pecking and sperm competition in dunnocks Nature, 302 (5906), 334-336 DOI: 10.1038/302334a0

Burke, T., Davies, N., Bruford, M., & Hatchwell, B. (1989). Parental care and mating behaviour of polyandrous dunnocks Prunella modularis related to paternity by DNA fingerprinting Nature, 338 (6212), 249-251 DOI: 10.1038/338249a0

Opportunistic crows

 Corvids are omnivorous birds and display a very flexible and opportunistic feeding behaviour: they will exploit coastal resources when near beaches, will actively search for nests in the spring looking for eggs or chicks, will take advantage of human scraps, feed on roadkill or other carrion, or look for worms and other invertebrates on fields and woodland floor. They show a remarkable inventiveness regarding looking for food, leading in some species to an understanding of the manufacture and use of tools. Rooks are able to add stones to a beaker with a little water where food is floating until the water level rises enough for them to reach the food. Caledonian Crows craft tools - including hooks - from leaves and twigs and use then to fish out grubs from trunks. Other species, like Jays, have extraordinarily precise spatial memory, which allows them to cache food surplus for use later.
  In general, corvids are dexterous birds. They have excellent foot-beak coordination and often use their feet when feeding, to hold down a piece of food so that they can pick bits with their beaks, as the Carrion Crow above. In experimental set ups, Ravens are able to reach meat tied to a a long string by alternatively pulling with their beaks and stepping on the loops of the string with their foot.
Their catholic food preferences and ingenuity have undoubtedly allowed them to adapt to living near people, and some crows species, like the Carrion Crow are common in towns, cities and agricultural land.

 Carrion Crows and a Greater Black-Backed gull feeding on a dead seal pup and afterbirths on a Grey Seal colony

I saw this Carrion Crow flying with a Woodpigeon's egg on its beak. It landed and quickly opened it up and ate the contents 

The predated egg shell

Carrion crow searching for a nest while a blackbird gave alarm calls nearby

Crow feeding on a chick 

Rooks and Carrion Crow feeding on a beach

A short video of a pair of Carrion Crows feeding on the leaf litter

More information
Bird, C., & Emery, N. (2009). Rooks Use Stones to Raise the Water Level to Reach a Floating Worm Current Biology, 19 (16), 1410-1414 DOI: 10.1016/j.cub.2009.07.033

Hunt, G., & Gray, R. (2004). The crafting of hook tools by wild New Caledonian crows Proceedings of the Royal Society B: Biological Sciences, 271 (Suppl_3) DOI: 10.1098/rsbl.2003.0085

Heinrich, B., & Bugnyar, T. (2005). Testing Problem Solving in Ravens: String-Pulling to Reach Food Ethology, 111 (10), 962-976 DOI: 10.1111/j.1439-0310.2005.01133.x

Extraordinary feral pigeons

When I make bird lists I do not often tick feral pigeons (Columba livia). They are always there, but I like to take photos of their behaviour.

The local flock sitting on the roof, waiting for the sun to rise in the morning,

 the flock scared by a movement or noise, suddenly taking off and flying in a few elegant circles before settling back; a few of them feeding on the bread that some old lady dutifully brings to the same corner of the street each morning; males in display flight holding their wings high in a V pattern and gliding in a circle.

Their leisure times, having a bath...

...or enjoying the sun on a cold day

Despite their ubiquity, feral pigeons are still extraordinary birds. They are descendants from the Rock Dove, a wild bird of river canyons and sea cliffs, domesticated millennia ago. Some have now have returned to the wild, although still quite attached to humans. Some feral pigeons (above) have very similar plumage patters to their wild ancestor: bluish grey, with a double black wing bars and iridescent neck. Many also keep a trademark white patch in their rumps (like the one bathing on the photo above).

Charles Darwin extensively bred many domestic pigeon breeds and dedicated a sizeable section of the first chapter of the Origin of Species to domestic pigeons. He used them to illustrate how domestication had generated distinct races through breeding together animals with certain preferred variations, producing a wide array of diverse shapes, colours, plumages, flight ability and voice from the original ancestral species, the wild Rock Dove. He stressed that this "selection by man" was essentially the same mechanism that his "natural selection".

 Male pigeons court females by cooing and bowing to them while walking alongside her with puffed feathers, inflated necks and fanned tails, pirouetting occasionally. The courting male, on the left, is also showing his white rump patch.

 A quick copulation may ensue...

 As other pigeons, Feral Pigeons lay two eggs per clutch. They nest on ledges inside abandoned buildings, inside roof spaces where they can gain access, or outside, under bridges. Fledged squabs, the young birds (below), have dark eyes, underdeveloped operculum - the white swelling a the base of the beak - and have a look of naivety about them. Both siblings often move about together.

Although mortality, even in town, is high. According to research by Alberto Pelleroni and colleagues, the wild-type blue plumage with a white rump affords a survival advantage to feral pigeons, as it decreases the chances of success in Peregrine Falcon attacks. When pigeons with and without the white rump patch had their feathers swapped, the success of the Peregrines reversed accordingly. The presence of Peregrines in the area increased the frequency of the feral pigeon wild type in the area. As for the possible reason for the increased failure of Peregrines in capturing wild-type feral pigeons:

All feral pigeons perform the same evasive roll during predation by falcons. The protective white patch may disguise the initiation of the pigeon’s evasive roll by contrasting conspicuous (white patch) and cryptic targets (grey wings and body). A fast flying falcon primed to a conspicuous target centered on the roll might fail to detect the dodge initiated by the cryptic wings as the predator closes from behind.

 I wonder who ate this squab? Sorry if you are squeamish!

Never underestimate pigeons. They have been found to outperform university students in a range of probability calculation tests (see this post for an example). According to Walter T. Herbranson and Julia Schroeder:

Pigeons might not possess the cognitive framework for a classical probability-based analysis of a complicated problem [...], but it is certainly not far-fetched to suppose that pigeons can accumulate empirical probabilities by observing the outcomes of numerous trials and adjusting their subsequent behavior accordingly.

Update: see this post on pigeon convergence


References

Herbranson, W. & Schroeder, J. (2010). Are birds smarter than mathematicians? Pigeons (Columba livia) perform optimally on a version of the Monty Hall Dilemma. Journal of Comparative Psychology, 124 (1), 1-13 DOI: 10.1037/a0017703

Palleroni, A., Miller, C., Hauser, M. & Marler, P. (2005). Predation: Prey plumage adaptation against falcon attack Nature, 434 (7036), 973-974 DOI: 10.1038/434973b

The hybrid goose in the park

I have known this goose since 2005. It is a hybrid between Canada Goose and possibly Greylag, and it turns up every winter in my local park. Today, it behaved gallantly, chasing away other geese and ducks while its partner, a Canada goose, fed.

The hybrid goose holds the pair of Canada away from the food source, allowing his partner to feed unmolested.

This goose is a large bird, with a mixture of features from both species (see photos of Greylag and Canada below), with the white patch on the face of the Canada, the pink legs of the Greylag and a beak half, black half pink.

A Canada goose

 A Greylag

 I think the hybrid is a male. He is paired with a female Canada and travels with a Canada flock. Hybridisation is a fascinating issue. In a puritanical view of species - which were created, would not change and could be nicely put into separate drawers - hybrids, were viewed as oddities: unnatural, often sterile, hard to put away in a neat drawer. This view has changed a lot lately. Recent genome sequencing data have shown that we humans seem to be a melange of different hominid isolates that mixed when they met; polar bears and brown bears have been shown to have often interbred during the Pleistocene; Carrion Crows and Hooded Crows do it; Willow Grouse and Rock Ptarmigan do it as well. There are many more examples. Natural climatic changes, such as during ice ages, often brings species together facilitating hybridization. Human introductions, placing species that previously didn't come together in the same areas can also promote hybridisation. Hybridisation is not only more frequent in nature than previously thought, but can also fuel species diversification and adaptation.
 Geese hybridisation is interesting as, when young, goslings undergo a phenomenon called imprinting. After hatching, goslings have a sensitive period during which they attach themselves socially to any moving stimulus, and follow it. This usually happen to be their parents, but they will eagerly follow people when goslings are hatced in an incubator. Even their mating preferences are determined to some point early in life, as goslings imprint on the adults that rear them - this is called sexual imprinting - and, when adults, will try and find a mate of the same species of the individuals that reared them. In nature, errors can happen. For example when individuals practice egg parasitism - laying eggs on another species' nest - or a brood from another species is adopted or brood amalgamation, when goslings from  different species pool together and the adults of one of the species take care of them.
 What happens to the fostered brood when they grow up? They imprint on their foster parents, which rear them as if they were their offspring, but they still look like their biological parents. Eric Fabricius carried out an experiment in the wild to examine the effects of sexual imprinting on hybridisation in geese. He swapped all eggs from two nests of Canada geese by Greylag eggs. The cross-fostered Greylags migrated with their Canada parents and upon returning to the breeding ground the following spring they paired up:

Of 35 returning birds, all 16 females paired with greylag goose males (100%) whereas of 19 males 5 paired with Canada goose females (14%) and the remaining 14 with greylag goose females (74%). The pair bonds generally persisted as long as both birds were present, but after loss of a partner, the remaining bird usually re-mated. Even when this happened several times during the lifetime of a male, the new mate was always a Canada goose female, showing that the males were sexually imprinted to this species. The Canada goose females which had mated with the greylag ganders also remated when widowed, but their new mate could be either a Canada goose or a greylag goose.

All cross-fostered females paired with Greylags. They looked like greylags, so they weren't courted by Canada males, just Greylag males. The pattern in the males is interesting, as they probably actively courted Canada females - and some accepted them-  but they were also found attractive by greylag females and they ended up pairing with them. Only the category of cross-fostered males pairing with Canada are likely to father hybrid offspring. What happens to the hybrid offspring is interesting as they do look intermediate between species. If is still unclear if hybrid CanadaxGreylags are fertile. The hybrid in the park then probably had a cross-fostered dad. He appears to have no trouble keeping a mate, and if he is always as considerate in his behaviour as today, she might as well keep him.

References
Fabricius, E. (2010). Interspecific Mate Choice Following Cross-fostering in a Mixed Colony of Greylag Geese (Anser anser) and Canada Geese (Branta canadensis). A Study on Development and Persistence of Species Preferences1 Ethology, 88 (4), 287-296 DOI: 10.1111/j.1439-0310.1991.tb00283.x


Edwards, C., Suchard, M., Lemey, P., Welch, J., Barnes, I., Fulton, T., Barnett, R., O'Connell, T., Coxon, P., Monaghan, N., Valdiosera, C., Lorenzen, E., Willerslev, E., Baryshnikov, G., Rambaut, A., Thomas, M., Bradley, D., & Shapiro, B. (2011). Ancient Hybridization and an Irish Origin for the Modern Polar Bear Matriline Current Biology, 21 (15), 1251-1258 DOI: 10.1016/j.cub.2011.05.058


Quintela, M., Thulin, C., & Höglund, J. (2010). Detecting hybridization between willow grouse (Lagopus lagopus) and rock ptarmigan (L. muta) in Central Sweden through Bayesian admixture analyses and mtDNA screening Conservation Genetics, 11 (2), 557-569 DOI: 10.1007/s10592-009-0040-9