Monday, 29 March 2010

Mate guarding in Mallards

There are just a handful of female mallards in the local park pond. Today I could only see a couple. As for males, there must have been at least two dozen around. This male skewed sex ratio seems to be a general phenomenon in the mallard, possibly due to a higher female mortality during migration and incubation, but given the time of the year, many females in the park are probably already incubating. The remaining females are followed everywhere by their zealous males. Many bird species form monogamous pairs, which help each other in raising the offspring. This is called 'social monogamy' but genetic analysis revealed that it does not follow that the social pair are the parents of the offspring they are raising. First, male cuckolding occurs with some regularity in many species, so that the female mates with other males and a proportion of the offspring is not fathered by her social mate. Secondly, females occasionally carry out 'egg dumping' on other pairs nests.
Mallard pair mounting in the autumn
 In the case of the Mallard, our commonest duck, there is social monogamy, with pairing observed at least for six months of the year and lasting at least several years. Pairing starts after the moult in the autumn, when even pair displays and mounting are common, even though breeding cannot occur. Males do not share the parental duties, the female being in charge of incubating, brooding, leading the ducklings and keeping them out of danger. Pairs can be seen carrying out a ritualised courtship, but a different mating strategy is commonly seen, in which one or several males chase a female and force her to copulate.
Group of drakes attempting copulation with a female
Females do try to escape these attempts by flying, diving or hiding and call loudly - presumably for their partner - and they are known to have been injured or even drowned in the process, probably due to the number of males trying to hold onto the females neck to secure copulation. In most cases females are not injured - other than losing some neck feathers - and forced copulations can result in fathered offspring, so it is in the interest of the drake to guard the female against this 'extra-pair' copulations to ensure that he fathers as much offspring as possible. Drakes maintain very close proximity to their mates during a critical period, - up to four days before the beginning of egg laying and up to four days afterwards - following the females everywhere. Males only swam away from their partners to chase other males away or initiate fights when other males approached his partner. Males also attempted to stop forced copulations on their partner with varied success. But it was also during the critical period that both paired and unpaired males initiated approaches to paired females. If the female was on its own, she called loudly if other males approached. Mate guarding also benefits females allowing them to forage more efficiently and possibly, decreasing their mortality due to predation because of their mate's alert state. Males with high testosterone levels in spring are more efficient guarding their females, and females show a preference to pair with such males during the autumn.

More reading
Cunningham, Emma J. A. (2003) Female mate preferences and subsequent resistance to copulation in the mallard. Behav. Ecol. 14: 326-333. here.
Davis, Ellen S. (2002) Female choice and the benefits of mate guarding by male mallards. Animal Behaviour, 64: 619-628. here.
Goodburn, S. F. (1994) Mate Guarding in the Mallard Anas platyrhynchos. Ornis Scandinavica 15: 261-265. here.

Monday, 22 March 2010

A gentle side of crows

The resident pair of crows were sitting on an aerial this morning, and the individual to the right was cawing. I was waiting for it to call again camera in hand - I like the arched necks and up and down powerful movements of crows cawing. Instead, after a few moments, the other individual approached its mate and began preening. Carrion Crows, as other corvids, form long-term typically life-long partnerships, where both pair members share most of the duties. In fact, the only noticeable division of labour is incubating and feeding the young nestlings. Pairs remain and defend their territory together year round. Usually, the pair defend the same territory year after year. Pair cooperation takes the form of aggressive territory defence from other crows but also other species which might compete for food. Both members of the pair approach intruders and engage in duet cawing, apparently inciting each other to attack them. They will also carry out 'pincer tactics' in which each member of the pair try and face each other keeping the intruder in the middle. Carrion crow intruders are the main nest predators, with up to 50% of their nests being robbed by other crows,  so it is important to keep them away from the territory.

More information
Bossema, I and R.F. Benus (1985) Territorial defence and intra-pair cooperation in the carrion crow (Corvus corone). Behav Ecol Sociobiol. 16:99-104. here.
Nicola S. Clayton, Nathan J. Emery, The social life of corvids, Current Biology, Volume 17, Issue 16, 21 August 2007, Pages R652-R656, ISSN 0960-9822, DOI: 10.1016/

Saturday, 20 March 2010

Uneasy neighbours: Carrion crows and Kestrels

A kestrel has taken residence in the neighbourhood, I have seen it flying almost every other day. The resident pair of crows does not seem happy about this. A couple of weeks ago I saw them mobbing the kestrel while regularly uttering their characteristic 'krrr' call high over the garden. The same happened yesterday. In both occasions, by the time I had taken the camera out they were already quite distant. As I wanted to write a post about mobbing, I decided to take out my Wacom Bamboo tablet and sketch the behaviour. Mobbing is found in many bird species and often is a communal affair by which potential prey call and harass a predator. Mobbing is a costly behaviour, it takes a lot energy to chase and call and dive bomb the predator. The crows can spot the kestrel in the distance and go out of its way to mob it. Once a kestrel was gliding quite high, and I watch a crow laboriously gaining hight in broad circles in order to mob it. During the actual mobbing, the crow flies slightly over the kestrel - a safe way to do it - and chases it, occasionally taking dives at it, while the kestrel avoids each dive, and occasionally turns to the crow. In rare occasions, the mobber can turn into a meal for the mobbed, but I am not sure this can happen in crows, which are quite robust and bigger than kestrels. Why do birds mob? what do they gain from this behaviour? There are many hypothesis and it is likely that a combination of some of them is the reason. One of them is that mobbing allows birds to transmit information on who the enemy is to the next generation (a form of cultural transmission). In some classic experiments with blackbirds, birds watching a harmless, novel bird being mobbed by another blackbird started to mob the new bird themselves. Mobbing can have also direct benefits for the mobbing individual, it could be showing off is good condition, a form of advertising that if wont be easy prey. Mobbing could confuse or distract a potential nest predator from their nest seeking behaviour. When the predator is chased and engaged it cannot pay attention to the local territory and gather information on the nest location, for example. In seagull colonies, mobbing on crows - yes, crows can be mobbers or mobbed themselves - decreased the chances of egg predation and colonially nesting gulls enjoy lower rates of nest predation than solitary nesting gulls. In addition, mobbing tends to intensify during the breeding season in solitary nesting birds.

This crow is gathering nest material, soft bark from the lime nearby. The photos were taken trough glass.
Crows are indeed nest building now. Yesterday and this morning we watched the pair gathering nest lining material, stripping pieces of lime bark dextrously using their feet and beaks. Their zealous harassing of the kestrel might well reflect the fact that they would rather not have him as neighbour when their vulnerable offspring hatches.

Tuesday, 9 March 2010

Nesting Rooks

Rooks are not common birds in the city, they suddenly appear when houses make way for the first grassy fields in the outskirts. Somewhat surprisingly there is a small rookery in the university grounds. Probably the large mature trees attract them. They are not present year round though. I heard a group in January and I hadn't heard them again until today. Suddenly, they have taken over the place having decided that it is nesting time. Their constant, excited drawn out single 'kaah' calls - quite different from the repeated, harder and sharper calls of the Carrion Crow - filled the air. There were at least five evident nests in construction - all of previous years nests all fell out with storms and high winds - , one just a little jumble of sticks. Rooks break off twigs and branches from trees instead of collecting nest materials from the ground, and they were coming and going from their nests to a nearby Alder where they broke off twigs for their nest. It has been suggested that collecting branches from living trees allow for better size and strength selection This description taken from Bayne, 1929 was very fitting today, with squabbles at some nests possibly as a neighbour tried to steal a precious stick.

"Most birds do their nest-building quietly and as secretly as possible. But the rooks cannot hide theirs, so they talk about them freely. Every time they come back with a twig they shout the news to the whole rookery and to any one else who may be able to understand them. But it is dangerous for them to boast too much about a stick, for it may be stolen by a neighbour when they are gone to fetch another." 

This series of photos taken on a few minutes illustrates the activity in the colony.
A Rook leaves its nest calling
This three Rooks were collecting sticks atop an Alder. The bird on the right has already got its stick.
This bird spend a good few minutes wandering aroung the tree undecided and left with nothing.
Bird returning with a branch
For this one a little twig will do

Friday, 5 March 2010

The surprising moorhen

Moorhens breed every year in my local park and, for moorhens, they are quite tame and allow close approximation. In the countryside moorhens are more difficult to see, their loud "kirr-up!" call, often giving away their presence while they feed in marginal vegetation of canals and rivers. Moorhens are unusual in several ways. First, contrary to most birds, females compete for males, engage in female-female fighting and initiate courtship. In most bird species, females carry most if not all of the burden of parental care, they make energy-rich eggs, do most or all of the incubation and in several groups they protect and or feed the chicks for some time after hatching. No surprise then that males compete for such good assets and female are quite choosy as to which males should father their offspring!
Incubating moorhen
In contrast, Moorhen males are devoted dads and carry out most of the incubation and tending of the chicks. Some males, those with good fat reserves are better at incubating than others. Small males then to accumulate more fat reserves, and females fight to pair with those small fat males. There are also male fights but they are for territorial defence, instead of fighting for access to females.
An adult searches for food while a young chick begs on the water edge
Both males and females tend the chicks, which are quite mobile from birth, but need feeding and are shown what to eat by their parents. The other unusual habit of Moorhens is that chicks from the first brood often stay around in their natal territory and help feed their siblings from the second (or third) brood. This cooperative breeding is also found in Swallows and Long-tailed tits in the UK. As territory availability is very constrained due to aggressive territory defence during the summer, young first-brood moorhens are forced to stay in the parental territory and cannot easily disperse. The more food resources are available in their natal territory, the more likely is that first brood chicks will help raise their siblings and the more effort they put into it. Helpers can make a difference in the number of surviving chicks for the second brood and also reduce the parent burden when feeding the second brood.
Courting pair just before mating. A young chick was in their nest nearby.
A tiny chick still in the nest with its parents
More mobile chicks follow one of their parents
These fledglings were chased up the tree by an adult
Pair of moorhens foraging on frozen pond
Eden, S.F. (1987) When do helpers help? Food availability and helping in the moorhen, Gallinula chloropus Behavioral Ecology and Sociobiology, 21: 191-195. Here.
Petrie M (1983) Female moorhens compete for small fat males. Science 220: 413-415.