The British Magpies

One for sorrow, two for joy but what do you get for 100 or even 150 Magpies Pica pica? Groups of this size are not uncommon, and indeed the antics of large gatherings of Magpies were described by the Reverend Darwin Fox to his cousin, Charles Darwin, who subsequently wrote:

“The common magpie used to assemble from all parts of Delamere Forest, in order to celebrate the 'great magpie marriage'. They had the habit of assembling very early in the spring at particular spots, where they could be seen in flocks, chattering, sometimes fighting, bustling, and flying about the trees. The whole affair was evidently considered by the birds as one of the highest importance. Shortly after the meeting they all separated, and were then observed to be paired for the season.” (Darwin 1871)

The Magpie is a small-to-medium-sized member of the crow family (Corvidae).  The adult males weighed 248 g and adult females 223.3 g. The sexes are identical in plumage, but the size difference between the sexes is often apparent if they are seen side by side. The Magpie has a wide geographic distribution, breeding throughout most of Europe and Asia, North Africa, and western North America.

Geographic variation exists, generally in terms of body size and the relative amounts of black and white plumage. Inevitably, a species with such a broad geographic range occupies a wide range of habitats, from the semi-arid desert in North Africa, and the prairies of North America, to Alaska's boreal forests. In Britain, its habitat includes the lush farmland of lowland England and the windswept moors of the Peak District. The estimated breeding population of Magpies in Britain and Ireland to be about 250,000 to 500,000 pairs.

This is a resident population, and there is no immigration into Britain from the Continent. In Britain, the Magpie has undergone a population increase during the past 40-50 years. Therefore an increase has also resulted in the spread of Magpies into the urban areas.

This increase in abundance has been met with a variety of responses: obviously, Magpie enthusiasts have welcomed it, but they are pretty thin on the ground. The commonest response has been concern over song-bird populations (see page 598). Ringing recoveries show that Magpies rarely undertake long-distance movements. That the median distance between the natal nest and first-breeding location was just 447 m, equivalent to moving 1.8 territories.

As with other passerine species, females tended to move farther (497 m,) than males (350 m), although this difference was not statistically significant. The median distance between annual breeding attempts was even less: males 15 m and females 27 m. A significantly higher proportion of females bred in more than one territory during their life (13/48; 27%) than did males (5/ 64; 8%). This occurred because males which lost their partner were still able to defend their territory alone and invariably remained there.

In contrast, a lone female could not maintain a territory, so, if she lost her partner, she usually moved to another territory to repair. These results probably reflect Magpie dispersal fairly accurately; we regularly searched all surrounding areas for color-ringed birds, and, in ten years of enthusiastic Magpie recording by the Sheffield Bird Study Group, we had only four reported sightings of individuals more than 2 km from where they had been ringed. Magpies are monogamous, and a breeding pair defends an all-purpose territory.

In most studies of Magpies in Britain and the Continent, territories average about 5 ha in extent. The population can be divided into two sectors, the breeders and the non-breeders. In some areas, non-breeding individuals may comprise 20-40% of the total Magpie population.

Non-breeders are usually one-year-old and two-year-old birds (rarely three- and four-year-olds) that live as part of a loose flock ranging over the territories of established pairs. The flock is organized into a dominance hierarchy, with males generally being dominant over females.

This occurs partly as a result of the difference in body size between the sexes. The situation is rather more complex than this: the date that a young Magpie entered the non-breeding flock also had a marked effect on its status, with late arrivals achieving the lowest status. The pattern is by performing an ambitious experiment that involved hand-rearing and releasing 44 Magpies into the study area at different times. This result also explains why late-hatched young had the lowest likelihood of surviving to breed.

Providing that they survive long enough, Magpies start to breed in their first or second spring: mean ages of first breeding were for males 1.6 years and for females 1.4 years (this difference is not significant). Breeding Magpies in our study area (based on re-sightings of color-ringed birds) had a life expectancy of 2.0 (female) to 3.5 years (male).

The oldest Magpie we recorded died in its 9th year. However, the ringing recoveries provide a similar longevity record: 9.7 years. These data were obtained in an area where Magpies were unmolested; obviously, mortality rates will vary in different areas.

Diet and feeding behavior

Magpies are omnivores and will eat pretty well anything: we have seen them eat pears, dog faces with relish, attack a mole Talpa europaea, catch, kill and eat voles (Microtinae) as well as eating the berries of whitebeam, acorns, and household scraps.

It is difficult to build up an accurate picture of any species' diet because different study techniques have different biases associated with them. The stomach contents of shot Magpies and analyzed Magpie pellets and droppings. During summer, the bulk of the adult and nestling diet comprised grassland invertebrates (beetles, caterpillars, spiders, leather jackets, and earthworms). In winter, much more vegetable matter (e.g. seeds, bulbs) was eaten. A very little evidence that Magpies took many songbird eggs.

Like many other crows, Magpies hoard excess food. There is, however, not the slightest evidence that Magpies are specifically attracted to (or steal) bright objects such as money or rings. The food-hoarding behavior of the Magpies has some detail. Most food-hoarding by Magpies is short-term, with items recovered within just one or two days, unlike the Jay ‘Garrulus glandarius’ and nutcrackers Nucijraga, which cache acorns and pine seeds, respectively, in the autumn and eat them in the following breeding season. All but one of 3,184 caches by Magpies were made in the ground, usually in areas of short grass.

A Magpie typically filled its pouch with food, flew or walked to the cache site, and then made a hole in the ground with its bill. It then ejected the food into the hole and covered it with a stone, twig or piece of dead grass. We found caches extraordinarily difficult to relocate. On several occasions, we watched a Magpie hoarding food and maintained a fix on the hoarding place through a telescope while the other person walked into the field of view. This narrowed our searching area to about 0.2 m2, but we rarely found caches without a lot of trouble.

So how do Magpies relocate their caches? Although some experiments that Marmioc their sense of smell to find hidden food, they probably rely mainly on their well-developed spatial memory to relocate caches. We regularly saw Magpies fly directly to a point in their territory and recover cached food; unlike us, they found the food immediately, without having to dig up half the field. This strongly suggests that they could remember its precise location.

Squirrels and the Acorn Woodpecker store food in a single 'larder' which they then defend against other animals, but Magpies are 'scatter hoarders', dispersing their caches over a fairly wide area. Territorial Magpies hoarded within the territory and (by definition) non-breeders cached food within their home range, but these two categories of birds had different hoarding strategies.

Territorial Magpies made their caches much closer together than did non-breeding Magpie. To determine whether this spacing was adaptive, some field experiments; to test the idea that the difference in cache spacing was important in terms of the birds' likelihood of recovering them.

Grids of artificial caches at different densities were (laboriously) set out. Each cache, which consisted of 7.5 g of grain, had a two-pence piece placed under it, so that it could be relocated using a metal detector, but without providing any visual cues for Magpies. After four days, the grids were re-examined to record the proportion of caches still present. The experiment showed that the caches placed closest together suffered the greatest losses.

Subsequent observations showed that Magpies spaced their caches according to the rate at which the artificial caches had been lost in those same territories. In other words, the Magpies were able to assess the intensity of cache loss (probably through the number of other Magpies in the immediate vicinity) and space their caches accordingly. A territorial Magpie could afford to space its caches close together because no Magpies (other than its mate) were likely to enter the territory and raid the caches.

On the other hand, non-breeders generally foraged in a flock, and, once one Magpie discovered a food item (from a cache or otherwise), other flock members rapidly congregated in that area. In other words, if a non-breeder made its caches as close together as a territorial Magpie, it would almost certainly lose much of its hidden food to other flock members. By caching at a low density, flock Magpies reduced this risk.

Breeding biology

Nest-building may start very early; we have occasionally seen Magpies building on mild days in late December and early January, even though eggs are not laid until late March. Magpies may build a new nest or they may re-use or add to an old one. If a new nest is built, it is constructed in four distinct stages: an anchor, a superstructure, a mud bowl, and a lining. Once the Magpies have decided upon a nest site, the first place twigs there; these are followed by mud or clay, which forms the anchor.

Twigs are added to this to make the superstructure, including the domed roof. Once the superstructure nears completion, the mud bowl is started; and when this is finished the lining of hair and rootlets is added. Nest-building may be spread over two months, or a new nest may be knocked up in less than a week, Well-constructed nests are very durable and may last many years. One can tell quite a lot about the owners just by examining the nest.

One-year-old Magpies and first-time breeders generally build very poor nests—often not much better than bulky nests of Woodpigeon Columba palutnbus. In contrast, established breeders sometimes build fortress-like nests. One of our long-lived color-ringed birds built a particularly robust nest every year; the roof was especially dense, such that the nest contents were completely concealed from the outside.

Moreover, the roof was interlaced with spiky twigs of hawthorn Crataegus, making nest inspection particularly difficult for us. Most Magpie nests are roofed and found that 61 out of 79 nests (77%) were roofed and that 66% of those produced at least one chick, whereas only 5% of nests without a roof produced young.

Nests are built in a variety of locations, from the tops of 30-m beeches to scrubby willows Salix or hawthorns just a meter or so high. In some parts of the study area, Magpies had a wide choice of trees in which to nest; in others, at higher altitudes, there was little or no choice, since many territories contained only a single tree or bush. In treeless urban areas of Sheffield, we have found Magpie nests in British Rail watchtowers, on electricity pylons, and inside factories.

Egg-laying commenced in late March, and the annual median laying date (the date on which 50% of pairs had laid their first egg) varied from 10th to 24th April. Much of this variation was attributable to spring temperatures: in warmer springs, laying started earlier, and this, in turn, was almost certainly the result of warmer temperatures increasing the availability of the Magpie's invertebrate food.

Magpies are single-brooded, and the clutch consists of three to nine eggs; the mean number varied only slightly between years, from 5.5 to 6.3 eggs. Clutch size also varied within a season, with the earliest breeders generally producing the largest clutches. This in turn was a result of older Magpies breeding earlier and producing larger clutches than young Magpies.

If the first clutch is lost, a replacement will be laid, and, in our study area, almost always in a new nest. Replacement nests were always within the territory, but were often difficult to find, because, unlike first nests, they were usually built when the trees were in leaf. Magpies would lay up to four clutches in a season if necessary.

Incubation lasts for about 18 days and is undertaken entirely by the female. The male feeds the female on the nest, and the extent to which he does this helps to determine their likelihood of success. Females that are not looked after by their partners have to leave their eggs more often to feed themselves, and this increases the risks of the eggs being taken by predators.

There were two main causes of egg loss: small boys (which we cured of the habit rapidly) and Carrion Crows. Carrion Crows and Magpies are arch enemies; they compete for food and probably nest sites, and Carrion Crows will eat Magpie eggs or chicks at any opportunity. As a result, Magpies breeding close to a pair of Carrion Crows generally had relatively low breeding success. In one year, we estimated that over 30% of all Magpie breeding attempts failed as a result of Carrion Crow predation.

The Carrion Crow catches an adult Magpie in flight and kills it. Magpie nestlings leave the nest after about 24 days. We weighed, measured and color-ringed nestlings at 14 days after hatching; later than this and we would have risked them 'exploding' from the nest. After fledging, the young remain with their parents for about six weeks; in one or two cases (out of about 1,000) young Magpies stayed with their parents in the territory until the following breeding season. In our study, breeding success averaged about two nestlings per pair, although some pairs were very successful and raised large broods, whereas others failed to rear any young at all.

This is reflected in the lifetime reproductive success of Magpies: 60% of females and 40% of males failed to produce any young during their lives. During the course of our study, the Magpie population doubled, examined the effect of this increase in Magpie density on breeding success. Interestingly, there was no effect on either clutch size or breeding success, but there was a strong effect on juvenile survival.

The proportion of Magpies surviving through their first year was negatively (and significantly) correlated with breeding density. This indicates that density-dependent mortality of juvenile Magpies is one of the factors regulating Magpie populations.

Reproductive behavior

Many bird species, like the Magpie, traditionally regarded as monogamous, are now known to be less monogamous than once thought. The female’s trouble they went to protect their paternity. If similar things went on among birds: it soon became apparent that they did. Established pairs of Magpies live in their territories all year round, but the male and female may operate independently for much of the time.

From just before and during egg-laying, however, the male never lets the female out of his sight, remaining close beside her and following her every move. If she flies, he follows. If she walks behind a wall, he moves so he can still see her. Why should the male be so keen to keep close to his partner at this time?

The answer is that he is guarding her against the sexual advances of other males. Male Magpies are remarkably randy and regularly try to mate with neighboring females. On several occasions, we saw a male sneak (literally) into the adjacent territory, using the vegetation as cover, approach the female, and attempt to mate with her.

Most of these extra-pair copulation attempts were unsuccessful because they were cut short by the male partner chasing the intruding male. The closest that I saw a male come to being successful occurred when a guarding male fell asleep. His head lolled onto his breast, and, in so doing, fell below the top of the wall so that he could no longer see his female, who was foraging close by. Within a second or two of this happening, the male from the adjacent territory flew across and mounted the female. I think that local contact was not achieved; the female called during the mating attempt and this alerted her partner, who woke up and flew down and chased the other male and his partner.

This would be the end of the incident, but there was a further development. When this extra-pair copulation attempt occurred, the female had already laid part of her clutch, but her partner immediately started to build a new nest in a tree adjacent to the original nest. The female laid the rest of her clutch in the original nest but never started to incubate. Instead, she subsequently produced a repeat clutch in the new nest. The interpretation was that the male partner did not want to risk the fact that his female may have been fertilized by the other male. Hence, waste his effort by rearing chicks that may not have been his own.

Subsequent studies have shown that both mate-guarding and extra-pair copulations are widespread among many bird species and that such extra-pair mating can result in extra-pair paternity. Magpies showed some interesting extra-pair responses to a tame, caged female placed in their territory.

If the male approached the tame female alone he invariably courted and attempted to mount her. But if the pair approached the 'intruder' together, they were always aggressive towards her. Often the male would approach on his own and start to court the female, only to be followed a minute or two later by his partner.

As soon as he heard his female approaching, the male's courtship switched to aggression. The human analogy is obvious. It is perhaps surprising that we have actually seen as many, or probably more. The extra-pair mating attempts then have pair mating. The courtship between Magpies is infrequent. It consists of the male circling the female with his wings flapping and tail held high and twisted to one side. Mating is brief (1-2 seconds) and very rarely seen. On the basis of prolonged and detailed field observations, Magpies probably copulate only about three times for a clutch.

Groups and Territories

Magpies are regularly seen in groups, from half a dozen to over 100 birds, and these groups may occur for a variety of reasons: mobbing, feeding, roosting, and 'ceremonial purposes. Magpies may assemble to mob predators such as foxes, cats, weasels, rats, or owls, but mobbing groups are usually small (up to ten birds) and disintegrate fairly quickly.

Non-breeding Magpies live as members of a flock, but these flocks are very loose, and all members come together only if food is locally abundant (e.g. if muck has recently been spread in a field). In our study area, the Magpie population density was high; the largest feeding flocks that we saw consisted of about 50 birds, but feeding groups of 20-30 were not unusual. Groups also occur where territories abut, and the pairs perform parallel walks along their respective boundaries.

If the territory boundaries were unknown to the human observer, these birds would appear to be a 'group'. The largest groups occur at roosts or pre-roost gatherings, where over 100 may congregate. In our study, most Magpies roosted in small groups (of about 30 individuals) close to or within their daytime range, but, just 2 km away, a roost of 150-200 Magpies existed throughout the autumn and winter. These birds roosted in hawthorn bushes in an area of dense scrub, but they entered the bushes only just before dark. Prior to this, the birds assembled in a pre-roost gathering in a field some 200 m away.

The observations support the idea that Magpie communal roosts contain mainly non-breeding Magpies while established breeders roost singly or in pairs within their territory. The final category of the group is that about which Darwin wrote, now generally known as 'ceremonial gatherings'. These groups are rarely as large as pre-roost gatherings, although they have sometimes been depicted as such.

The average size of 225 ceremonial gatherings is to be nine Magpies (range 3-24). Gatherings occurred at all times of year but were most frequent during January to March (i.e. just before the breeding season). They also tended to occur more frequently in the morning than in the afternoon. The function of ceremonial gatherings has been discussed for a long time and suggested functions included pair formation and competition for nest sites or territories. Only through having individually color-marked birds of known status were we able to work out what was going on in these gatherings.

Initially, most gatherings that we observed were in progress when we first saw them, but we soon discovered that the key to understanding them was to see them from their inception. Gatherings were usually initiated by one or two non-breeding Magpies, usually the most dominant members of the non-breeding flock. Single initiators were always males, and 80% of groups of two were paired birds. Gatherings were started in the following way: the bird would fly unusually high and would then swoop directly down into occupied territory.

There they would instantly be met by the territory owners, who would call and chase them. This in turn would attract non-breeders in the vicinity and the holders of adjacent territories (only rarely did breeders 'leapfrog' territories to join a gathering). Almost all the 'action' during a gathering occurred between the initiators and the territory owners. The other birds were present as mere spectators. Sometimes, however, one of these would be accidentally drawn into the chasing and calling if it was attacked by mistake, resulting in the apparent 596

The Magpie chaos typical of these gatherings means the duration of gatherings was ten minutes and the maximum was 70 minutes. In most cases, the gathering soon subsided and the initiators left the territory, followed by the other individuals dispersing. What is happening is as follows. Competition for territories is intense. In our study area, all available space was occupied by territorial Magpies and a large pool of potential breeders (the non-breeding flock) existed. Instead of passively waiting for a vacancy to arise, say through a territory owner dying, the most dominant flock birds attempted to obtain a territory by force.

They visited territories in a deliberately provocative manner, confronting the owners in order to monitor the strength of their aggression. In most cases, the initiators quickly backed down and left, but, occasionally, they found a pair whose territory was less strongly defended. They then appeared to press home an attack, harassing the owners with repeated visits and prolonged chasing. In a few cases, the territory owners were evicted, in others, the gathering initiators carved out a very small piece of ground for themselves. If this occurred, it was usually at the junction of two or three territories (the point where the defense was weakest).

Once they had obtained a foothold there, the invaders gradually expanded the territory until, after a week or so, it was sufficiently large for breeding. We estimated that a third of the territories were obtained in this way. Evicted territory holders usually disappeared (which probably means they died) or became members of the non-breeding flock (which was genetically equivalent to being dead, for none of these birds ever bred again). Why did other Magpies turn up as spectators at gatherings? I think the reason is that it was in the interests of all nearby birds to know what was going on. A change in territory ownership could, through a domino effect, lead to a change in fortunes for these other birds as well.

A change in the ownership of one territory sometimes resulted in a succession of subsequent changes. There were two other ways in which Magpies could obtain territories:

(i)                 Replacement

(ii)               What we called 'squeezing in.

The replacement was straightforward: one or both members of a territorial pair died or disappeared and their places were taken by non-breeders, without a gathering.

Squeezing-in consisted of pairs taking advantage of the decline in territorial aggression that occurred after egg-laying. These birds simply squeezed in at the junctions of existing territories, without a gathering. Those that squeezed in, however, usually did so too late to breed in that season.

Bird and territory quality

The effect of bird and territory quality on reproductive success was examined in detail. This showed that some territories were occupied more (and others less) than expected by chance, a result that suggests that territories differed in their quality.

Furthermore, analysis of territory composition showed that the number of years in which the territory was occupied was positively (and significantly) correlated with the relative amount of grazing land (short grass with cattle or horses) in the territory. Several other lines of evidence indicate that this is a good measure of territory quality:

(i)                 Magpies obtain most of their food from such areas above.  

(ii)               The breeding success was positively correlated with the relative amount of grazing land in the territory. It is also found territory quality to be important in affecting the Magpie's breeding biology: his studies indicated that food availability determined the quality of a territory.

The importance of the experiment is by providing some Magpies with the additional food prior to egg-laying. Fed birds lay earlier, produced larger eggs and clutches, and reared more young than did unfed birds. In our study area, territory quality was obviously important, found that bird quality played a bigger part than territory quality in determining breeding success.

The disentangle the effects of territory and bird quality on breeding parameters (such as clutch size and laying date). That is by looking at these parameters when the same territories were occupied by different birds. However, these parameters for the same birds breed in different territories.

This analysis showed that about 60% of the variation in clutch size, egg size, and breeding success was attributable to bird-quality effects, whereas territory quality accounted for less than 10% of the variation in breeding success.

Magpies and song-birds

Magpies have been much in the news in Britain in recent years: their increasing abundance in urban and suburban areas has evoked some strong reactions regarding their possible effects on garden songbirds. Some city councils have even contemplated Magpie culls.

Also, the magpie killing and eating a young Blackbird “Turdus merula” while the Blackbird parents fly around frantically calling is not a pretty sight. But many of those who would happily shoot Magpies for such behavior tell of their excitement at seeing a Sparrow-hawk “Accipiter nisus” take a Starling “Sturnus vulgaris” from the bird-table.

Such double standards mean that there can be no grounds for condemning Magpies for the emotional trauma they cause bird-lovers. The real question must be whether Magpies take sufficient numbers of songbirds (adults, young, or eggs) to reduce their populations significantly. More research is required to resolve this. Though, there are several points that should be kept in mind:

(i)                 The results from the Common Birds Census (CBC) show that Magpie numbers have increased. Hence, they do not show a decrease in the numbers of Blackbirds, Song Thrushes “Turdus philomelos”, Dunnocks “Prunella modularis” or Collared Doves “Streptopelia decaocto”, are the main targets of Magpie attacks. The most CBC plots are in rural rather than suburban or urban areas, however, so we still need more information on this topic.

(ii)               The suburban song-bird populations exist at densities much greater than they do in traditional, woodland habitats;

(iii)              The ringing recoveries and other types of the study point out that the main predator of garden birds is the domestic cat. How many bird-lovers also own a cat? Virtually all the information which we have obtained on Magpies during the last few decades has been obtained through the use of color-marked individuals. These studies have been conducted primarily with rural Magpies. Hence, a similar investigation of Magpies in suburban and urban areas would provide an interesting comparison and would allow us to assess the impact of Magpies on songbirds. Clearly, we should reserve judgment on this issue until we have some hard facts.

 

Reference - British Birds VOLUME 8 2 NUMBER 12 DECEMBER 1989