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
