The chalk pit has long been known as an attractive winter roost site. I first visited more than 30 years ago, when the flooded quarry held a more sizeable reed bed, albeit still with a fair amount of wet scrub and mature trees surrounding the reed bed. Back then Berengrave had a reputation among local birdwatchers as a ‘hot spot’ for good numbers of roosting Redwing, Fieldfare, Chaffinch, Brambling and Goldcrest, especially in harder winter weather.
I have been making regular dusk visits to record species and numbers going to various roosts there. The make-up has certainly changed over the decades. The Redwing and Fieldfare have switched to a different site, Chaffinch numbers have dropped by more than 75%, and a sniff of a Brambling is now a red-letter day. There is always something to see, however, and for me the star bird is now the Magpie.
Objective
Birkhead's excellent monograph 'The Magpies', provides a short but detailed section on roost behaviour, and my objective was simple; could I see these same behaviours for myself at Berengrave?
I also wished to make a reasonable estimation of roost size, as it appeared to be large in comparison with both Birkhead, and with roosts noted in the annual county Bird Reports.
Methodology
The resultant totals cannot be taken as an exact number, as birds can enter, unseen, along several flightpaths, so there
could easily be more using the site. (Similarly birds could be leaving!). Any series of counts can give an indication, however, and can show roost patterns well. As
well as counting, on each visit I have noted any interesting behaviour and
interpreted against known information (there is a list of all the main references I have used at the end of this page, should anyone who would like to read more).
In order to collect reasonable data I use the same methodology for each count. I arrive at the south-western viewpoint in the quarry to start counting an hour before sunset, and remain until 30 minutes after sunset.
(If that length of time sounds a little daunting for a casual visit, the greatest period of activity is usually in the 15 minutes or so either side of sunset, so you can easily witness the gathering yourself.)
(If that length of time sounds a little daunting for a casual visit, the greatest period of activity is usually in the 15 minutes or so either side of sunset, so you can easily witness the gathering yourself.)
The first count of the season is not until the second half of October (which allows for some leaf-drop so as to see into the chalk pit) and counts then continue each half-monthly period until the end of April (13 counts per year).
The roost habitat
A communal roost has to offer safety. As a reedbed/wet scrub habitat, Berengrave’s present condition, where scrub has encroached and now dominates much of the reedbed, provides several common features needed for a safe roost sites- tightly packed willow and sallow, and deep standing water intermixed with reeds. Magpies using such a roost will be relatively safe from ground predators, and from the worst of the weather. In addition, being a deep quarry Berengrave provides considerable shelter from the effects of wind.
The purpose of a roost
Magpies are territorial birds staying, more often than not, singly or paired mainly within their small home range throughout the year. Thanks to a large population growth vacant territories are now much harder to come by. Occupied territories will not always have a safe, warm, hidden roost site and so breeding birds start to use a local communal roost after the breeding season.
In ornithological terms the Magpie is a 'highly sedentary' species. Studies have shown a peak dispersal distance of just five kilometres among young birds up to the age of about 10 months, and for older birds an average just under one kilometre. Young birds seeking out Berengrave may have come a short distance, adult Magpies using Berengrave will not be travelling far at all.
Nationally there has been a 97% increase in Magpie numbers in the 40 years to 2010, with reduced control thought to be a significant driver of this change, so it is no surprise larger roosts may now be more usual. Increased food availability in our towns, together with suitable nest sites (mature trees with a mosaic of short grass close-by for feeding in parkland/gardens) makes for highly desirable nesting territories. The suburbs of Rainham, with Berengrave on their northern edge, provide perfect habitat for Magpies.
Nationally there has been a 97% increase in Magpie numbers in the 40 years to 2010, with reduced control thought to be a significant driver of this change, so it is no surprise larger roosts may now be more usual. Increased food availability in our towns, together with suitable nest sites (mature trees with a mosaic of short grass close-by for feeding in parkland/gardens) makes for highly desirable nesting territories. The suburbs of Rainham, with Berengrave on their northern edge, provide perfect habitat for Magpies.
In addition to the breeding population, overlapping a number of breeding territories local to a roost will be a large area home to a population of non-paired birds. These travel together within own larger territory scouting for vacancies in the breeding pairs. They also need a safe roost somewhere within their area. So the roost at Berengrave is never vacated completely- in the summer months there will still be a small nightly roost of some 20 or so Magpies in Berengrave.
When the urge to defend/seek breeding territories is low, many birds in a local population can come together at one site. Large communal roosts build. During published studies birds joining such roosts were found to be mainly from usually only up to a kilometre away, more rarely up to four kilometres distant, to reach a safe site. A communal roost can be a very good indicator of local population size.
The daily routine
Previous studies have shown
birds might start to arrive as early as two to three hours before darkness,
although the vast majority only arrive during the last 40 minutes of daylight.
Even for someone as keen as myself, arriving three hours pre-sundown seemed
somewhat excessive, so I decided on starting counts exactly an hour before
sunset. Birds will already be in attendance at this time, but only on a handful
of occasions have I recorded double figures already in the Chalk pit on my arrival.
Dependent on daylight density, these earlier studies hinted at last birds
arriving up to 40 minutes past sunset (most latest birds not arriving more than
20 minutes after sunset).
For Berengrave, after a little
trial and error, I settled on finishing any count exactly 30 minutes after
sunset, as I found I hardly ever recorded an arrival later than about the
20 minute post-sunset mark.
At the start of any count nearly all the Magpies avoid entering the roost area immediately on arrival. They tend to arrive in small parties of up to three to six birds (one to three more usual) to just sit quietly in the larger trees around the quarry edge, or in the tallest trees in the quarry. They can then remain on view for some considerable time before entering. Such groups can eventually nigh fill their more favoured trees, and this behaviour is known as 'pre-roost gathering'.
Several types of recognised activity can be observed among these pre-roost gatherings, including aggression (‘chase hopping’ and ‘chase-flight’ displays), advertising (‘tree-top sitting’) and ‘roost circling’.
At Berengrave small numbers of birds will also regularly desert, moving a short distance to the largest trees at the edge of the pit or, more unusually, leave the reserve for nearby gardens/fields
Most birds do not start to enter the roost itself until about fifteen minutes before sunset. The main roost is in low wet
scrub in the middle of the chalk pit and Magpies do not start to enter it until
it is in shadow. Pre-roost trees always seem to make the most of any
sunlight.
Once birds have started to enter the roost proper until around sunset, later arrivals often do not bother with alighting in pre-roost trees, instead choosing to drop straight in, using an impressive 'wings-back' dive approach.
It is not possible to view the many of the birds once they have dropped into roost, but some may stay for five-ten minutes at the top of this denser scrub before descending to nearer to water level (the scrub height in the chosen roost site is about two to three metres, water depth in winter varying between one to one and a half metres).
Once birds have started to enter the roost proper until around sunset, later arrivals often do not bother with alighting in pre-roost trees, instead choosing to drop straight in, using an impressive 'wings-back' dive approach.
It is not possible to view the many of the birds once they have dropped into roost, but some may stay for five-ten minutes at the top of this denser scrub before descending to nearer to water level (the scrub height in the chosen roost site is about two to three metres, water depth in winter varying between one to one and a half metres).
From studies of marked birds elsewhere, it has long been known lower social ranking birds arrive earliest, while birds from better-quality territories arriving latest. This is hinted at by many of the late-arriving Berengrave birds travelling in pairs.
Studying birds leaving roost is extremely hard, as thanks to their eyesight, their light-gathering abilities mean birds can leave routinely from around 30- 60 minutes before sunrise, and an observer will miss many leaving silently.
Seasonal trends
Adults eventually drive the young of the year from their home territory, when they become part of the loose wandering groups at the same time they may start to roost communally. Peak communal roost numbers will usually be in mid-winter, when colder weather has driven more of the breeding pairs to the site.
It is not a smooth increase. There is sometime a 'blip' in the autumn with an obvious peak and a trough in numbers. This may be due to problems for younger, inexperienced birds. There is a clear rise in the mortality rate among young birds at this time as they struggle to cope with shorter days for feeding and colder weather. This makes for peak dispersal distance time as well, as some are driven further in an attempt to survive.
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| Berengrave roost trends (to December 2015) |
Roosts usually start to disband once the higher social birds start nesting in earnest. Some consider a main purpose of communal roosts to be for choosing breeding partners, most feel they serve just as usefully as an information exchange on food resources.
In the second half of winter as well as arriving in pairs, Magpies certainly more often sit in pre-assembly trees in pairs, especially around sundown. From February onward, larger (male?) birds may be noted soliciting. Birds can be seen collecting, or flying in with, nesting material such as twigs and displaying/presenting to prospective mates.
Changes in the numbers on various approach directions are also evident, with the autumn and early winter majority being ‘town’ birds from the south-west which seemingly become 'country' birds as the season wears on, approaching from the north and east more. Reasons for this are as yet unclear. It might merely reflect improving feeding situations nearer the roost once milder weather starts, as from early afternoon groups of up to a dozen or so can be found ground feeding in horse paddocks north and east of the reserve. In February 2015 there appeared to have been a dramatic disturbance, as the vast majority of birds switched to entering the roost from the north for a short period.
The fields to the north and east are certainly suitable for 'ceremonial gatherings' in the early spring, when up to around 20 or so birds can engage in ritual display involved with choosing mates. However, I have observed locally that suspected ceremonial gatherings can be more preoccupied with feeding, perhaps due to a lot of ceremonial posturing able to take place around the chalk pit edge?
Local Comparisons
In previous studies, adjacent roosts have been found to be up to about some two kilometres apart. To compare roost behaviour at a local level, I have also made counts at two other roosts, one 1.35 kilometres distant, the second 3.75 kilometres. The first, just into the 'greenbelt' by the border with the parish of Upchurch, is in undisturbed dense conifer and closed canopy scrub over dry ground (itself a more unusual habitat). The second, in Lower Halstow, is much more similar to Berengrave- by the edge of housing, in dense scrub by a reedbed. Both attract many fewer birds at the beginning of the winter period (only up to c. 40 and 30 respectively). Both also show a different trend to Berengrave, a shallow but steady declines in numbers as the winter progresses. This is perhaps down to both being much closer to areas of routine systematic corvid control throughout the period. For urban Magpies threats of routine ongoing control are much less.
This is is one reason which has, perhaps, led to highest Magpie densities being associated with urban/sububan areas. Instead of having a declining number throughout the winter, Berengrave shows a build to a mid-winter peak, with a strong decline only as longer days and warmer weather arrives, which matches studies quoted in Birkhead.
Natural threats
At the end of March 2014 saw an
obvious increase of birds arriving in pairs. One brought a food offering to
impress a potential mate, only to be chased off by hungry rivals. (The only
other time I have noted food being carried into the roost in mid-winter was December
2015, when one bird carried a large crust, which he proffered to no other bird,
instead dropping quickly out of sight.)
It is known that Magpies and Carrion Crows have aggressive interactions, with the larger Crow the dominant species. In early 2013 the Quarry also held a Crow roost, peaking at 67 individuals in late February. Although close to the Magpie roost, it was situated high on the western edge of the quarry in higher trees. No interaction was noted, the Crows prone to arriving later, at/after dusk. It may be this was a secondary roost site; Crows are more often noted roosting at Rainham Docks East/Motney Reedbed some 800 metres north-northeast of the Quarry site. Birds were always seen to arrive from the north and east, and were presumed to be coming from Horsham Marsh, where a commercial game farm provides easy feeding (albeit with a high risk of predator control). The Crows also feed on/along the nearby estuary creeks for a great deal of the day. In 2013-14, the small number of Crows seen over the Quarry always continued to the south and west to utilise an unknown roost site. They only once used it as a pre-roost gathering site. In December 2015 it was interesting to see Crows started to roost again on the western edge of the reserve; although just in single figure numbers, this was enough to shift Magpie roost flightlines away from any potential threat.
Foxes and cats are frequent around the edge of the quarry, but pose no real threat to the roost. Sparrowhawks do hunt the quarry at dusk, but mainly take smaller prey; their flights through the roost will disturb birds, but mainly in response to panicking Woodpigeon, which also roost in large numbers here (regularly 300 to 500 birds).
Once a Buzzard roosted in one of the pre-assembly trees, and was surrounded by Magpies until dark; most were silent, a few gave agitation calls and one or two were bold enough to fly at the Buzzard, one or two even tweaking at its tail.
The seasonal break-up of the roost
Previous studies have shown roosts start from mid-October, dropping away dramatically from mid-March (the time when the majority of territorial pairs begin nesting in earnest). The count made in early March 2015 followed several days of unseasonably warm weather (up to thirteen degrees), and while the roost number reamined high it featured a large early assembly number of birds, not only enjoying the warmth of the sun, but also indulging in a high level of inter-pair aggression.
By the end of April 2015, 29 out of 33 roosting birds arrived singly, indicating breeding birds to be settled on territory.
Magpie age at first breeding is usually two, so many of these singletons still using Berengrave may be last year's young. It is highly likely a larger proportion may be male, as young females can replace lost paired females whereas young males trying to inherit and defend a vacated territory find it harder to defend against older birds looking to establish themselves, or neighbours extending their own territories into the vacated site.
The future for the roost
Any changes are likely to be dynamic, and it will be interesting to note any effect in numbers. The expected change most likely to bring about the biggest impact will be vital conservation work. The reedbed is important habitat locally, and at some point the ever-encroaching scrub will have to be managed. A number of the older trees are thought by some to be dangerous and unstable, as well as having a detrimental effect on the ground flora of the chalk pit edges.
Flooding occurs each year, to varying degrees. The quarry does have an outlet to the north, but with recent increasing annual seasonal rainfall the water level can vary by about one to one and a half metres, and may remain high during the summer months, leading to a dieback of some of the more mature trees.
Human disturbance levels are also likely to increase. The number of walkers, especially dog walkers, using the circular path around the quarry after dark, is noticeable from the level of anxiety calls. The quarry is also used sometimes for nocturnal gatherings, and only higher police presence from time to time stops Berengrave becoming a haunt of recreational drug users. Both these disturbance types occur more at the beginning/end of the winter roost period. (Annual fireworks activity will cause a small number of birds to vacate the roost. Thankfully, unlike the estuary, the enclosed nature of the quarry has not led to any unauthorised displays in the reserve itself.)
Control. Many see Magpies as a threat to other local bird populations. Legal trapping has takes place for many years within half a kilometre of the roost. Increased local knowledge of the growing size of this roost has led to some now running larsen traps for longer time each year. In 2014-15 it was known one local resident was known to have run a larsen trap on the boundary of the quarry, but this seemed to have no real effect on numbers- most birds arrive intent on roosting rather than feeding from baited traps.
Roosting behaviour during a partial solar eclipse
Although not a totality, this event gave an opportunity to watch how birds can be fooled into roosting. Unfortunately the day itself, March 20th 2015, was dull and overcast, so the observable change in light intensity was not as dramatic as might have been, but a small amount of noteworthy gathering was seen.
On my arrival one hour before the fullest extent of the eclipse, two birds were on view. A further 22 birds arrived during the next hour, but none adopted the 'pre-roost gathering' behaviour of sitting together until at first three moved into one of the routinely-used trees to sit out the eclipse, another five joining them at the dullest time. Only one bird, two minutes before the time of the eclipse, actually entered the roost.
Other species roosting at Berengrave
Visitors at dusk will not just see Magpies. In addition to resident birds, Berengrave is used as a safe roost by many other species, and during the three winters to date I have recorded more than 40 species coming in to the quarry to roost. These will be covered in separate article later.
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Amendments
Please note this is an ongoing personal study, and so this page will continue to be revised and updated regularly. The list of revision dates is as follows:
Original posting- 04/01/16
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References
Balmer, D.E. et al., 2013. Bird
Atlas 2007-11; the breeding and wintering birds of Britain and Ireland, BTO
Books, Thetford.Birkhead, T.R., 1991. The
Magpies. The Ecology and Behaviour of Black-billed and Yellow-billed Magpies.
Poyser, London.
B.T.O., 2014 Birdfacts-
Magpie. as retrieved February 12th 2014 from http://blx1.bto.org/birdfacts/results/bob15490.htm.
B.T.O., 2014 Birdtrends-
Magpie. as retrieved February 12th 2014 from http://blx1.bto.org/birdtrends/species.jsp?&s=magpi.
Cramp, S et al., 1994. Handbook
of the Birds of Europe the Middle east and North Africa The Birds of the
Western Palaearctic Volume VIII. Oxford University Press, Oxford.Wenham C.V. et al., 2002. The Migration Atlas: movements of the birds of Britain and Ireland. Poyser, London.
