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 open reed bed, albeit still with a fair amount of wet scrub and mature trees surrounding the reed bed, meaning a fair chance of a local 'goodie' like Bittern. More importantly, back then Berengrave had a reputation among local birdwatchers as the ‘hot spot’ for good numbers of roosting Redwing, Fieldfare, Chaffinch, Brambling and Goldcrest, especially in harder winter weather.
But the site had become under-watched since then. An obvious local project, then. I started making regular dusk visits to record species/numbers going to various roosts there. The make-up has certainly changed over the decades. The Redwing and Fieldfare have switched to a different local site, using Berengrave more as an alternate refuge when disturbed. Chaffinch numbers have dropped by more than 75%, and, initially, a sniff of a Brambling was now a red-letter day. But there is always something to see when low-carbon birding, and for me the star bird quickly became the Magpie.
Out for the count
Birkhead's excellent monograph 'The Magpies', provides a short but detailed section on roost behaviour, and my objective was simple (I'm a simple guy); could I see these same behaviours for myself here at Berengrave?
I also wished to make a reasonable estimation of roost size, as it appeared to be large in comparison with both the roosts studied in Birkhead, and roosts noted in the annual county Bird Report.
This 'ticking off behaviours' approach was to become my low-carbon birding mantra; to really appreciate the birds on my doorstep.
Trending
My resultant totals could never be taken as an exact number, as birds can enter, unseen, along several flightpaths, so there could easily be more (many more) using the site. (Similarly, birds could be leaving switching roost!). Any series of counts could only give an indication, but would show roost patterns well. As well as counting, on each visit I noted interesting behaviours 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).
The trick to collecting reasonable data was, of course, using the same methodology for each count. I chose to arrive at the south-western viewpoint in the quarry to start counting an hour before sunset, and continue 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 was set for the second half of October (to allow for some leaf-drop so as to see into the chalk pit) with fiurther counts each half-monthly period until the end of April (making 13 counts per year).
The roost habitat
A communal roost has to offer safety. As a reedbed/closed canopy wet scrub habitat, Berengrave’s condition, where scrub had encroached unmanaged and now dominated much of the reedbed, provided several common features needed for a safe roost sites- tightly packed willow and sallow in deep standing water intermixed with reeds. Magpies using such any such roost will be relatively safe from ground predators and, deep in the old pit, safe,from the worst of the weather. Being a deep quarry Berengrave is a considerable shelter from the weather.
The purpose of a roost
Magpies are territorial birds staying, more often than not, singly or paired mainly within their small home range throughout a year. Thanks to a large population growth over the decades, vacant territories in and around Rainham are now much harder to come by. Occupied territories will not always have a safe, warm, hidden roost site, why breeding pairs often 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) make for highly desirable nesting territories. The suburbs of Rainham, with Berengrave on their northern edge, provide perfect nesting 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) make for highly desirable nesting territories. The suburbs of Rainham, with Berengrave on their northern edge, provide perfect nesting habitat for Magpies.
In addition to any breeding population, overlaying breeding territories close to a non-breeding roost site, will be a large area home to a population of non-paired birds. These travel together within a larger larger core areas scouting for vacancies in/among the established breeding pairs. During the breeding season they also need a safe roost somewhere within this core area. So the roost at Berengrave is never vacated completely- during the summer months there is 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. 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 (mad?) as myself, arriving three hours pre-sundown seemed
somewhat excessive, so I decided on starting counts exactly an hour before
sunset. Yes, birds would already be in attendance at this time, but only on a handful
of occasions have I recorded high double already in the Chalk pit prior to my arrival (the odd raptor can disturb a pre-roost assembly).
Dependent on human frailty, our daylight density abilities, published studies hinted at last birds
arriving up to 40 minutes past 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 small main roost area immediately upon their arrival. Instead 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 out to the largest trees around the edge of the pit or, more unusually, leave the reserve for nearby gardens/fields
Most birds do not start to enter the core roost area itself until about fifteen minutes before sunset. The main roost was in low wet
scrub in the middle of the chalk pit and the Magpies did not start to enter it until
it was in shadow (in contrast, regular pre-roost trees often seem to those in sunlight).
Once birds have started to enter the roost proper until (around sunset), the 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. An exciting highlight of any roost- birds dropping from the sky.
It is not possible to view the majority 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 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 two metres).
Once birds have started to enter the roost proper until (around sunset), the 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. An exciting highlight of any roost- birds dropping from the sky.
It is not possible to view the majority 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 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 two 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 - they have stayed longer on their home territory as part of their routine defence.
I should point out studying birds leaving roost is extremely hard, as thanks to their eyesight, their light-gathering abilities, birds can leave routinely from around 30- 60 minutes before sunrise, and a pre-dawn observer will invariably miss many leaving sneaking out silently.
Seasonal trends
Adults eventually drive the young of the year from their home territory, to 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 best sites.
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. You need a few years' viewing to get to really get to grips with such annual 'curves', helping make long-term local birding more of a delight than just a species list.
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| Early 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 is for choosing breeding partners, most feel they serve just as usefully as an information exchange on food resources. By the second half of winter as well as arriving in pairs, more Magpies sit in pre-assembly trees in defined pairs, especially around sundown. From February onward, larger (male?) birds may be noted soliciting. Plus bBirds can be seen collecting (or flying into roost carrying) nesting material such as twigs, often then used in display, sticks presented to prospective mates.
Changes in 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 on open ground a little north and east of the reserve.
The fields to the north and east were 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? Sadly, these fields became a new housing estate in the early 2020s.
Local Comparisons
In published studies adjacent roosts have been found to be up to about some two kilometres apart. To compare roost behaviour at my local level, I became keen to note counts from two other roosts, one 1.35 kilometres distant, the second 3.75 kilometres. The first, just into the 'greenbelt' by the Rainham border with the parish of Upchurch, is in undisturbed dense conifer and closed canopy scrub over dry ground (itself a more unusual habitat, another area that became a new houing estate within a few years of starting to count.). The second, a little further out in Lower Halstow, was much more similar to Berengrave- by the edge of housing, in dense scrub close-by a reedbed. Both show similar behaviours to Berengrave, eg. attracting lower numberss at the beginning of the winter period (only up to c. 40 and 30 respectively). Both also showed different trends to Berengrave, eg. shallow but steady declines in numbers as the winter progresses. Perhaps down to both being much closer to areas of routine systematic corvid control throughout the period? For urban Magpie roosts threats of routine ongoing control are lower.
One theory for highest Magpie densities being associated with urban/sububan areas. (Such fun musing on them!) 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, matching studies quoted in Birkhead.
Natural threats
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 were more often noted roosting at Rainham Docks East/Motney Reedbed some 800 metres north-northeast of the Quarry site, and 2013 was a 'one-off'. 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, scavenging among rocks and seaweeds. 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 remained 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. One expected change likely to bring about the biggest impact would be vital conservation work. The reedbed is important habitat locally, and at some point the ever-encroaching scrub will have to be managed. That scourge of modern life and getting things done, 'Health and Safety', has so far put paid to large-scale work in the wet depths. Around the pit 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, and are being cleared, piecemeal.
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 have been increasing. The number of walkers, especially dog walkers, using the paths in the quarry after dark, is noticeable c/o anxiety calls, doggy noises and shouty owners seeking their off-leads. After a few years the boardwalk closed (too expensive to maintain) and the circular route disappeared, but new housing around the pit has led to new paths. The quarry is also used from time to time for interesting nocturnal human gatherings, and only the odd police visit stops the Berengrave viewpoints becoming routine haunts of recreational drug users. Both these disturbance types occur more at the beginning/end of the winter roost period. Plus the year-round problems of illiterate fisherfolk failing to read the 'no fishing' signs.
Annual fireworks activity (Diwali, Bonfire night, New Years' Eve, and many varied self-entitled individually important dates inbetween) cause varying numbers of birds to vacate the roost. Thankfully, unlike the estuary, the enclosed nature of the quarry does not lead to unauthorised displays within the reserve itself, but the disturbance can be high. Being (i) tee-total and (ii) miserable, my own New Year celebration quickly became standing in the pit at midnight listening to the big flight exodus as the whizz-bang assault started.
Then there's 'control'. Many humans see Magpies as a threat to other local bird (game) populations. Legal trapping has taken place for many years within half a kilometre of the roost. Increased local knowledge of the growing size of this roost has led to people running Larsen traps for longer time each year. In 2014-15 it was known one local resident was running a Larsen trap on the boundary of the quarry, but this seemed to have no real effect on numbers- most birds arriving intent on roosting rather than feeding from baited traps.
A one-off: roosting behaviour during a partial solar eclipse.
Yup, watch a local spot long enough and a real oddity comes along. 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. Wimp(!)
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 first three winters I recorded more than 40 species coming in to the quarry to roost. This more than doubled over a decade. Even watching Magpies, there's listing fun to be had.
If I were to write up another species, that would have to be Woodpigeon. Don't worry, that's for another time. But common species really do become all-consuming the harder you look, the more you question.
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Post-script: Growing pains
One thing long-term studies never prepare you for - trees continue to grow. By the time of the first Covid outbreak it was already becoming hard to view the roost from the viewpoint. A couple of winters later and the ability to get any comparative count to early years was gone. Sure, the flights in were still exciting, but the purposeful element was gone. Now if I go, it's just to anjoy watching Magpies being Magpies.
The roost deserves a better written account, and it might have got it if I had taken up an offer to write a chapter for the excellent 'Low-carbon Birding' book. I turned it down. Local birding politics left me feeling anything that my name attached to it would do more damage than good. In a decade of singing the praises of watching the roost I'd been joined once, by one birder (by accident, taking them a good 15 minutes to escape my verbose clutches). No matter the fun to be had learning about Magpies, the fun of the odd Yellow-browed warbler roosting, or that one-off (much wanted patch-tick by some) Hawfinch flyover, I really couldn't sell the idea. The Medway birding scene is too affluent, the draw of county lines too strong. The draw of routinely driving a couple of hundred miles a weekend around Kent, for the same common birds you could see here but plus some rare/scarce list-padders, remains too strong. Even in a climate crisis.
Enough, I rightly expect to have such negative thoughts edited out of a short chapter in an anthology encouraging Low-carbon birding. Such personal negativity is best left festering here in a dusty old blog-post. I'm sure in a few years such negativity will be unnecessary. Birders will do what's best for the birds, right? Until then, it's me and the Magpies.
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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.
