Black-headed and Herring Gulls, and one Oystercatcher with the Geese, with Cormorants out to sea. A Pied Wagtail and a Blackbird by St Mary's. I thought there was a Pipit along the wall by the Oyster Farm. At least a score of Linnets by Coldharbour, feeding on the bank by the bunded pools, together with half a dozen Redshank, both seen and heard, and two Black-headed Gulls.
I was moving fairly quickly, but thought I might also have seen a Stonechat behind Coldharbour, just from an impression of a shape, coupled with a perch high on a bush - weak evidence I know!
About 540 Dark-bellied Brent Geese on the fields and commuting to and from a gravel bank on the beach, lovely to see, and hear! Their almost grunting (the BBC suggests "gobbling") calls are surprisingly attractive and restful. BWP has them characteristically noisier on water, and indeed it was while they were on the sea, or flying from the sea that I heard them calling (BWP = hronk, hronk)! I also thought I saw one Pale-bellied Brent Goose, as regularly reported here, but it was difficult to be really sure. The birds were quite mobile, first disturbed from the field, and then moving in smaller groups backwards and forwards from the shingle bank by the Coldharbour outfall into the fields, including the one by the Green Bank. It was noteworthy that they flew quite low and in loose "brunches" or long "lines" rather than V-shaped skeins.
They lead lives that are so close to the edge. Average annual winter mortality in the UK is estimated to be about 15%, although a typical life span may be as long as 11 years, and the current BTO record being over 28 years old. On their long migration it is essential that they arrive on the breeding grounds in as good a condition as possible. On the breeding grounds they may first eat mosses and lichens, then turning to grass as it becomes more available. They need to have nutritional reserves to see them through problems such as any remaining snow and ice. It seems that individual birds that have better access to food within the flock, and therefore build up heavier body masses on the spring staging areas show better reproductive success. The females seem to get about half their energy for incubation from stored endogenous sources, the other half from intensive feeding episodes the length of which depend on the threat from weather, etc (Inger et al, 2008). It is suggested that an increase of body mass of one sixth on migration may lead to raising breeding success from 8% to 65%. However I think birds should not be too heavy before long distance flights, or they will have potential problems migrating.
Marine habitats seem to be greatly preferred by Brent Geese over terrestrial food sources for quality. Best food sources are
Zostera spp., Eelgrass,
Ulva sp, Sea Lettuce and
Enteromorpha spp, The rationale is that these algae are easier to digest than the fibre-rich grass, particularly important in geese, which have rather inefficient digestive systems.
Increasingly as the migratory populations have grown over the years from the nadir of the '30s, (10 fold perhaps?), Brents are apparently being forced onto (or are turning to) farmland rather than the
Zostera or
Enteromorpha rich intertidal zone. This may in the longer run have impacts on the average reproductive success of the populations. I wasn't paying proper attention today or I would have been surer of their detailed behaviour. The little I did see suggested that the Brents at Reculver were perhaps feeding a bit while they were on the autumn sown cereal (wheat?) and definitely often interacting aggressively. While they were on the seashore, they didn't seem to be feeding, or indeed showing too much aggression to each other, but I should have been actively checking. The flock was fairly densely packed on the sea and I wouldn't have thought there was anything for them to eat where they were anyway, perhaps a little
Enteromorpha. They seemed more to be having a bit of a "little seaside break".
Normal food sources start off with
Zostera, Eel Grass. In early winter the
Zostera spp. beds are relatively intact and the rhizome, in particular, provides an important food resource (Mathers et al. 1998). In late winter, however, the availability of
Zostera spp. declines, as a result of foraging and senescence, and it is more patchily distributed. Presumably there is
Zostera, Ulva or
Enteromorpha somewhere around off Reculver or Minnis Bay?? Perhaps they spend most of their feeding time on the fields?
At this time of year the flocks may consist of mixed ages, including the yearlings, if any are present - again I didn't really look at the time! The first winter birds are similar to the adults, although the white neck markings are less conspicuous, the white tipped coverts of the juveniles are largely retained and there is rather less white edging to the flanks. In retrospect I think I did see the white-tipped coverts indicating first winter birds in a number of cases.
The social inequities will mean that the higher-ranked individuals get the better resources in a feeding flock. It seems that in the winter period at least, families with any number of juveniles dominate pairs, pairs dominate singletons, and males dominate females. Several of these characters correlate with breeding success, and may be difficult to disentangle. Another paper suggested that large families do control more feeding resources than small ones, leading to less cost to the increased parental investment, so that things balance out. However by springtime arrival on the breeding grounds, the yearlings will apparently have been excluded from the family and the more mature birds will have settled into their pairbonds again. The pairings give advantages to the females of the pair, as the males protect them from aggressive behaviour from the other birds, allowing the females longer to feed up. BWP also notes that the birds are monogamous and pair for life!
The behaviour of the birds on the Arctic breeding grounds is extremely interesting. Laying is quite synchronous over a wide area - but do young birds lay later than experienced ones? The sites chosen will generally be on the drier hummocks to avoid melt flooding, perhaps on islands. The nests shaped like shallow bowls are about 30 cm. across, the internal bowl about 6 cm deep, of about 20 cm. diameter. They may be roughly 25 m. apart. Both sexes build the nest, using nearby materials, lining the nest with moss and grass, and covering with a fair amount of down. Incubation starts when the last egg is laid, leading to synchronous hatching. The nests may be re-used from year to year. Breeding success ranges are the most variable of all Arctic Geese, as calculated by the numbers of yearlings in winter flocks in the UK ranging from 1% to 45%. Up to 4 eggs are laid, generally only in one batch.
Like most, but not all, Arctic geese, the females have to eat as well as incubate. They cover the eggs with down and then move off the nest to feed, pecking virtually continuously and quite close to the nest, thus maximising food intake. The males do not incubate, but take on a protective role, against predators and conspecifics, being relatively more alert while the female is feeding, and feeding relatively more while the female is incubating. Once the chicks are hatched, both parents remain protective, although they do not need to feed the young as the chicks are precocial, nidifugous and self-feeding.
They tend to have two brood attempts, and then moult all in a narrow time window, before migrating, quite a challenge!?
The recorded numbers of these geese at Reculver do seem to fluctuate considerably. Chris Hindle's blog suggests that numbers in November 2014 were up to a thousand or so, but they seem to be down to 400 - 500 now in December 2014. They may disperse quite widely, moving onto other areas according to the weather, or food resources. They may perhaps move to Chichester or Harbour on the South Coast, or the French coast at Morbihan for example.
Boorman and Ranwell (1977) summarised much of information held about Brent Geese at that time, in their ITE paper scoping the possible environmental impact of the construction of Maplin Airport on the East Coast, never implemented in the end thank goodness. They showed that at least 20% of the world's population of Brent Geese (40% of the UK population) used the area over at least part of the winter, normally the first part.
In the 1930s the Eelgrass
Zostera species had been hit by a disease that greatly lowered their populations, particularly
Z. marina. Anecdotal accounts suggested that the Brent Geese had been hit as well, but the reduction cannot now be quantified. Between the 1950s and 1970s populations increased to about 25,000 - 30,000, contemporaneously with the introduction of shooting bans in the 1950s in the main European countries used as migration routes. It was thought that the population increases were being, or would be, limited by land reclamations in various European countries, particularly the Netherlands. However between 1968 and 1973 the world population more than doubled, shooting up to about 68,000 birds, while at the same time they started using farmland sources of winter food!
In the Maplin study, in the first half of winter, September to January, over half the Essex population fed on the
Zostera beds of Foulness and Leigh, then moving on to the
Enteromorpha beds of the other estuaries. In January/February '72-3, with peak counts double the previous year, about 30% of the birds turned to farmland, a behaviour only previously noted in the freezing winter of '62-3. In 1973-74, at very high populations, weight losses in birds averaging about 9% were noted. These losses could significantly impact upon reproductive success. By contrast in '62-63 in Denmark, with about a third of the 73-74 populations, despite the terrible weather, no weight losses were found.
ITE then organised some ground-breaking netting and ringing to try to understand the movements of the geese. Cannon netting had to be used on the open flats, based on siting the cannon nets in advance, hoping to guess where the geese might be. Only about 16 individual Brent Geese had ever been ringed before, so little was known of their winter distribution changes. In this exercise one family was noted to move North to the Blackwater, then South to Chichester Harbour, and then finally on to the Waddensee within 24 hours of leaving Chichester - interesting to see how they stuck together. Of course such familial cohesion is essential. Juveniles feed for up to 95% of their time presumably being partly watched over by their parents, who feed for about 75% of their time early in the winter, although this trends to 95% later in the year, as food quantity and quality decreases. The ringing demonstrated that at least some birds remained relatively faithful to their sites on particular estuaries at least for some time.
Brent geese have very variable breeding results according to the weather and predation. In the year 1977 it was estimated that successful breeding had occurred in only 8 of the previous 15 years. No other goose species has such a variable breeding success leading to unbalanced age class structures
in the population from year to year, likely to affect social behaviour. In years with success, the family groups migrate together, adults with that year's juveniles. In unsuccessful years the (non)breeding adults migrated before the two year old birds.
The approximate age composition of the British population of Dark-bellied Brent geese was calculated for 1973-74 by the ITE. The total population was estimated then at roughly 41,000, with about 15,400 first-winter birds, 7,600 breeding adults and 18,000 full grown birds without young (from the estimated mean brood size of 4.0 and the estimated percentage of first winter birds as 37.5). The full grown birds without young were probably mostly in their second winter, for Brent geese do not usually breed until their third or fourth summer.
It is worth noting that the latest BTO population estimate is in the region of 95,000 Dark-bellied birds wintering in the UK! There are also about 27,000 Canadian Light-bellied Brent Geese and 3,400 Svalbard Light-bellied Brent Geese. The main concentrations of Dark-bellied birds are in the Wash, the North Norfolk coastal marshes, Essex estuaries, the Thames Estuary and Chichester and Langstone Harbours.
The ITE trials demonstrated that disturbance by humans or light aircraft caused the geese to stop feeding or fly, reducing their feeding success by 3 - 4 percent, which might seem low, although any reduction may be very significant. Using a nightscope it was established that the Brent Geese did feed at night throughout the winter, sometimes in cloudy weather, as well as during the day, and in some cases on quite sparse
Enteromorpha. However, Brent geese appeared not to feed so intensely at night as during the day, and were almost exclusively confined to the tide edge.
Clearly the farm cereals and grasses are not going to be anywhere near as nutritional as
Zostera, and the balance between energy expanded, and energy gained while feeding may be greatly narrowed. It is also worth remembering that extended periods of frost may mean that the farmland resources become completely unavailable for a period! I wonder how the Reculver Geese did over the severe winters of 2011/2012 and 2012/13. The impact of switching to farm resources on individual fitness is certainly of some potential concern.
The amount of
Zostera, or
Enteromorpha, likely to be available in future years is not necessarily predictable. Particular species appear to have very finely attuned requirements for drainage/drying out of the substrate, and the balance between accretion and erosion. These factors may be affected by other changes along the coast, and climate.
Zostera noltii of the intertidal zone appears to be the currently preferred species for Brent Geese, with the role of the subtidal
Z. marina var.
angustifolia now less well understood. Coastal squeeze and shore steepening may impact on
Zostera beds as well as salt marsh availability.
These are monthly peak counts for Brents at Reculver. 2008/09 was a relatively poor year across North Kent, but not too bad at Reculver. Interestingly the winter of 2011/12 gave high numbers, but the significantly colder winter of 2012/13 (at least in the East and Southeast) gave only the "normal" moderate numbers.
Bibliography:
Boorman, L.A. and Ranwell, D.S. (1977) The ecology of Maplin Sands and the coastal zones of
Suffolk, Essex and North Kent. Institute of Terrestrial Ecology, Cambridge.
Birds of the Western Palaearctic (BWP).
