First birds I saw were Black-Headed Gulls, Great Crested Newts and Herring Gulls on the sea, and then Oystercatches and Turnstones on the rocks as the tide came in.
However my interest in these were soon eclipsed by fantastic sightings of some roosting Purple Sandpipers,
Calidris maritima, at Rhos-on-Sea at almost exactly high tide today!
The birds are overall quite dark above, streaky breasts and flanks with short orange-yellow legs and an orange-yellow base to the bill. Having shorter legs is possibly an adaptation to their tough environment, keeping them closer to the ground and therefore keeping them warmer due to the lower wind speed in the boundary layer - Canadian results do seem to show they have shorter legs the further North they breed. These photos are a bit blurry - and the rock in front kept getting in my way! He does seem to be enjoying this little rock splash pool though!
The first individual above at least seems to have a darker splodge just in front of his eye, more obvious in these next two photos:
They have an all dark tail and a narrow white wing stripe running down the middle of the wing, quite visible here - at the edge of the greater coverts and then perhaps a secondary fringe along the edge of the primaries? See also how white the flank under the wing looks, and the bird actually uses several forms of wing-lift displays, exposing the white flank and underwing as a white "flash" - quite dranatic!
The Purple Sandpiper is almost entirely a winter visitor to the UK, with only a really tiny breeding population in Scotland, that is known to have survived for over 25 years, and is thought to have originally transferred their affections from Norway. In this breeding colony the return rate for adults during a study was 65%, thought normal for small sandpipers. The fledging rate was about one chick per pair of adults, just keeping the colony afloat.
However evidence from an over-wintering population in Cleveland indicated almost 100% winter survival rates, with the spring-autumn absent periods being the dangerous times, ranging from 57 - 77% death rates. On Helgoland, in a resident (?) population, summer mortality was mainly from sparrowhawk predation. Helgoland is also used for over-wintering by several populations that breed to the North.
Generally birds in their over-wintering areas are fairly site-specific, and only move if forced to. This poses interesting questions about this small and specific high tide roost in Rhos - is it, by now, the descendants of the original birds? Or do birds aggregate from others that just turn up? The suggestion is that these Welsh birds may be from somewhere like Canada or Greenland - one bird ringed on Hilbre Island turned up later in Greenland.
Research shows that getting the best position in a roost is possibly quite important - birds are aggressive over getting the best positions. They are increasingly aggressive as the wind speed increases, although the frequency of aggression decreases as the wind speed gets very high and the birds appear to have to concentrate on just keeping their balance! Bigger and possibly older birds are the most dominant.
The analysis of the gut contents of Purple Sandpipers from eastern and northern Scotland showed that most of the diet at low tide consisted of molluscs, particularly littorinids (winkles), but also
Mytilus edulis (mussels),
Nucella lapillus (dog-whelks) and
Rissoa interrupta. Crustaceans, annelids and algae were also eaten. At high tide Purple Sandpipers fed on the larvae, pupae and adults of
Coelopa frigida (the kelp fly) when waves broke into banks of rotting algae along the high water mark. As the molluscs were swallowed whole and the shells crushed in the gizzard there may be limitations on the size of molluscs that can be eaten, below the size that can be swallowed. Purple Sandpipers with longer bills (females) took larger
Mytilus edulis and littorinids than those with shorter bills (males). The evolution of sexual size dimorphism and resource partitioning by the sexes is quite possibly of interest.
On a Svalbard beach birds preparing for migration to the over-wintering grounds appeared to feed quite indiscriminately on springtails and amphipods.
On the island of Helgoland (German Bight) Purple Sandpipers Calidris maritima feed mainly in the intertidal range of piers and rocky shores. The main prey species are
Littorina saxatilis and
Mytilus edulis, complemented by crustaceans, polychaetes, other molluscs and green algae. Beach habitats are used as alternative feeding sites during storms. Feeding sites seem to be selected according to rates of assimilated energy intake. The most profitable habitat (wrack beds on the high-tide line with kelp-fly larvae, 16.8 W) is used after arrival in October but is not available during winter. Because of high intake rates in rocky habitats (13.1 W on piers, 5.5 W on mussel beds), which allow short daily feeding times, and available alternative feeding sites during storms, Purple Sandpipers do not need to carry fat reserves in winter like other waders wintering in central and western Europe. This, and the ever accessible food supply of epibenthic macrofauna on rocky shores, may enable Purple Sandpipers to winter further north than other wader species.
The foraging activity of the Purple Sandpiper, with respect to its utilization of habitat and prey, was studied on the shore of a small bay on West Spitsbergen (Norway) at low tides during a 10 day period. Based on sediment type and prey species, the shore could be subdivided into three parts: a muddy beach with rich infauna, a cobble/sediment beach inhabited by high densities of both amphipods and Collembola (springtails), and a cobble/rock beach with a high density of amphipods only. All parts of the beach and all prey types were utilized, with no clear preference for any of them. However, water level and weather affected the availability of prey differently. The infauna on the mudflat and the amphipods on the cobble beach were preyed upon if the water level at low tide (infauna) or waves (amphipods), did not prevent the birds probing activities, and Collembola was taken on calm overcast days when they settled down to graze algae upon stones on the beach.
A study in the UK showed some interesting stats. First-year birds were lighter than adults by 2 g. There was only a slight increase in mass in mid-winter, in contrast to other waders wintering in Britain, suggesting that Purple Sandpipers are less at risk to cold weather and food shortages. Their winter fat reserves were low but their breast muscles were relatively larger than other small waders wintering in Britain. Their plumage was slightly heavier and their breast and belly feathers were longer. There was a large increase in mass in May by first-year and adult birds prior to migration. The 'long-billed' population showed a greater relative increase in May, compared with the 'short-billed' population, perhaps reflecting different migration distances.
A comparison between Purple Sandpipers in Iceland and Svalbard, plus a review of studies elsewhere, discovered relatively few geographical differences, despite the differences in breeding environments. One difference was the timing of the breeding season: Purple Sandpipers breeding at lower latitudes bred earlier and had a longer laying season than the high Arctic birds, and birds breeding at sea-level bred earlier than those in the mountains. Early breeding may have advantages, such as the chance to relay after the failure of the first clutch. Breeding densities were generally low, with the lowest occurring in the most severe conditions in Franz Josef Land. Higher densities occurred where birds could make use of coastal feeding habitats while breeding on the tundra or breed within the presumed protection of tern colonies (Iceland). Egg size varied geographically, and this was reflected in the variation in body (wing) size; eggs and birds from southern Norway were smallest, and Icelandic eggs and birds were largest. The factors determining this pattern, which is also apparent in other waders, need further study.
Most of the population of Purple Sandpipers that overwinters in North East Scotland appears to come mainly from the population that breeds in Norway, supposedly a relatively short migration for these birds that are able to complete their breeding early. These tend to be shorter-billed birds. However it is very likely that at least some come from Canada, and they tend to be longer-billed birds. Orkney results, where the majority of birds are long-billed and therefore appear to come from Canada, indicate that the islands also act as a stopping-off point on the return spring migration back to the breeding grounds. The birds feed up quite rationally, increasing mass from about 70 g to about 90 g, thus taking on sufficient food reserves to get to Iceland if they are headed for Canada, or to get to Norway if they are destined for there.
The Dutch/Belgian over-wintering population apparently comes from further North, and are most likely to be from the Canadian (or perhaps Russian) Arctic, having perhaps had to leapfrog the earlier migrants. Canadian breeding birds appear to over-winter all along the East Atlantic flyway, Greenland, Iceland, Northern Scotland (migrating via Iceland) and Norway.
However there are also birds that over-winter throughout Norway that appear to come from Novaya Zembla and probably to a lesser extent from Svalbard. These birds were expected to have a higher metabolic rate and to require greater insulation than birds over-wintering in Scotland for example, but upon examination the higher metabolic rate appeared to be more significantly supported by a larger stomach and liver, indicating that they possibly just ate more, according to one paper based mainly killed birds?? This in turn might imply that the amount of food a bird eats is at least partly limited by its ability to process the food eaten.
This species is quite strongly sexually dimorphic, with the females being larger than the males - and therefore tending to be more dominant in winter roosts. There are quite a few more males than females, in the approximate ratio of 65:35. this does not appear to be determined at birth, but seems to arise owing to higher juvenile mortality among females soon after fledging. This may result from the relatively lighter mass of female chicks.
Purple Sandpipers are similar to most shorebirds in that males actively court females, establish territories and vigorously defend their territories from intruders. As the eggs are laid the males are the less attentive at attending them. However as the eggs develop further, the males get increasingly responsible, and are the better parents in the second half of the period before the eggs hatch. Then, very unusually for a bird species, the adult females (the more dominant ones?) often abandon the nest soon after the eggs hatch - leaving their males to take care of the young in about 90% of the nests. Despite this, when experimentally deserted by the males (not sure how they did this?), most female purple sandpipers assumed brood care.
However there have still been instances in this species where the pair were faithful both to the bond and the site the following year - despite wintering apart (a parental influence?), and this faithfulness may not be too unusual in this species. In addition there appears to be a very low level of extra-pairing copulations in this species (less than 2%) , so the parental effort put in by the male at least seems to be fairly reliably being spent on his own offspring.
In most monogamous sandpiper species, females share parental care but leave the brood earlier than males, a feature unusual among birds in general. Purple sandpipers therefore appear to simply take this behaviour even further than the other members of the genus.
Timing and duration of primary moult in three populations of Purple Sandpipers
Calidris maritima were described and discussed in relation to the birds’ need to complete moult before the onset of winter, when resources are required for survival. Researchers predicted that moult would be completed earlier by birds wintering at higher latitudes. The south Norwegian breeding population, which moults and winters along the coast of east Britain (54–57°N) had a mean starting date of 21 July for primary moult (16 July for females and 24 July for males), a mean duration of 61 days, and completed on 20 September. Resident Icelandic (64–65°N) birds had a mean starting date of 22 July for primary moult (17 July for females and 25 July for males), a mean duration of 51 days, and completed on 11 September. Birds moulting in north Norway (70°N) arrived in north Norway in suspended primary moult or without having started moult, and completed it there. They had a mean completion date of 2 November for primary moult (31 October for females and 3 November for males). Starting date and duration could not be estimated because some suspended moult for an undetermined period, but it was thought that they started in late August. It is likely that most originated from Russia. The onset of moult appears to be set by the end of breeding and there is little overlap in these two events. The earlier start of moult by females in all three populations may be because they abandon the males when the chicks hatch, leaving the males to attend the chicks. Although the duration of primary moult followed the expected trend, being fastest in north Norway and slowest in Britain, the onset of moult was so late in north Norway that they had an unexpectedly late completion date, despite their rapid moult. The late completion of primary moult in north Norway suggests that wintering in the far north may not pose the energetic constraints on Purple Sandpipers that had previously been supposed.
Climate change is likely to impact on the British over-wintering population by allowing the birds to over-winter further to the North East as the risk of freezing conditions there decreases. However there may also be changes in populations as well as distributions, and there is a lot of guesswork here.
Declines in numbers by several wader species in Britain have been linked to climate change, but the mechanism for the declines has rarely been fully explored. Britain lies at the northern end of the East Atlantic Flyway, and supports 1.3 million out of the Flyway's 8.5 million coastal waders (Charadrii) in winter and the Purple Sandpiper is one of the species whose numbers have declined. Here, we examine the dynamics of the decline as observed in the Moray Firth, northeast Scotland, investigating whether the decline was due to poorer apparent survival (return rate) or poorer recruitment of young birds. The maximum number in the Moray Firth declined from 860 in 1987/88 to 236 in 2006/07, with some increase during winters 2007/08 and 2008/09. At the three main high-tide roosts (Balintore, Lossiemouth and Buckie) the maximum combined number declined from 574 to 90. Changes in survival and recruitment (percentage of first-year birds) were examined at these roosts from captured samples, which were ringed and recaptured. There were no significant changes between winters in survival rates, nor were there differences between the survival rates of age groups (first-year and adult) or bill size groups, which represented birds of different sex and breeding origin. Annual survival estimates for the three roosts ranged from 72 to 77%. The percentage of first-year birds varied among roosts and years; the lowest values were during the late 1980s/early 1990s and early 2000s. A free-running population model incorporating varying percentages of first-year birds and constant mortality for each roost provided a plausible explanation for the decline. Although modelled numbers followed the observed pattern, a discrepancy in one year was carried forward in subsequent years, so that the fit with the observed numbers was parallel rather than similar. However, it seems that the decline in numbers was largely due to poorer recruitment. We discuss whether breeding success had declined, whether the population had responded to changes in the local sewage treatment systems, which could affect invertebrate food for Purple Sandpipers, or whether fewer birds chose to winter in Scotland. The Moray Firth population is derived from Norway and possibly Canada, and there is evidence that the Norwegian population was disproportionately affected. The reason for poor recruitment requires further study, and other wader species require examination to test if poor recruitment is a common feature of decline in numbers.
