Sewage sedimentation ponds – a lost bird paradise : a nostalgic return visit and case study

Sewage ponds with exposed sludge are known to be excellent feeding sites for waders during migration. They still exist in parts of the world but are disappearing in pace with introduction of modern methods which do not involve open exposure of the sludge. Here I report a five-year study from a Swedish plant with open sludge ponds in the 1950s when many similar ones were still active. I counted the waders with frequent visits from 18 April through 2 November, 1952–1956. Ninety-three percent of the dates had a visit in at least one of the years giving an almost complete combined coverage of migration. Nineteen species were recorded at least once. The total average annual number of bird-days was 2868 (5% in spring). Most abundant were the Ruff Calidris pugnax with 35% and Wood Sandpiper Tringa glareola with 32% of all bird days. Temminck’s Stint Calidris temminckii showed the highest spring proportion of bird-days, 30%.

Having little spare time for birding when visiting a foreign city, a travelling birdwatcher knows what to inquire about.How do I find the way to the nearest garbage dump, water work or sewage plant?That's where the birds are!This is still the case in many parts of the world but no longer in wealthier countries.As economies improved things changed.Human health and sanitation as well as an environment without pollutants and excess nutrients became important.New methods of garbage and sewage treatment are now predominant, and neither garbage nor sewage is longer available as feeding stations for birds as they once were.
During the twentieth century, at least the early part, many sewage works had only the primary step of treatment, which involved open sedimentation ponds where the sludge was temporarily deposited to permit biodegradation.Excess water was released into seas, lakes or streams.The degraded and dried sludge was usually used as fertilizer and for soil improvement in adjacent farmland.In the 1950s, fifty percent of the sewage in Sweden was treated in sewage works but only with the primary step (Naturvårdsverket 2000).These sedimentation ponds became excellent habitats for waders because the biodegradation process involves an abundance ORNIS SVECICA 28: 81-86, 2018 Sewage sedimentation ponds -a lost bird paradise: a nostalgic return visit and case study Reningsverkets sedimenteringsdammar -ett förlorat fågelparadis: nostalgiskt återbesök och fallstudie SÖREN SVENSSON of many potential food species such as worms (potworms Enchytraidae, sludge worms Naididae particularly Tubifex tubifex, and earthworms Lumbricidae), insects, and mites.
There is a number of studies on birdlife in sewage treatment plants and sewage farms.An early one is about British farms by Boyd (1957), and it is worth to cite a part of his introduction: "… modernization of one after the other of these artificial marshes is little short of a calamity, necessary as it may be for the community as a whole.It is because future ornithologists, not fortunate enough to have known these farms in their glory during the last 50 years, may fail to understand their nature and condition, and their fascination for bird and man, that these notes are written."I am writing in the same spirit as it was at about the same time that modernization of the sewage plants began in Sweden.When I started to watch birds in the early 1950s, I was fortunate enough to live only minutes from such a plant.The sewage ponds became one of my most frequented haunts, and during five years I made almost daily counts of the waders during the migration seasons.Many younger bird-watchers will never experience the combined stench from degrading sewage and the wealth of waders feeding on its surface.Hence, it is worthwhile to present this summary from one https://doi.org/10.34080/os.v28.19535 of many similar sites, although the counts were made sixty years ago.

Study area
The study area was the sewage treatment works at the municipality of Hallsberg, Örebro county, southern Sweden (approximate centre of the study area: 59°04'15"N; 15°07'30"E).It was delimited by railways in west and south, farmland in the north, and leather industries including a tannery in the east.About ten rectangular ponds were used for the sludge treatment before the water was released into the Rala stream.The area covered by the ponds and some adjacent waterlogged grassland was about seven hectares.One or two large pond received the fresh sludge and other ponds contained successively drier sludge.

Method
The site was carefully surveyed by covering the whole area on foot.Only waders were counted (all species in this report belong to the families Charadriidae and Scolopacidae).The waders were easy to count either when still resting or feeding on the ground or when flushed.Binoculars were used as necessary.The area was small, and all parts were easily accessible and I am certain that no waders that were present could have been missed.Only minor counting errors may have occurred.Records were entered in a notebook at site.

Temporal coverage
The site was surveyed on a variable number of days in each of the five years 1952-1956 (Figure 1, Table 1).The earliest visit was on 22 April and the latest on 2 November, a period of 195 days.The number of years with a visit on a specific date was: 31 (16%) of the dates had a visit in all five years, 30 (15%) in four, 39 (20%) in three, 40 (21%) in two, and 42 (21%) in one year.Only 13 (7%) days were not visited in any of the years.Coverage in the different months was 35% during 22 April-31 May, 27% in June, 65% in July, 86% in August, 80% in September, and 30% during 1 October-2 November.In the different years the coverage was 47%, 64%, 58%, 43%, and 52% of the possible 195 days each year.In total, 514 visits were payed which is 53% of the total number of 975 possible days.But thanks to the fact that the visits were paid on different days in different years, as much as 93% of the dates had a count in at least one of the years.Table 1.Number of visits in different months and years during the period 22 April-2 November.The percentages are the proportion of days with a visit out of the total number of days in the period (195).Hence, 93% of the days had a visit in at least one of the five years.The same data are shown in Figure 1.Antal besök olika månader och år under perioden 22 april-2 november.Procentvärdena är andelen dagar med besök av de totalt möjliga under perioden (195).Således hade 93% av dagarna besök under minst ett av de fem åren.Samma data visas i Figur 1. Visits were payed at different hours, usually in the early morning or in the evening, but visits were also payed about noon or at any other hour except when dark.Almost all counts were made by myself, but on a few days two friends of mine helped to make the count.

Results
Nineteen species were recorded on at least one day during all five years.In each of the years, the number of species was 16, 16, 16, 17, and 19.The maximum number of species in a single day was 10, 10, 11, 11, and 11, and the maximum number of individuals in a single day was 65, 121, 91, 73, and 60 in 1952-1956, respectively.The average number of species on each date during all five years is given in Figure 2 (calculated with only days with a visit included).The maximum value was four species in spring and eight species in autumn.There was a period between 12 July and 30 August when an average of more than five species was recorded almost every date.
The average number of individuals (also with only days with a visit included; Figure 2) demonstrated a similar pattern with mean counts of more than 40 waders on most of the dates between 22 July and 27 August.Peak average was 64 waders on 25 August.The spring and autumn migration periods were well separated with few birds during the first two weeks of June.Apart from very few visitors of other species it was the resident Little Ringed Plover Charadrius dubius that was regularly observed in early June.
Table 2 provides a summary of the number of "bird-days" for all species, also separately for spring and autumn.The number of bird-days in spring was only six percent of that in the autumn.In terms of total number of bird-days, two species stand out, Ruff Calidris pugnax with 35% and Wood Sandpiper Tringa glareola with 32% of all bird-days, together with as much as 67%.Third position is taken by the Common Sandpiper Tringa hypoleucos, but with only 6% of all bird-days.
Figure 3 provides three examples of daily mean counts of individual species.Two of these species are the two most numerous ones, the Ruff and the Wood Sandpiper.Having somewhat different migration periods, the two-peak pattern of the mean occurrence of all species (Figure 2) is caused by this difference.The third example is Temminck's Stint.This species stands out in being a rather prominent spring visitor compared to its autumn numbers.The number of spring bird-days was 30%.Only the Wood Sandpiper came close to this proportion (20%).

Discussion
In several countries there are a few publications about the birds of sewage treatment plants or sewage farms.Another early one, apart from that of Boyd (1957) cited in the introduction, is that from Cambridge by Nisbet (1957).Another more recent study has been made by Anthes et al. (2002) at Münster.In this case the sewage ponds are no longer used for their original purpose but has been turned into a nature reserve with many open water ponds.A more extensive literature search may have yielded more examples.
When searching for information about wader migration at sewage works in Sweden, I was surprised about finding no compilation of bird records.And this in spite of the fact that numerous bird-watchers must have spent much time at such sites.Consequently, for most former sewage works it is more or less impossible to reconstruct their importance as feeding sites for waders.The records are probably widely spread in different bird reports, most often as rather anecdotal accounts of unusually large numbers or rare species, or lost altogether.
One example for which some notes exist is the sewage works at Örebro where a new treatment plant was established in the 1950s bordering an attractive site for birding called Oset, the mouth area of Svartån into lake Hjälmaren, an area that was made famous by a book by Rosenberg (1934).At the site there was also a garbage dump and fields of ruderal vegetation, all providing food and shelter for birds all year round.Hence, birdwatchers visited the area frequently, and more ambitious surveys were made at least once a week.Källander (1959) writes for the first time about the waders visiting the new sludge ponds, and later he (Källander 1961, p. 336) presented a summary table of wader counts at "Oset" in the autumn season of 1960; he gives the highest count of each species every autumn week.The bird counts at Oset included walks through grassland, wetland, reeds and a stretch of lakeshore and not only the sludge ponds.However, Hans Källander (in mail) recalls that most of the wader records presented in this particular table must have come from the sludge ponds.With this assumption it is possible to make a simple comparison of the Hallsberg and Örebro ponds.I used the maximum count on any day at Örebro and the five-year average of the maximum counts at Hallsberg for the same weeks.The total of all species maxima was about two times higher at the Örebro ponds (387 vs. 198 at Hallsberg).The two species with highest maxima were the same at both sites (Ruff with peak numbers of 75 vs.78 and Wood Sandpiper with 63 vs. 55 individuals).But the other species were mostly more abundant at Örebro, particularly Common Ringed Plover (26 vs. 2), Common Snipe (48 vs. 10), Little Stint (32 vs. 3) and Dunlin (50 vs. 5).As there are some uncertainties involved in the Oset counts (only the sludge ponds or also other parts included and the absence of daily counts), it is possible that my study at Hallsberg is the only careful one of sewage sedimentation ponds in all Sweden.

Figure 1 .
Figure 1.Number of years (maximum all five) with a visit on different dates between 22 April and 2 November.Grey bars represent dates without a visit in any of the five years.Date is given in the form month and day.Antal år (maximalt alla fem) med besök på olika datum mellan 22 april och 2 november.Grå staplar anger dagar utan besök något av de fem åren.Datum anges som månad och dag.

Figure 3 .
Figure 3. Three examples of mean counts of individuals on different dates: the two most common species, Ruff and Wood Sandpiper, and the only species with comparatively strong occurrence also in spring, Temminck's Stint.Tre exempel på enskilda arters antal olika datum: de två vanligast arterna, brushane och grönbena samt den enda arten med jämförelsevis stark förekomst även på våren, mosnäppan.

Table 2 .
Average number of bird-days different species, for the whole study period and separately for spring and autumn.Medeltal fågeldagar för olika arter, för hela undersökningsperioden och separat för vår och host.