VOL. 13 (8) DECEMBER 1990 243 AUSTRALIAN BIRD WATCHER 1990, 13, 243-248 The Status of Ducks (Anatidae) in the Darwin Area, Northern Territory, 1974-1986 by K.S. SHURCLIFF1 and J.L. McKEAN2 1 11 Queens Road, Railway Estate, Townsville, Queensland 4810 2 1 Avondale Street, Hampton, Victoria 3188 Summary Twelve species of Anatidae were recorded during the study period 1974-1986 in the Darwin area, Northern Territory. Five species (Magpie Goose Anseranas semipalmata, Radjah Shelduck Tadorna radjah, Wandering Whistling-Duck Dendrocygna arcuata, Pacific Black Duck Anas superciliosa and Grey Teal Anas gibberifrons) showed significant and consistent seasonal patterns in their population sizes, with the first four species showing similar patterns. Radjah Shelduck and Wandering Whistling­ Duck have resident populations. Hardhead Aythya australis and Pink-eared Duck Malacorhynchus membranaceus did not show statistically significant seasonal patterns. Plumed Whistling-Duck Dendrocygna eytoni, Green Pygmy-Goose Nettapus pulchellus and Garganey Anas querquedula occurred irregularly, with levels too low to test for significance. Black Swan Cygnus atratus and Blue-billed Duck Oxyura australis were recorded only once; the latter was the first record for the Top End region. Population numbers showed considerable variation from year to year, however these variations were not correlated with the local rainfall patterns. Pink-eared Duck and Hardhead were first recorded in 1978 and 1976 respectively. Magpie Goose has shown a steady decline in numbers, which may perhaps be cyclical. Breeding records were obtained for three species, and all occurred during the late wet season (February-May). Introduction This is the sixth of a series of papers summarising data on birds in the vicinity of the Sanderson Sewage Ponds, Darwin, Northern Territory, from 1974 to 1986. The objective of the counts was to determine monthly and yearly variations of bird populations in the study area. The study area was chosen because of its richness of habitat types within a small accessible area, located within the rdatively unstudied wet-dry northern tropics (e.g. Ridpath 1985). The habitats include intertidal beach, sewage ponds, mangrove and coastal floodplains. Both the sewage ponds and floodplains have permanent water. Rainfall patterns (Figure 1) for the study area are very seasonal, with a dry period from May-September and a wet period from October to the following April. The months October-November (referred to as the 'build-up') and April have significantly less rainfall than the intervening months, but more than the dry-season months. Methods Six sample sites involving the four habitat types were censused at least once each month, during the years 1977-1986. From 1974 to 1976, observations were not made for 8 of the 36 months. Observations were made by several observers, however one of us (JLM) and Hilary Thompson accounted for over 80% of the observations. Counts of all individual birds seen from vantage points were made at each visit. The time period spent on each count varied. The data from these counts were used to determine monthly population changes, as illustrated by a series of graphs. The monthly values were determined by using that count with the maximum number of individuals seen at a sample site. This value was used in an effort to reduce effects which might be attributable to different observers or to different numbers of counts per month. These maximum counts were then summed over the six sites. AUSTRALIAN VOL. 13 (8) 244 SHURCLIFF & McKEAN BIRD WATCHER DECEMBER 1990 Status of Ducks, Darwin Area, 1974-1986 245 500 Table 1 400 Total counts of waterfowl in the Darwin area (N.T.) for the years 1977-1986. ~ Species 1977 ~ 300 1978 1979 1980 1981 1982 1983 1984 1985 1986 Mean Std Dev. z <( Magpie Goose 11882 9059 12853 5665 2762 2559 2238 4554 2277 1423 5527 4027 UJ 200 Wandering Whistling Duck 11510 ~ 16900 13193 14579 6936 6524 13044 6881 5350 11354 11027 4031 Radjah Shelduck 4626 6239 6081 2856 3040 7718 3012 2966 1860 903 3930 2048 100 Pacific Black Duck 5459 6451 7547 5698 2209 4984 4032 5007 1118 162 4267 2261 Grey Teal 49 3061 4459 5186 1177 2167 718 1109 3525 689 2214 1669 0 Pink-eared Duck 0 142 139 968 175 66 6 34 538 II 208 295 F M A M J J A s 0 N D Hardhead 9 92 1152 131 2 399 108 409 512 297 27 432 434 Green Pygmy-Goose 2 4 18 59 8 6 II 6 0 10 12 16 Figure 1. Mean monthly rainfall at Darwin (N.T.) for period 1941-1986. To test whether the population samples were significantly different from month years 1974-80 and 1983. The general lack of broods in recent years may result from to month, the data for the ten-year period 1977-1986 were tested by the non-parametric the colonisation of the ponds by Saltwater Crocodiles Crocodylus porosus. Friedmann two-way analysis of variance. The monthly figures are ranked within a given year from lowest to highest. The test, which is based upon the chi-square Radjah Shelduck Tadorna radjah distribution, determines whether the rank totals for the months differ significantly. The numbers for this species usually follow the same pattern as the last species For each species, a graph is given of the mean counts and sum of ranks for each month. (increasing before the onset of the wet season, followed by a rapid decrease; Figure Species are grouped according to whether seasonal changes in abundance were 2a) . There are regular occurrences throughout the wet and early dry season, indicating significant (Friedmann test, p(O.Ol, ll df) or not. that there is a substantial resident population, which is added to by birds which use the area as a dry-season refuge. This is the only duck species that has been recorded Results in every month of every sampled year. Our maximum count was 3500 birds in November 1982 and above-average years were 1977-1979 and 1982. Species showing significant seasonal changes Newly hatched broods were noted on the sewage ponds during February-July in the years 1974-1979, 1982-1984 and 1986. Birds are often seen moving between saltwater Magpie Goose Anseranas semipalmata habitats and freshwater habitats. Thus, numbers recorded during individual counts This species is clearly more common during the months of September to January were probably influenced by tidal conditions. (Figure 2a). It is virtually absent during any other time. The highest numbers correspond to the 'build-up' season, just before the commencement of heavy sustained Pacific Black Duck Anas superciliosa rainfall during the wet. A maximum number of7950 birds was recorded in November This species used the area as a dry-season refuge, with numbers peaking in October 1977. Exceptionally high concentrations occurred during the years 1976 to 1979 inclusive. and decreasing markedly through the wetter months with a virtual absence of the species There were no breeding records in the study area. in March and April (Figure 2a). Our maximum count was 2000 birds in July 1979. There was a sudden decline in numbers over 1979-80 and again over 1980-81 (Table Above-average years were 1976-1980, 1982 and 1984. Our only breeding record was l) . Since 1981, numbers have remained nearly constant except for an influx in December a brood of newly hatched ducklings on the sewage ponds in March 1979. 1984. During 1986 numbers were substantially lower than during any other year. This 'low' year occurred for most other Anatidae in the study area. Grey Teal Anas gibberifrons The changes in yearly populations appear to be cyclical rather than a recent trend, This species showed a slightly different pattern from the preceding species. Peak as 1974 counts showed levels similar to those of 1981-1985 (Thompson and McKean numbers were recorded in November, but September was usually the month with the pers. obs.). highest counts (Figure 2a). This is one to two months earlier than for most other species. Lowest counts were usually in February, two to three months before the lowest counts Wandering Whistling-Duck Dendrocygna arcuata for the other species. High annual totals occurred in 1978-80 and 1985. Low counts This whistling-duck is the most numerous of the duck species in the area. It uses occurred in 1976-77, 1983-84 and 1986. the area as a dry-season refuge. Numbers steadily increase over the dry season and 'build-up' season to a peak in November, and drop off sharply as the wet season Species not showing significant seasonal changes commences (Figure 2a). Few ducks occur in April and May, and there have been only 5 individual months during which no individuals were recorded, indicating a small Pink-eared Duck Malacorhynchus membranaceus resident population. The years 1977-1980 and 1983 had above-average numbers for This species has been recorded during each month of the year, but in numbers this species, although our maximum count was 6500 birds in November 1974 (Table too low and inconsistent to show any significant seasonal patterns (Figure 2a). High 1). Newly hatched broods were noted on the sewage ponds during March-May in the averages in the dry season result from very high counts during 1978, 1980 and 1985 . AUSTRALIAN 246 SHURCLIFF & McKEAN BIRD WATCHER Its numbers fluctuate widely, ranging from 0 in 41 separate months to 350 birds recorded in June 1985. This species was first recorded in the study area in 1978, but there are no patterns in the yearly totals. Hardhead Aythya australis Our counts indicate that this species can be present at any month of the year (Figure 2b), but that there are no patterns in its presence or absence.