The population dynamics of the Eider Duck Somateria mollissima and evidence of extensive non-breeding by adult ducks

The dynamics of an Eider Duck population have been investigated over 25 years, using census and capture-mark-recapture methods. During the study the population increased two and a half fold, with two periods of major increases in numbers, giving a stepped growth pattern. Mean clutch size showed considerable annual variation, the extremes being 5.40 and 3.78 eggs. The variation in clutch size was greater than that recorded in the Netherlands. Adult female Eiders had a high annual survival rate, averaging 0.895, and varying between 0.75 and 1.00 in individual years. The survival rate decreased markedly in the old ducks. There was no indication of any change in the survival rate during the study. Recruitment of ducks to the breeding group was irregular, with most years showing little recruitment and a few years high recruitment. However, recruitment, associated with good duckling survival, appears to have been the main factor associated with increase in the population. In many years, an appreciable proportion of the surviving ducks, which had already bred in a previous year, failed to nest. The extent of non-breeding increased during the study and in one year, 1973, this reached 65%. Lower clutch size and adult survival were associated with years of high non-breeding. The ‘red-tides’ in 1968 and 1975 appeared to have little effect on the Eider. It is suggested that the Eider missed breeding in years in which its survival was potentially poor, in order to maximize its reproductive output during its life span. This is supported by the smaller clutch size laid by those females which nest in years when many females fail to breed. It is suggested that young ducks may also miss breeding in the year after first nesting but this is not associated with the non-breeding in older ducks, although it may he related to body condition. It is suggested that non-breeding by adults of long-lived birds may be widespread. This has important implications in survey work based on nest counts.     

Nonbreeding and nests without eggs in the Lesser Black-backed Gull Larus fuscus

Estimates of the number of breeding Lesser Black-backed Gulls Larus fuscus at the Tarnbrook Fell gullery, Lancashire, have been made from annual counts of nests since 1981. During all of these surveys, the mean percentage of nests which did not contain eggs has been remarkably constant from year to year (54%± 1.9%). A study of specific nests at the colony in 1992 showed that the great majority of these "empty" nests did not subsequently receive eggs. in 1993, a study of a sample of territory-holding gulls was made on a 0.41-ha study plot to investigate and to quantify the occurrence of empty nests. Of the 62 pairs that defended a territory and constructed a complete nest, 27% subsequently failed to produce eggs. These birds attended their territory as a pair significantly less frequently than pairs that produced clutches and achieved significantly fewer apparently successful mountings. Sixty percent of pairs constructed more than one nest, and 58% of all nests built received no eggs. The construction of empty nests was not found to be related to the density of nesting gulls. Since 1981, estimates of the number of gulls in the colony have been based on the assumption that one nest represents one pair of gulls. This has been shown not to be the case, and a correction multiplier of 0.61 must be applied to counts of nests at the colony to obtain an estimate of the number of breeding gulls.     

Landscape partitioning among Triodia spp. (Poaceae) in the fire prone Kimberley,north‐west Australia

The processes which determine the structure of plant communities vary across spatial and temporal scales. Climatic factors are more likely to influence community structure at a regional scale with more transient environmental effects such as disturbance or demographic interactions having a greater influence at local scales. Understanding these differences is important for managing communities at a landscape scale. Triodia spp. grasslands are the most extensive plant community in Australia, covering 1.4 million km², and yet little is known about the processes which structure these communities. We collected data on six sympatric Triodia spp. at the regional, landscape and local scale across the 325 000 ha property, Mornington Wildlife Sanctuary, in the Kimberley region of northern Western Australia to investigate the processes which structure this community. Regionally we looked for correlations between species distributions and substrate or rainfall. At the landscape scale we collected data on substrate, drainage and vegetation type and at the local scale we determined the extent to which individuals form mono‐specific stands both along and across the contour gradient. Only one species, T. aeria, was found to be substrate specific and only T. epactia was restricted to the drier southern end of the property. The other species were not restricted by substrate or rainfall at the regional scale and were found to be habitat generalists at the landscape scale. All species grew in mono‐specific stands with little to no mixing at shared boundaries. However, this pattern broke down when crossing the contour gradient on hillsides. The results suggest rainfall may influence the distribution of some Triodia spp. at a regional scale with interspecific competition, due to differences in post‐fire regeneration niches, structuring the community at the local scale. At the landscape scale community structure appears to be influenced by feedback mechanisms involving differences in the post‐fire regeneration strategies of sympatric species and subsequent competition for establishment microsites.     

Masquerade is associated with polyphagy and larval overwintering in Lepidoptera

Masquerading animals benefit from the difficulty that predators have in differentiating them from the inedible objects, such as twigs, that they resemble. The function of masquerade has been demonstrated, but how it interacts with the life history of organisms has not yet been studied. Here, we report the use of comparative analyses to test hypotheses linking masquerade to life‐history parameters. We constructed a phylogenetic tree of the British species of the lepidoptera families Geometridae and Drepanidae, and compiled life history and coloration data from the literature. We found that masquerade is associated with the exploitation of a greater diversity of host plants whether measured by the number of families or genera. We found a positive relationship between body size and polyphagy among masquerading species, and no relationship among cryptic species. Among those species predominantly found on woody host plants, masquerading species are more likely to overwinter as larvae while cryptic species mostly overwinter as pupae. Polyphenism was associated with multivoltinism in masquerading species but not cryptic species. Taken together, our results show that masquerade must be viewed as a strategy distinct to crypsis and hence may provide insights into the evolution of both defensive strategies. Our study further demonstrates the utility of broad‐scale between‐species comparisons in studying associations between diverse life‐history parameters and sensory aspects of predator‐prey interactions. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 90–103.     

Protection by association: evidence for aposematic commensalism

Aposematism is a well known and widely used strategy for reducing predation by conspicuous signalling of unprofitability. However, the increased conspicuousness could make this strategy costly if there are no secondary defences to back the signal up. This has made the elucidation of the evolutionary mechanisms for aposematism and that of the closely‐related Batesian and Mullerian mimicry difficult. The present study aims to test whether cryptic and nondefended prey could reduce their predation risk by grouping with aposematic and defended prey. To do this, we used groups of artificial baits that were either cryptic and palatable or conspicuous and unpalatable, along with the corresponding control treatments. These were then presented in mixed and homogeneous treatment groups within a field setting and the local wild bird assemblage was allowed to select and remove baits at will. The results obtained show that undefended non‐aposematic prey can benefit by grouping with aposematic prey, with no evidence that predation rates for aposematic prey were adversely affected by this association. These results provide insights into the evolution of Batesian mimicry. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 81–89.     

Foraging guild membership explains variation in waterbird responses to the hydrological regime of an arid‐region flood‐pulse river in Namibia

1. Little is known about hydrological influences on tropical waterbird communities. We used a 16‐year data set (1991–2007) of waterbird censuses, together with a classification of observed species into foraging guilds, to explore the relationships between natural variations in flow regime, foraging guild and the community composition of waterbirds at the Okavango River in the Caprivi Strip of north‐eastern Namibia, southern Africa. 2. We addressed three hypotheses to explain variation in waterbird community composition: (i) exploitation (birds move towards resource‐rich patches to exploit periods of high food abundance); (ii) escapism (declines in regional habitat quality force birds to aggregate in perennial waterbodies); and (iii) interaction (bird assemblages are dominated by intra‐ and interspecific interactions, such as flock formation for breeding or moulting, that can be explained better by life history demands or competition than by resource availability). 3. Waterbirds in different foraging guilds responded strongly but at different periods to changes in the hydrological environment, creating a complex but predictable successional pattern in community composition through time. Deep‐water feeders responded fastest (abundance peaking 2 months post‐flood), followed by shallow‐water feeders (4 months) and emergent vegetation feeders (7 months). Species that forage on short vegetation or in mud showed a bimodal response with peaks in abundance at 3 and 8 months post‐flood. 4. Our results indicated a strong effect of the local flow regime and hence supported the exploitation hypothesis. The foraging guild approach allowed us to identify clear patterns in a highly complex ecosystem and shows considerable promise as an analytical tool for similar data sets. Our results further suggest that while the entire bird community will be affected by hydrological alterations such as impoundments, water extraction and climate change, deep‐water feeders may be one of the most vulnerable groups.