Responses of house mice to the removal of mammalian predators and competitors

Efforts to eradicate multiple mammal pests from offshore islands and fenced mainland ‘habitat islands’ often fail to remove mice, and such failures can result in a dramatic change in the food‐web whereby the removal of larger mammal pests facilitates a population explosion of mice through predator and competitor release. We investigated the ecological responses of house mice to the removal of mammalian predators from a 500‐ha fenced sanctuary at Tawharanui, northern New Zealand. Data on population structure and body condition of mice trapped in 2007, in four habitat types within the sanctuary, were compared with baseline data collected in 2001, before mammal control operations commenced. We hypothesized that: (i) in the absence of mammalian predators mouse densities would increase in all habitat types that provide vegetation cover, and (ii) in the absence of mammalian competitors mice would become heavier due to greater access to food resources. Mouse densities were significantly higher in 2007 than in 2001 in three habitat types. The high density of mice in forest – where none were trapped prior to control – suggests a competitive release, in which mice profited from the removal of ship rats. No mice were caught in the presence of ship rats on a forest trap‐line at a control site outside the sanctuary. Mice trapped in 2007 were significantly heavier than those trapped in 2001, and significantly heavier than mice trapped at the control site. Greater access to food in the absence of competing and predatory mammals probably explains the heavier body weight of Tawharanui mice. There has been a significant change in the mammalian food‐web at Tawharanui, such that the house mouse is now the primary pest. A rapid and dramatic increase in mouse numbers is likely to adversely impact invertebrates and seedling recruitment, which in turn could affect ecosystem functions.     

Herbivore feeding preferences in captive and wild populations

Investigations into the mechanisms underpinning plant selection by herbivores have often yielded conflicting results. Such inconsistency might stem from whether selection experiments are conducted with captive or wild populations, and upon the different measures of plant selection used to determine herbivore preference. Here we compared the feeding preferences of captive and wild kangaroos (Macropus fuliginosus) using a standard set of plant species (14 Hakea spp., Proteaceae) and several measures of herbivore selection to examine how environment influenced relative consumption. Three indices of herbivore consumption were measured: number of plants (NP), total shoot volume per plant (TV) and percentage available shoot volume (PV) consumed. NP and TV were closely correlated in the wild and captive populations and consistently correlated with six morphological and chemical plant attributes examined, the most notable being a strong negative correlation with shoot phenolic content. This uniformity suggests that plant selection by captive kangaroos is broadly consistent with that observed in field trials, and consequently that for macropods, at least, captive trials offer a valid way to determine the relative acceptability of different plant species. However, the fact that our third measure of herbivore selection PV was weakly correlated in captive and wild populations and showed no relationship with shoot phenolic content highlights the importance of which measure of plant selection is applied. We suggest that, while NP and TV are potentially confounded by plant size and availability, they offer the clearest insight into plant selection from the point of view of the herbivore, while PV is more suitable for plant‐centred studies.     

Diet specificity is not associated with increased reproductive performance of Golden Eagles Aquila chrysaetos in Western Scotland

Amongst raptor species, individuals with specialized diets are commonly observed to have higher reproductive output than those with general diets. A suggested cause is that foraging efficiency benefits accrue to diet specialists. This diet specificity hypothesis thus predicts that diet breadth and reproductive success should be inversely related within species. We highlight, however, that a prey availability hypothesis also makes the same prediction in some circumstances. Hence, when high diet specificity results from high encounter rates with an abundant, preferred prey, then prey availability may affect reproductive success, with diet specialization as an incidental correlate. Using three insular study areas in western Scotland, we examine diet specificity and reproductive success in Golden Eagles Aquila chrysaetos. Diet breadth and breeding productivity were not negatively related in any of our study areas, even though birds with specific diets did tend to have a higher incidence of preferred prey (grouse and lagomorphs) in the diet. Indeed, in two study areas there was evidence that diet generalists had higher breeding productivity. Our results therefore failed to support the diet specificity hypothesis but were consistent with the prey availability hypothesis. We highlight that although many other studies are superficially consistent with the diet specificity hypothesis, our study is not alone in failing to provide support and that the hypothesis does not provide a generic explanation for all relevant results. Diet specificity in predators can be at least partially a response to prey diversity, availability and distribution, and benefits associated with different prey types, so that being a generalist is not necessarily intrinsically disadvantageous. We suggest that the available evidence is more consistent with variation in prey abundance and availability as a more influential factor explaining spatial and temporal variation in breeding productivity of ‘generalist’ species such as the Golden Eagle. Under this argument, prey abundance and availability are the main drivers of variation in reproductive output. Diet specificity is a consequence of variation in prey availability, rather than a substantial cause of variation in reproductive success.     

Variation in the abundance of Red-breasted Mergansers Mergus serrator on a Scottish river in relation to season, year, river hydrography, salmon density and spring culling

Red-breasted Mergansers Mergus serrator were counted in the river North Esk, Scotland, and on the sea nearby, 1987–1990. Pairs arrived at the river estuary from early winter, but the main influx to freshwater took place in late April and in May, when breeding pairs dispersed far upriver. Females began incubation from late May. Most young hatched in July and fledged by late September. Males left the river in June and congregated at an offshore moulting site, their numbers peaking in August, and dispersed rapidly in September.
Breeding density and total duckling production decreased with increasing distance upstream, decreasing river width and increasing gradient and elevation. The total number of breeding pairs and their distribution on the river were similar from year to year, despite variable numbers killed, suggesting a stable breeding population near the upper limit the habitat would support in those years. It also suggested that killing mergansers in April was an ineffective way to reduce the population. The spatial variation in merganser breeding density was not correlated with the density of their main spring food, parr of salmon Salmo salar , but could have been related to its availability.
The production of well-grown ducklings varied annually and was inversely correlated with river flow during the main period of hatch. It is argued that Red-breasted Mergansers breed late in the year because the hatching of ducklings in July coincides with an abundance of their food, large aquatic invertebrates and tiny juvenile fish.     

Production and costs of nesting attempts in Snow Buntings Plectrophenax nivalis: why do they attempt second broods?

During 1988–1993, pairs of Snow Buntings Plectrophenax nivalis on our study sites in northeast Scotland reared an average of 1.2 broods. Clutch sizes of first and second broods were similar, but partial losses were greater in second broods, leading to a difference of at least 40% in overall nest productivity between successful first and second broods. Over and above this, total nest failure was four times higher in the second broods, and autumn sightings of ringed nestlings from second broods were only a third of those of first-brood nestlings. As a result, second broods produced a mere 10% of future recruits to the breeding population. However, there was also little evidence of costs associated with producing, or attempting to produce, second broods. Young from first broods were not less likely to reach independence if a second brood was attempted, between-year return rates of double-brooded adults were similar to those of single-brooded birds and double-brooded adults did not appear to be less fecund in the following summer. Therefore low costs to the adults of attempting second broods may allow the continuation of a strategy which appears to have only a marginal influence on overall reproductive success in the area studied.     

Origins of irruptive migrations by Common Crossbills Loxia curvirostra into northwestern Europe revealed by stable isotope analysis

We used analyses of stable hydrogen isotope (δ²H) measurements in Common Crossbill feathers (δ²H_f) to infer the region of origin of Crossbills collected from different irruptions into Britain, Iceland and the Faeroes, comparing these values with those from birds sampled in breeding areas in Britain and elsewhere in the western Palaearctic. No differences in δ²H_f values were found between different species or sexes of Crossbills that could be presumed to have grown their feathers in the same region, but juveniles had lower δ²H_f values than adults that had grown their feathers in the same region. On the basis mainly of museum skins, immigrant birds were sampled from 30 different irruption years, spanning the period 1866–2009, with annual samples varying from one to 29 individuals. The variation in δ²H_f values within irruptions was substantially less than the variation between irruptions, indicating that irruptions in different years originated in different parts of the western Palaearctic boreal zone. Birds with lower δ²H_f values tended to arrive later in the migration season, which was consistent with the idea that they had travelled further. In 17 of the irruption years, the birds had mean δ²H_f values more than ?120‰, suggesting that they had originated somewhere in the region extending from northern Scandinavia to northwestern Russia. In these years the birds arrived early, in June and July. In 10 of the irruption years, the mean δ²H_f values were between ?120 and ?130‰, suggesting origins further east, in northern Russia, east of Archangel (about 40°E). In only three of the 30 years (1898, 2002, 2009) the mean δ²H_f values were even lower (< 130‰), and these birds arrived in late July, August and September. Birds in these three irruptions had probably come from Siberia, east of the Ural Mountains. In at least three irruption years (1898, 1927, 1985) the observed range of δ²H_f values suggested that birds had come from more than one of these regions, including east of the Urals in 1898 and 1927.     

The diet of Grey Herons Ardea cinerea breeding at Loch Leven, Scotland, and the importance of their predation on ducklings

Regurgitations from nestling Grey Herons Ardea cinerea at Loch Leven, Scotland, April-July 1981–83, contained mainly three types of prey:perch, brown trout and ducklings. The diet varied markedly through the season but not between years. As the heron breeding season progressed, perch occurred in fewer regurgitations and ducklings in more. There was no seasonal variation in the occurrence of brown trout. Regular collections of duckling down from the heronry suggested that the consumption of ducklings peaked in early June. The ducklings taken by herons were less than 10 days old, Mallard ducklings predominating before mid June and Tufted ducklings later. Most ducklings were taken by only a few herons:those that bred earliest and which initially fed their young on perch. Ducklings became a major part of their diet in the late nestling period and broods fed on ducklings fledged no more or fewer young than others. Herons feeding their young on brown trout took ducklings rarely, if ever. It is argued that variation in the contents of regurgitations resulted from three interacting variables; the type of feeding habitat used by individual Grey Herons, the date at which they bred and the date that regurgitations were produced by their nestlings. The numbers of ducklings taken by herons were calculated to be about 230 in 1981 and 291 in 1982, about 5% of Mallard and 3:b of Tufted ducklings estimated to have hatched annually. Assuming herons continued to consume ducklings at the same rate after their young had dispersed, the figures for Tufted ducklings would be higher but still only about 4% in 1981 and 6% in 1982. Compared with total duckling losses of over 75%, predation by Grey Herons was minor and did not affect duckling production in the years concerned.