Ian Newton

Lifetime reproduction in birds

Thanks to long-term studies of marked individuals, information is now available on the lifetime reproduction of many bird species. This book brings together results from most of these studies and presents much new information and many new analyses of classic data sets.

Habitat and landscape factors influencing the presence of individual breeding bird species in woodland fragments

Bird species distributions in 151 woods (0.02-30 ha) in a lowland arable landscape in eastern England were investigated using logistic regression analysis. Of 31 species included in the study, only Marsh Tit Parus palustris, Nightingale Luscinia megarhynchos and Chiffchaff Phylloscopus collybita failed to breed in woods of <0.5 ha. For many woodland species, the probability of breeding was positively related to woodland area and other variables decribing the woods themselves. For other species, including Blackbird Turdus merula and Sparrowhawk Accipiter nisus, woodland perimeter, rather than area, was significant. Variables describing the landscape surrounding the woods were important for both woodland species and those more typical of open country. The length of hedgerow in the surrounding landscape was positively related to the breeding presence in particular woods of Long-tailed Tit Aegithalos caudatus, Robin Erithacus rubecula, Bullfinch Pyrrhula pyrrhula, Garden Warbler Sylvia borin and Blackcap Sylvia atricapilla, and the area of surrounding woodland was important for Long-tailed Tit, Great-spotted Woodpecker Dendrocopos major and Chaffinch Fringilla coelebs. The presence of Treecreeper Certhia familiaris was negatively related to the distance to the nearest wood, whereas that of Tree Sparrow Passer montanus was negatively related to the amount of woodland in the surrounding landscape. Thus species breeding distributions were influenced by factors associated with the wider landscape, on a scale larger than that of their immediate habitats.

EXPERIMENTS ON THE LIMITATION OF BIRD NUMBERS BY TERRITORIAL BEHAVIOUR

(1) This paper examines removal and other experimental studies on the role of territorial behaviour in the limitation of bird densities. Experimental design is discussed, as are the types of conclusions that can be drawn.

Conservation: top predators and biodiversity.

The charisma of top vertebrate predators is often used by conservationists as a lever for financial support, to raise environmental awareness and in planning protected areas — a strategy that has been criticized. Here we use information collected from five raptor species that differ widely in their diet and habitat associations to show that sites occupied by these predators are consistently associated with high biodiversity. The biodiversity at these sites is more extensive than it is at sites selected at random, or at sites occupied by species from lower down the trophic pyramid (insectivorous or herbivorous species, for example). Our results indicate that conservation focusing on top predators can be ecologically justified because it delivers broader biodiversity benefits.

Factors influencing bird species numbers in small woods in south-east England

151 woods, ranging in size from 0.02 to 30 ha and with differing degrees of isolation, were surveyed to determine the numbers of breeding bird species present in each of three consecutive years. Woodland and edge species were analysed separately. Numbers of woodland species showed a strong relationship with woodland area which explained c. 70% of the variation in breeding species numbers. Several other features of the woods and of the surrounding landscape also had small additional influences on species numbers. Numbers of edge species showed a poor relationship with woodland area, but length of perimeter explained 30-40% of the variation in species numbers. The number of structural features within a wood and isolation from other woodland also had small additional positive influences on edge species numbers. Small woods held more edge species than equivalent areas of large woods, but tended to hold fewer woodland species than equivalent areas of large woods.

Rodenticides in British barn owls.

Out of 145 Barn Owls found dead through accidents (66%), starvation (32%), shooting (2%) and poisoning (<1%), 10% contained residues of rodenticides, difenacoum or brodifacoum, in their livers. Difenacoum was in the range 0.005-0.106 microg g(-1) fresh weight, and brodifacoum was in the range 0.019-0.515 microg g(-1). Minimum levels of detection were about 0.005 microg g(-1) for both chemicals. Mice fed for 1 day on food containing difenacoum and brodifacoum died after 2-11 days. Within these mice residues were present at greater concentration in the liver than in the rest of the carcass. The mean mass of residue in a whole 35g mouse was estimated at 10.17 microg (range 4.73-20.65 microg) for difenacoum and 15.36 microg (range 8.07-26.55) for brodifacoum. Such poisoned mice were fed to Barn Owls for successive periods of 1, 3 and 6 days. All six owls fed on difenacoum-dosed mice survived all three treatments, in which up to an estimated 101.7 microg of difenacoum was consumed, and the coagulation times of their blood returned to near normal in less than 5-23 days. Four of the six owls fed on brodifacoum-dosed mice died 6-17 days after the 1-day treatment, but the survivors also survived the 3-day and 6-day treatments. Those that died had each eaten 3 mice, with a combined weight of about 105g and a total brodifacoum content of about 46.07 microg, which was equivalent to a dose of 0.150-0.182 mg kg(-1) of owl body weight. After death these owls had 0.63-1.25 micro g(-1) of brodifacoum in their livers. Blood from the survivors would not coagulate at 9 days post-treatment, but did so at 16 days in one bird and between 38 and 78 days in the other. It is concluded that: (1) Barn Owls in Britain are now widely exposed to second-generation rodenticides; (2) not all owls exposed to these chemicals are likely to receive a lethal dose; (3) brodifacoum is more toxic to owls than difenacoum; and (4) while there is yet no evidence that rodenticides have had any appreciable effect on Barn Owl populations in Britain, further monitoring of residue levels and population trends in desirable.

Lead concentrations in birds of prey in Britain

This paper reports on lead (Pb) concentrations in the livers of 424 individuals of 16 raptor species found dead and sent for analysis to the Institute of Terrestrial Ecology, Monkswood, from the early 1980s to the early 1990s. Elevated Pb concentrations in liver (>20 ppm dry wt), within the range associated with Pb poisoning mortality in raptors, were recorded in one peregrine (4% of species sample) and one buzzard (2% of species sample). These birds are likely to have ingested lead gunshot in the flesh of their prey. Another one each of these species had liver Pb concentrations of 15-20 ppm dry wt, reflecting unusually high absorption of Pb. No individuals of any other species had >15 ppm dry wt liver Pb, although some had 6-15 ppm. The source of Pb in these birds was unknown, but it could have resulted from high Pb concentrations in prey items, including some containing lead shot. Median liver Pb concentrations were generally very low (ranging from <0.07 to 1.61 ppm dry wt for species with sample sizes exceeding 10). In sparrowhawks, for which a large sample was available, liver Pb concentrations in all but one individual were low (<2.6 ppm dry wt). Pb concentrations in juveniles were significantly lower than in adults, and were lowest in recently fledged birds.

Habitat loss and raptor predation: disentangling long- and short-term causes of red grouse declines.

The number of red grouse (Lagopus lagopus scoticus) shot in the UK has declined by 50% during the 20th century. This decline has coincided with reductions in the area of suitable habitat and recoveries in the populations of some avian predators. Here we use long–term records of shooting bags and a large–scale manipulation of raptor density to disentangle the effects of habitat loss and raptor predation on grouse populations. The numbers of grouse harvested on the Eskdale half of Langholm Moor in southern Scotland declined significantly during 1913–1990 and grouse bags from the whole moor from 1950 to 1990 exhibited an almost identical but non–significant trend. Hen harriers (Circus cyaneus) and peregrine falcons (Falco peregrinus) were absent or bred at low densities on this moor throughout this period but heather–dominant vegetation declined by 48% between 1948 and 1988. Harrier and peregrine breeding numbers on Langholm Moor increased to high levels following protection in 1990 whilst grouse density and grouse bags declined year after year until shooting was abandoned in 1998. The prediction of a peak in grouse bags on Langholm Moor in 1996 based on the patterns of bags during 1950–1990 was supported by the observed peaks in 1997 on two nearby moors with few raptors which formerly cycled in synchrony with Langholm Moor. This study demonstrates that, whilst long–term declines in grouse bags were most probably due to habitat loss, high levels of raptor predation subsequently limited the grouse population and suppressed a cycle. This study thus offers support to theoretical models which predict that generalist predators may suppress cycles in prey populations.

Age-dependent diet choice in an avian top predator

Age-dependent breeding performance is arguably one of the best-documented phenomena in ornithology. The existence of age-related trends has major implications for life-history theory, but the proximate reasons for these patterns remain poorly understood. It has been proposed that poor breeding performance of young individuals might reflect lack of foraging skills. We investigated this possibility in a medium-sized, powerful raptor—the northern goshawk Accipiter gentilis. Male goshawks are responsible for providing their females and their offspring with food. We hypothesized that young males may generally show poor breeding performance or even delay breeding, because they lack the experience to hunt efficiently—especially, their principal avian prey, the feral pigeon Columba livia. Our study exploited a rare ‘natural experiment’, the expansion phase of an urban population, where intraspecific interference was negligible and many young males bred successfully. This enabled us to examine the improvement of foraging skills in a larger sample of young individuals, and in more controlled conditions than usually possible. Using data from individually identified male breeders, we show that, consistent with our hypothesis, the proportion of pigeons in the diet increased significantly with male age, for at least the first three years of life. Other studies have shown a parallel increase in productivity, and a positive effect of a pigeon-rich diet on brood size and nestling condition, stressing the potential fitness relevance of this prey species for goshawks. Our results suggest a causal link between patterns of age-dependence in foraging ecology and reproductive performance. Furthermore, our study is, to our knowledge, the first demonstration that prey choice of breeders, which might reflect individual hunting skills, is age-dependent in a raptor.

Influences of population size and woodland area on bird species distributions in small woods

Distributions of individual bird species in 151 small woods (size range 0.02–30 ha) were investigated in 3 consecutive years during which the abundance of certain species varied markedly. Relationships between the probabilities of certain bird species breeding and woodland area were described using incidence functions derived from logistic regression analysis. In general, for species which were largely dependent on woodland and seldom occurred in other habitats (such as hedgerows and gardens), the probability of breeding approached 100% only for woods of 10 ha and more, whereas species with less stringent habitat requirements occurred in the majority of woods, including those of 1 ha and less. The sensitivity of incidence functions to changes in regional abundance and the size distribution of the study woods was examined. For some species, distribution patterns could not be distinguished from those expected if pairs had been distributed in proportion to woodland area (random placement), but the majority did not conform to random placement in at least 1 of the 3 years. This nonconformity was consistent across all 3 years for some species, such as wren (Troglodytes troglodytes), despite substantial fluctuations in population sizes between years, while for others, such as robin (Erithacus rubecula), distribution patterns changed with changes in regional abundance. The results suggested that some species, such as wren and blackbird (Turdus merula), preferred small woods, while other species, such as chiffchaff (Phylloscopus collybita), preferred large woods. For several other species, including robin, great tit (Parus major), long-tailed tit (Aegithalos caudatus) and marsh tit (P. palustris), small woods appeared to be sub-optimal under at least some conditions.