No abstract

2000. The prevalence of non-breeders in raptor populations: evidence from rings, radio tags and transect surveys. -Oikos 91: 271 -279.Age-specific survival and breeding (ASSAB) models were developed with data from 146 common buzzards (Buteo buteo) radio-tagged in southern Britain during 1990-1998, in a 120-km 2 study area that had on average 25 egg-laying pairs. Survival checks were aided by philopatric behaviour and a maximum annual tag failure rate of 7%: minimum survival rates, that were estimated by assuming death of buzzards with lost tags, were close to maximum rates that were estimated using only the recorded deaths. First-year survival rate estimates for 35 buzzards fitted in 1990-1991 with 25 -30-g backpack radios were 69 -74% (minimum-maximum), close to the 61-71% for 16 buzzards with 12-g tail-mount radios; the backpacks transmitted for 2-4 yr. Overall survival rates were 66 -73% in the first year, 91 -97% in the second and 88-91% thereafter. Survival estimates from 288 recent British ring recoveries were lower in the first and second years, at 55% and 75%, but similar (88%) thereafter. Most deaths were from natural causes (40%) or interaction with artefacts (36%). ASSAB models, from radio-tracking and the observed 1.71 young clutch − 1 , predicted breeding by 16 -21% of all the buzzards present in spring, or up to 25% with the minimum likely productivity of 1.4 young clutch − 1 or 12% net emigration. Ringing data predicted breeding rates of 33 -38%. The models were tested with density data from nest surveys and new radio-corrected-transect and truncationmark-resighting estimates of buzzard numbers. Surveys in autumn and late winter estimated breeding rates of 21 -25%. The high non-breeder density in spring, of three other buzzards for each paired bird with eggs, has important implications for understanding evolutionary fitness, predation and population ecology.

Methods used to estimate home ranges from point locations are based either on densities of locations or on link distances between locations. The density‐based methods estimate ellipses and contours. The other class minimizes sums of link distances, along edges of polygons or to range centers or between locations. We propose a new linkage method, using nearest‐neighbor distances first to exclude outlying locations and then to define a multinuclear outlier‐exclusive range core (OEC) by cluster analysis. The assumption behind exclusion of outliers, that movements inside and outside range cores involve different activities, was supported by data from radio‐tagged Common Buzzards (Buteo buteo). We compared the new method with other techniques by using location data from each of 28 goshawks, 114 buzzards, 138 gray squirrels, and 14 red squirrels. Range structure statistics from OECs showed marked differences between species in numbers and extent of core nuclei. Range analysis displays illustrated relationships of range area with age categories, food supply, population density, and body mass within species. The OECs gave highly significant results in three of five within‐species tests, perhaps because animal movements in these cases were affected by coarse‐grained habitat boundaries. When movements were likely to have been influenced by diffuse social interactions and foraging for scattered prey, the most significant results were from density‐based estimators, especially kernel contours that had been optimized by least‐squares cross validation. We recommend use of both density and linkage estimators of home range until a basis for a priori choices has been established.

No abstract

Radio‐tags were used to track 146 Buzzards Buteo buteo during 1990–1996. Each bird was tracked for up to 4 years; of 74 Buzzards tagged since 1992, 72% were monitored for more than 3 years. Among the 87 Buzzards tracked for more than 1 year, 46% settled after one dispersal movement, 37% dispersed and then changed their ranges, 17% did not disperse and one Buzzard alternated between a summer and a winter range. Natal dispersal occurred in two waves, one in the first autumn and the second in the following spring. Initial dispersal distances in the autumn were significantly greater than those in the spring. For 73 Buzzards that dispersed in their first autumn, 96% settled within 100 km of their natal nest, and their distances were similar to 76 records from ringing. After their second spring. Buzzards rarely changed ranges and were significantly more often to the east than the west, especially those that had dispersed more than 20 km. Buzzards that had dispersed farthest were most likely to be detected returning towards their natal area each spring and returned earlier as they got older. However, none was detected returning once it had started to breed. Nine early breeders were significantly farther from their natal nests than 44 nonbreeders.

Summary 1.Information on the effects of wildlife predation on game and livestock is required to allow improved management of all organisms involved. Monitoring of prey, predators and predation mechanisms each suggests important methods, illustrated here by data from common buzzards Buteo buteo and ring-necked pheasants Phasianus colchicus . 2. Location data from 136 radio-tagged common buzzards, together with prey remains from 40 nest areas, records from 10 gamekeepers and vegetation surveys, were used to investigate raptor predation at 28 pens from which pheasants were released in southern England. 3. Among 20 725 juvenile pheasants released in 1994-95, gamekeepers attributed 4·3% of deaths to buzzards, 0·7% to owls, 0·6% to sparrowhawks, 3·2% to foxes and 0·5% to other mammals. 4. Fresh pheasant remains were found on 7% of 91 visits to buzzard nests, and 8% of radio-tagged buzzards had significantly more association than other buzzards with pheasant pens. 5. Predation by buzzards was most likely to be recorded at release pens with little shrub cover, deciduous canopies and a large number of released pheasants. The number of pheasants killed was greatest in large pens with extensive ground cover, and the highest proportion of released pheasants was killed in large pens where few were released. However, only 21% of 55 releases had > 2 pheasant kills per week. 6. Radio-tagged buzzards were located most often at pheasant-release pens with open, deciduous canopies. Pens were most likely to be visited by buzzards that had fledged nearby, but the proximity of buzzard nests had little influence on how much predation occurred. 7. Only a minority of buzzards associated frequently with pheasant pens, and predation was heavy at only a minority of sites, where pen characteristics and release factors probably made it easy for individual buzzards to kill pheasants. We suggest that the occasional heavy losses could be avoided by encouraging shrubs rather than ground cover in pens, by siting pens where there are few perches for buzzards, and perhaps also by high-density releases.

The demography of red and grey squirrels was studied by live-trapping and radio-tagging at 14 deciduous and conifer sites in southern Britain and at eight conifer sites for one year in northern England. Densities and productivity correlated with tree seed crops for both squirrel species in deciduous and conifer habitats. Productivity was reduced by high density of full-grown squirrels relative to seed abundance. In oak±hazel woods, demography of grey squirrels correlated with abundance of acorns but not of hazel-nuts, whereas density and productivity of red squirrels correlated with hazel-nut abundance. Correlations of female density and productivity with pine-cone crops did not differ between red and grey squirrels. Predators ate many radio-tagged grey squirrels in conifers, and annual survival was only 50% compared with 80±82% for both species in other habitats. Grey squirrel populations in southern conifer sites were sustained by immigration, and at northern sites female density correlated with oak abundance within 500 m. Failure to exploit acorn crops puts red squirrels at a competitive disadvantage in deciduous woodland. Red squirrels had higher survival than grey squirrels in conifers, which may give them an advantage in that habitat, but could also have been explained by a lack of predators on their island study site.

Summary 1.Movements of many animals along a life-path can be separated into repetitive ones within home ranges and transitions between home ranges. We sought relationships of social and environmental factors with initiation and distance of transition movements in 114 buzzards Buteo buteo that were marked as nestlings with long-life radio tags. 2. Ex-natal dispersal movements of 51 buzzards in autumn were longer than for 30 later in their first year and than 35 extra-natal movements between home ranges after leaving nest areas. In the second and third springs, distances moved from winter focal points by birds that paired were the same or less than for unpaired birds. No postnuptial movement exceeded 2 km. 3. Initiation of early ex-natal dispersal was enhanced by presence of many sibs, but also by lack of worm-rich loam soils. Distances travelled were greatest for birds from small broods and with relatively little short grass-feeding habitat near the nest. Later movements were generally enhanced by the absence of loam soils and short grassland, especially with abundance of other buzzards and probable poor feeding habitats (heathland, long grass). 4. Buzzards tended to persist in their first autumn where arable land was abundant, but subsequently showed a strong tendency to move from this habitat. 5. Factors that acted most strongly in 1 / 2 -km buffers round nests, or round subsequent focal points, usually promoted movement compared with factors acting at a larger scale. Strong relationships between movement distances and environmental characteristics in 1 / 2 -km buffers, especially during early ex-natal dispersal, suggested that buzzards became primed by these factors to travel far. 6. Movements were also farthest for buzzards that had already moved far from their natal nests, perhaps reflecting genetic predisposition, long-term priming or poor habitat beyond the study area.