Paul E. Bellamy

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.

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.

Influence of Woodland Area on Breeding Success in Great Tits Parus major and Blue Tits Parus caeruleus

Reproductive performance of Great Tits and Blue Tits breeding in highly fragmented woodland was investigated in relation to woodland area in a landscape dominated by intensive arable agriculture in eastern England. Breeding performance was recorded for five years (1993-1997) for tits breeding in nestboxes in up to 43 woods ranging in size from 0.1 to 157 ha. Trends in the components of breeding success with woodland area were investigated using a linear, mixed model including both fixed and random effects, and logistic regression analysis. In both Great Tits and Blue Tits, breeding performance declined with decreasing woodland area. For Blue Tits, the decline was accounted for by the timing of breeding which was later in smaller woods. Great Tits showed a similar trend in laying date with woodland area, but over and above this effect of timing on breeding success, they also reared lighter young in smaller woods. The variation in first egg date accounted for by woodland area was about 25% for Great Tits and 33% for Blue Tits. In some years, failure at the nestling stage was frequent for Great Tits (but not Blue Tits) in small woods, but was not recorded to the same extent in large woods. The results are discussed in the context of previous studies of tit breeding success in relation to individual bird quality, habitat quality and availability, and the consequences of habitat fragmentation.

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.

The Application of Lidar in Woodland Bird Ecology

Habitat quality is fundamental in ecology, but is difficult to quantify. Vegetation structure is a key characteristic of avian habitat, and can play a significant role in influencing habitat quality. Airborne lidar provides a means of measuring vegetation structure, supplying accurate data at high post-spacing and on a landscape-scale, which is impossible to achieve with field-based methods. We investigated how climate affected habitat quality using great tits (Parus major) breeding in woodland in eastern England. Mean chick body mass was used as a measure of habitat quality. Mean canopy height, calculated from a lidar digital canopy height model, was used as a measure of habitat structure. The influence of canopy height on body mass was examined for seven years during which weather conditions varied. The slopes and correlation coefficients of the mass/height relationships were related linearly to the warmth sum, an index of spring warmth, such that chick mass declined with canopy height in cold, late springs, but increased with height in warm, early springs. The parameters of the mass/height relationships, and the warmth sum, were also related linearly to the winter North Atlantic Oscillation index, but with a time lag of one year. Within the same wood, the structure conferring “best” habitat quality differed between years depending on weather conditions.

Local extinctions and recolonisations of passerine bird populations in small woods

This paper considers, for eight species of woodland bird, the factors that influenced both local extinctions and recolonisations in 145 woods over 3 years. In all species, probability of local extinction was inversely related to population size; most local extinctions occurred in woods containing one to three breeding pairs. However, considerable variation in extinction probabilities occurred between species and between years. In addition, the suitability of habitat within a wood (more extinctions in less suitable woods) was important for wren Troglodytes troglodytes, song thrush Turdus philomelos and blue tit Parus caeruleus; also, the structure of the surrounding landscape was important for blue tit, great tit Parus major, and chaffinch Fringilla coelebs (more extinctions in localities with less woodland). In only two species was the probability of recolonisation related to any of the measured variables. Wrens were more likely to recolonise larger woods, whereas song thrushes were more likely to recolonise woods with a high habitat suitability rating and those which are more isolated from other woodland

Effects of broadleaved trees on birds of upland conifer plantations in North Wales

Abstract Five-minute counts at 253 points at ecotones in exotic conifer plantations contained 3515 records of 34 songbird species in the breeding season. Habitat data were collected at these points and at another 155 where singing individuals of selected scarcer species were located. Fourteen of 22 species with sufficient data showed habitat selection, including all the migrants, which were relatively uncommon. The main trends of vegetation were due to succession and the extent of non-crop broadleaves. The latter had a greater effect on bird species occurrence. A relationship between size of patch of broadleaves and density of habitat-selective birds was such that, for a fixed area, there would be more individuals of these species if broadleaves were dispersed throughout the conifers rather than concentrated in few larger blocks. The study has methodological interest and practical application.

Influences of Habitat Fragmentation on Bird Species Distributions and Regional Population Sizes

A simple modelling approach was used to investigate the influence of habitat fragmentation on the distributions and regional population sizes of passerines breeding in 145 small woods in an arable landscape. Observed distributions in three consecutive years were compared with those predicted by random selection of woods by pairs (based on a Poisson distribution), and by a simulation model based on the area of habitat available and estimates of individual species minimum area requirements and territory sizes. None of the species studied were distributed randomly; some were distributed according to the area of woodland available, whereas others preferred either small or large woods. However, species distributions were influenced by their regional population sizes. When species population sizes were small, observed distributions could be predicted using algorithms based on area alone, but as numbers increased, algorithms using territory parameters were required. An absence of detectable territorial interaction suggested that space for more pairs of the species concerned had been available in our woods. Of the 14 species investigated, the models for only two consistently required the use of territory parameters, indicating that the available habitat could probably have accommodated more pairs of most of the species. Thus habitat fragmentation may reduce regional population sizes more than expected from the loss of habitat alone, to the extent that patches of suitable habitat may be unused, or occupied at low density.

Modelling the Effects of Dispersal and Landscape Configuration on Population Distribution and Viability in Fragmented Habitat

Landscape configuration and dispersal characteristics are major determinants of population distribution and persistence in fragmented habitat. An individual-based spatially explicit population model was developed to investigate these factors using the distribution of nuthatches in an area of eastern England as an example. The effects of immigration and increasing the area of breeding quality habitat were explored. Predictions were compared with observed population sizes in the study area. Our model combined a nuthatch population simulator based on individual behaviour with a grid-based representation of the landscape; nuthatch life cycle and immigration parameters were user selectable. A novel aspect of the model is user-selection of habitat perceptual range. Using a realistic set of parameters, the number of breeding pairs predicted by the model matched observed numbers. According to model simulations, the main cause of nuthatch scarcity in the study area was the inability of patches to support viable populations without immigration from elsewhere. Modelled habitat management, which increased breeding quality habitat in existing woods, lowered the threshold above which the study area population became self-sustaining. The existence of a large core habitat area was critical in producing a self-sustaining population in this landscape, the same area in dispersed small woods failed to sustain populations.

Dispersal, ranging and settling behaviour of Marsh Tits Poecile palustris in a fragmented landscape in lowland England

Capsule Natal dispersal was rapid and distances were short. Winter ranging and breeding dispersal were limited. Few birds undertook large movements. Aims To investigate the natal and breeding dispersal of Marsh Tits, including the timing of dispersal movements. Methods Nestlings, juveniles and adults were ringed and searched for over 4500 ha during summer, autumn–winter, and spring over six years. Dispersal distances were measured as metric distances and multiples of territory widths. Ranging distances were compared with dispersal distances. Results Median distances of natal dispersal were 2.6 territory widths for males (704.5 m) and 3.1 territory widths for females (1065.0 m). Median distances of breeding dispersal were 0.2 territory widths for males and females (58.6 and 53.1 m respectively). Most natal dispersal was completed soon after independence, with further movement in spring. Breeding dispersal was also detected during these periods. Median ranging distances were short, and some winter floaters were identified. Conclusion Marsh Tits had short dispersal distances, with most dispersal activity occurring in June. Results suggested that dispersal behaviour was sensitive to habitat fragmentation, resulting in poor settling success outside of the natal wood. Habitat fragmentation may, therefore, be a contributory factor in the decline of the Marsh Tit population in Britain.