Stuart J. Marsden

Estimating bird abundance : making methods work

Summary In many bird monitoring surveys, no attempt is made to estimate bird densities or abundance. Instead, counts of one form or another are made, and these are assumed to correlate with bird density. Unless complete counts on sample plots are feasible, this approach can easily lead to false conclusions, because detectability of birds varies by species, habitat, observer and many other factors. Trends in time of counts often reflect trends in detectability, rather than trends in abundance. Conclusions are further compromised when surveys are conducted at unrepresentative sites. We consider how to avoid these problems. We give a brief description of distance sampling methods, which allow detectability to be estimated. We consider strategies to ease their implementation, to enhance their reliability, to adapt the methods for difficult species, and to deal with circumstances in which representative sampling is problematic. We also consider some of the common problems encountered, and suggest solutions.

Biodiversity in small-scale tropical agroforests: a review of species richness and abundance shifts and the factors influencing them

Although small-scale agroforestry systems (swiddens, complex and single-crop-dominated agroforests, and homegardens) form a diverse and important tropical land use, there has been no attempt to collate information on their value for biodiversity. This paper reviews 52 published studies that compared species richness and/or abundance between agroforests and primary forest, and 27 studies that compared biodiversity parameters across agroforests. The former covered a broad range of taxa and geographical areas, but few focused on homegardens, while those comparing across agroforestry systems were biased towards studies of plants (21 studies) and homegardens (13 of 27). Of 43 studies comparing species richness or diversity across habitats, 34 reported lower richness in agroforests than in adjacent forest. There was also high β diversity between primary forests and agroforests. Patterns of abundance shifts were less straightforward, with many species traits (for example diets) being generally poor indicators of response to agricultural disturbance. Among the few trends identified, restricted-range or rare species, and terrestrial and some understorey vertebrates tended to decline most, and open country species, granivores and generalists increased most in agroforests. Variability in biodiversity retention across systems has been linked most strongly to economic function, management intensity and extent of remnant forest within the landscape, as well as more subtle cultural influences. Species richness and abundance generally decrease with increasing prevalence of crop species, more intensive management, decreasing stratum richness and shortening of cultivation cycles. Increasing holding size did not necessarily reduce α diversity. Knowledge of the general effects of small-scale agroforestry on biodiversity is substantial, but the great diversity of systems and species responses mean that it is difficult to accurately predict biodiversity losses and gains at a local level. Further work is required on the influence of spatial and temporal structure of agricultural holdings on biodiversity retention across agriculture/succession/forest mosaics, how β diversity across individual holdings influences biodiversity across landscapes, and ultimately on how agricultural intensification can be best managed to minimize future losses of biodiversity from tropical landscapes.

Bird diversity and abundance in forest fragments and Eucalyptus plantations around an Atlantic forest reserve, Brazil

Little of Brazils remaining Atlantic forest is protected, so it is important to assess how well the regions wildlife can persist in areas/habitats outside reserves. We studied bird diversity and abundance during 546 point counts in the Sooretama/Linhares reserve, 200 point counts in 31 forest fragments (10–150 h), and 50 point counts in <30-year-old Eucalyptus plantations, within 7 km of the reserve. Only eight bird species were recorded in Eucalyptus, and this impoverishment, as compared to some Eucalyptus plantations elsewhere in Brazil may be a result of intensive clearance of understory vegetation. Species diversity in forest fragments was significantly lower than in the reserve. Twelve, mostly non-forest or edge species, were significantly commoner in the fragments, but nineteen species were frequent in the reserve but rare or absent in forest fragments. These included two Pyrrhura parakeets, a Brotogeris parakeet, a trogon Trogon, a jacamar Galbula, woodpeckers Piculus and Campephilus, Myrmotherula antwrens, and Hemithraupus and Tachyphonus tanagers. Bird species richness at points in forest fragments did not decline with fragment size, distance from the reserve, or forest quality. However, forest in fragments was more heavily degraded than forest within the reserve and poor forest quality may be the cause of declines in some species. Whilst protection of forest within reserves is a priority, management of forest fragments may aid conservation of some threatened species.

Population sizes, status and habitat associations of the restricted-range bird species of Sumba, Indonesia

The island of Sumba was visited in 1989 and 1992 with the aim of collecting data on its avifauna. The endemic and other restricted-range bird species are very poorly known and, potentially, at great risk from extinction due to habitat change. Using standardized methods, habitat and bird census data were collected in eight forest areas. Analysis of the habitat data shows that most of the restricted-range species are forest-dependent. The exception is Turnix everetti (Sumba Buttonquail), which is found in open grassland. Discriminant Function Analysis was used to define habitat associations in a more precise and objective way. The species with the most specific requirements are Ptilinopus dohertyi (Red-naped Fruit-dove) and Zoothera dohertyi (Chestnut-backed Thrush), which are associated with primary forest at high altitudes, and Cacatua sulphurea (Sulphur-crested Cockatoo) and Rhyticeros everetti (Sumba Hornbill), which prefer evergreen primary or mature secondary forest at low altitudes. The results of the bird censuses were combined with data on habitat cover from satellite photographs to produce estimates of total population sizes. Among the rarest and most endangered species on Sumba are three which are represented by endemic subspecies: C. sulphurea (estimated population 3,200 birds), Eclectus roratus (Eclectus Parrot) (1,900), Tanygnathus megalorynchos (Great-billed Parrot) (1,700). The rarest endemic species is R. everetti , with a population of approximately 6,500. It is suggested that the census method used – point counts With distance estimates to bird contacts – is the best compromise for multi-species surveys in tropical forests.

The nesting requirements of the parrots and hornbill of Sumba, Indonesia

Abstract A total of 132 nests of Sumbas parrots and endemic hornbill were located. Nearly half were in trees containing other parrot or hornbill nests (one tree contained five nests). All but one bird species selected very large trees to nest in (mean height = 38 m). Deciduous trees of the family Datiscaceae were very strongly preferred, with between 8% (Geoffroyus geoffroyi) and 83% (Eclectus roratus) of nests in two species of this family. Discriminant analysis indicated that if nests of the heavily-traded Cacatua sulphurea were high up the tree, they were safe from being raided by parrot catchers. A nest site availability index was calculated for individual bird species in each of six forest areas. There was some evidence for a correlation between potential nest site availability and parrot and hornbill abundance at the forest sites.

Parrot populations and habitat use in and around two lowland Atlantic forest reserves, Brazil

We estimated population sizes of parrots in a large (Sooretama/Linhares) and a small (Porto Seguro) lowland Atlantic forest reserve, and examined their habitat associations within the reserves, and their use of forest fragments. In Sooretama, most species had estimated populations between 1000 and 20,000 individuals, but in the smaller reserve population densities were low and all but one species had population estimates <500. Two Amazons, including the ‘Endangered’ Amazona rhodocorytha, were strongly linked to primary forests whereas the ‘Vulnerable’ Pyrrhura cruentata was associated with non-pristine forest. There was considerable movement of parrots between the Sooretama reserve and the surrounding country with Amazona species tending to fly into the reserve during the mornings, and the macaw Propyrrhura maracana flying out. Other species, notably P. cruentata and P. leucotis, were never recorded away from the reserve. Most parrot populations in the region are likely to be small, and the substantial populations around Sooretama make this reserve a parrot stronghold. It is crucial that fire, illegal logging and parrot capture are adequately controlled within Sooretama, and there is a strong argument for extending conservation management eAorts to areas immediately outside the reserve. # 2000 Elsevier Science Ltd. All rights reserved.

A Comparison of Predictive Methods in Extinction Risk Studies: Contrasts and Decision Trees

Over the last two decades an increasing emphasis has been placed on the importance of controlling for phylogeny when examining cross-species data; so-called comparative methods. These methods are appropriate for testing hypotheses about correlations between evolutionary events in the history of a clade and adaptive responses to those changes. When this approach is applied to extinction risk, possible correlations between evolutionary changes in, for example, body size or habitat specialisation and some measure(s) of current threat status are examined. However, there may be a mismatch here between the results of such studies, and the real, pragmatic needs of species conservation. This kind of approach certainly adds to our knowledge of some fundamental processes, but it is more difficult to see how this can be applied to conservation decision-making. For more practical purposes a decision-tree approach can be extremely useful. This paper illustrates the use of a contrasts based analysis of extinction risk compared with a decision-tree analysis for Galliformes (Aves). While the contrasts analyses concur with some general macroecological trends found in other studies, the decision-tree models provide lists of species predicted to be more at risk than current assessments would suggest. We argue that in practical terms, decision tree models might be more useful than a macroecological linear model-based approach.

ABUNDANCE, HABITAT USE, AND MOVEMENTS OF BLUE- WINGED MACAWS (PRIMOLIUS MARACANA ) AND OTHER PARROTS IN AND AROUND AN ATLANTIC FOREST RESERVE

Abstract The Blue-winged Macaw (Primolius maracana) has disappeared from most of southern Brazil, Argentina, and Paraguay; its remaining southern stronghold is the 2,179-ha Caetetus Reserve, São Paulo state, Brazil. We estimated the macaws population inside the reserve (88 individuals) and examined how it and other parrots use the extra-reserve landscape, which is dominated by coffee plantations and pasturelands. Flight activity of the macaw and Scaly-headed Parrot (Pionus maximiliani) declined with distance from Caetetus, although many macaws flew to the vicinity of the reserve to roost. Two other species, Canary-winged Parakeet (Brotogeris versicolurus) and White-eyed Parakeet (Aratinga leucophthalmus), used the landscape independent of the reserve itself. We recorded parrots in 90% of our 1-km2 study plots outside (<12 km) the reserve, but no species was recorded using pasture, coffee or rubber/orange plantations, or scrub habitats, which composed 80% of the landscape around the reserve. Only four habitat types were used by any species. Primary and secondary forests were the habitats most preferred; Eucalyptus plantation habitat was the only totally anthropogenic habitat used. Clearly, protection, and preferably augmentation, of forest cover around Caetetus may be crucial for the macaws survival at this important site.

A technique for measuring the density and complexity of understorey vegetation in tropical forests

The dense understorey regrowth after disturbance of tropical forests has been linked to serious declines in many understorey and terrestrial animal taxa. We devised a method of measuring changes in vegetation characteristics using standardized photographs of the forest understorey. We describe the method using a sample dataset of photographs taken at 28 stations in primary forest and forest that had been logged selectively eight years previously, at the Danum Valley Field Centre in lowland Borneo. The understorey photographs were digitized and eight measures of vegetation density and complexity generated for each image using FRAGSTATS. Parameters included vegetation area, total vegetation edge, and mean fractal dimension of vegetation patches. There were no significant correlations between the vegetation density measures and those representing vegetation complexity showing there to be two unrelated components to understorey variability. We used PCA to derive two principal axes from the eight vegetation parameters. There were no significant differences in our measures of understorey density or complexity between the primary and logged forest stations. There was, however, a greater range of understorey characteristics in primary forest than in logged forest indicating a loss of understorey heterogeneity following logging. Most apparent was a loss of very open and non-complex understories and this may affect some animals in a number of ways, including altering the abundance of their prey and altering their ability to forage successfully in logged forest.

Variability in roost size in an Amazona parrot: implications for roost monitoring

Abstract Roost counts may be a useful method for assessing and monitoring parrot populations as long as counting regimes can detect real differences in abundance above the noise of daily variability in roost size. We studied a roost of up to 85 Red-tailed Amazons (Amazona brasiliensis) for 28 consecutive mornings and evenings from 12 July to 7 August 2001, and recorded bird behavior and associated weather data. The roost declined significantly over the survey period as the breeding season drew nearer. There was no significant difference between evening and morning roost counts, but we suggest that as long as misty mornings are avoided, morning roost counts were more effective as birds left more quickly and predictably. It took longer for birds to arrive on evenings when the roost was large, but birds left quicker in the morning when there were large numbers in the roost. Weather influenced both roost size and timing of arrival, with larger than expected numbers in the roost, and birds arriving later in the afternoons of sunny, warm days. We tested the reliability of four roost counting regimes: counts from five consecutive nights, five counts, one every fourth night, five nights picked at random, and 10 randomly picked nights. Counts from every fourth day performed significantly worse than all the other regimes in estimating the mean numbers in the roost. The 10-d random sampling regime performed significantly better than the 5-d regime in detecting very large roosts which occurred occasionally through the month.