Avian Demography Research Articles

Application of a global age-coding system (“WRP”), based on molts and plumages, for use in demographic and other studies of birds

Abstract Determination of a bird’s age or cohort is critical for studies on avian demography, occurrence patterns, behavior, and conservation management. Age designations have largely been developed in north-temperate regions and utilize calendar-based or seasonally based codes; however, in tropical regions and in the southern hemisphere, these coding systems have limited utility at best. To address these issues, we had previously devised the “WRP system,” based on the nomenclature of Humphrey and Parkes (H–P) and Howell et al., which defines molts in an evolutionary context applicable to birds globally. Here we refine and build upon core concepts and definitions of the WRP coding system, resolving key limitations that were identified during its first decade of use. The WRP system employs a three-letter alpha code in which each letter describes a different aspect of H–P terminology: the molt cycle (which informs a bird’s age) and molt and plumage status within the cycle (each of which can also inform age). Here we recommend the continued use of most of the original (“core”) WRP coding while augmenting the system with an optional adjunct-code entry for comprehensiveness, clarity, and flexibility, and we clarify a few additional codes to cover less common molting and plumage strategies. For most users, from 7 to 13 core and 1 adjunct WRP code will be sufficient to describe all plumages and provide molt status and ages for demographic studies or other purposes. The revised WRP system is flexible enough to be adapted to the specific goals of programs while also providing core codes that can facilitate the comparison of avian age, molt, and plumage status on a global basis. We anticipate that our revised and standardized version of the WRP system will be easily adopted and could eventually replace calendar-based and seasonally based coding.

Dry season intensity has equivocal effects on the nutritional condition of understory birds in a Neotropical forest

Abstract Rainfall regime, the amount and timing of annual precipitation, can influence the breeding phenology, individual fitness, and population dynamics of tropical birds. In Neotropical regions with rainfall seasonality (i.e. wet and dry seasons), the warm phase of the El Niño Southern Oscillation (ENSO) can exacerbate seasonal drought and negatively impact avian survival and reproduction. However, the mechanisms underlying associations between seasonal drought conditions and avian demography are largely unexplored. One hypothesis is that nutritional condition mediates demographic responses to seasonal drought: individuals in poor condition may be less capable of balancing their energy budgets and consequently suffer reduced survival, lower reproductive output, or both. We estimated nutritional condition (i.e. scaled mass index, percent hematocrit, plasma lipid metabolites) as a proxy of energy balance in understory forest birds with contrasting population-level responses to dry season length. This study took place across two dry seasons of differing intensity in central Panama: an El Niño dry season (2016, severe drought) and a more typical dry season (2017). Scaled mass index remained relatively constant throughout both dry seasons and across years for 5 common focal species and among 4 foraging guilds (22 additional species, 27 species total). Three of 5 focal species did exhibit reduced nutritional condition (i.e. lower hematocrit and/or higher β-hydroxybutyrate) during the El Niño dry season but not during the more typical dry season. However, foraging guilds did not show consistent nutritional responses to seasonal drought and we found little evidence of the reduced nutritional condition at the guild level, suggesting that many Neotropical forest bird species are capable of tolerating seasonal drought.

Developing and validating a nestling photographic aging guide for cavity-nesting birds: an example with the European Bee-eater (Merops apiaster)

BackgroundAccurate estimation of nestlings’ age is essential in avian demography studies as well as in population ecology and conservation. For example, it can be useful for synchronizing nest visits with events of particular interest, such as the age at which young can be safely ringed, or in choosing the best period to attain the most accurate calculation of laying or hatching dates.MethodsWe constructed a photographic guide for aging European Bee-eaters (Merops apiaster) nestlings to 3-day age classes and evaluated the aging method by performing a validation exercise with several observers with no previous experience in aging bee-eater nestlings.ResultsThe aging guide for bee-eater nestlings allowed estimating age to within 3 days with an average accuracy of 0.85. We found the optimal period for aging nestlings was between days 13‒18 (with accuracy between 0.94 and 0.99), during which the status of feather development was more easily distinguishable from the preceding and subsequent age classes. During the first 3 days after hatching, nestlings could also be aged with high accuracy (0.93). The small size of the nestling in relation to the eggs and the nestling’s inability to raise its head during these first days allowed for good discrimination from the subsequent age class. Between days 25 and 28, nestlings were correctly aged in only half of assignments (0.55 sensitivity) and nestlings belonging to class 7 (days 7‒9) were the least correctly identified (0.38 sensitivity). However, by visiting the nests at 12 days intervals it is possible to achieve the highest accuracy in age estimation with the smallest disturbance and logistic investment.ConclusionsThis study highlighted how indirect methods and a simple protocol can be established and employed to quickly estimate nestling age in cases where handling nestlings is challenging or impossible, while minimizing disturbance in and around the nest.

Timing of natal nests is an important factor affecting return rates of juvenile Great Reed Warblers

In many bird species, return rates tend to be greatest in the vicinity of their hatching site, thus natal philopatry is an important trait in avian demography. In the present study, we examined how conditions in natal nests affect return rates and dispersal distances of a long-distance migratory passerine. From 2008 to 2016, we studied a colour-ringed population of the Great Reed Warbler Acrocephalus arundinaceus breeding in southeastern Czech Republic. Almost 10% of the 1430 ringed individuals returned to the study site within 2 years after fledging. Birds originating from earlier broods were more likely to return than birds from delayed broods. The variation in local dispersal distances was explained by the sex of the recruits: males dispersed shorter distances than females. There was no evidence that return rates and dispersal distances in Great Reed Warbler are related to other factors closely linked to the condition in the natal nest, namely brood social status and brood size. While the higher return rates of the juveniles from earlier broods is in concordance with the findings of other studies, the sex-related difference in dispersal distances does not correspond with what was found in Great Reed Warbler juveniles in other studies and adults at the same study site. Our results show that early breeding produces more local recruits and that sex-biased dispersal may be site specific.

Diet flexibility in a declining long‐distance migrant may allow it to escape the consequences of phenological mismatch with its caterpillar food supply

Climate‐driven shifts in prey phenology may lead to asynchrony with the timing of peak resource requirements of their predators, leading to a reduction in productivity and population declines. Migrant species that cannot adjust their arrival times may be particularly at risk, especially those that breed in seasonal environments and for which a temporarily super‐abundant prey source is important, such as insectivorous passerine birds that take advantage of the seasonal flush of caterpillars to feed their young. We assess whether population declines of the trans‐Saharan migratory Wood Warbler Phylloscopus sibilatrix are likely to have been caused by phenological mismatch. We measured seasonal invertebrate biomass and various fitness parameters, including the timing of breeding and breeding success, in two time periods: 1982–1984, prior to the species’ decline in the UK, and 2009–2011, as the reduction in numbers continued. Although birds bred on average a week earlier in 2009–2011 than in 1982–1984, this was not adequate to track the more rapid advancement of peak caterpillar biomass, which advanced by 12 days and was closely correlated with spring temperatures. Moreover, although caterpillars were the dominant prey fed to nestlings, there was only limited evidence that productivity was positively related to caterpillar biomass in the environment. Considering only successful nests, synchrony with the food peak did not produce heavier nestlings and had only a small positive effect on fledging success, although there was a seasonal decline in productivity when all nests were considered. We conclude that the lack of a marked effect of the observed mismatch is due to Wood Warblers’ generalist diet, enabling them to breed successfully on prey other than caterpillars. Although other studies have demonstrated that climate‐driven asynchrony of predator and prey populations can have impacts on avian demography, this study highlights the importance of investigating the generality of those findings.

The EURING Data Bank – a critical tool for continental-scale studies of marked birds

ABSTRACTThe European Union for Bird Ringing (EURING) coordinates bird ringing at a continental scale and operates the EURING Data Bank (EDB) to facilitate large-scale analyses of movements and demography. The EDB contains over 10 million individual encounter records which are summarised on a publicly available website, the EDB index. EURING welcomes applications to analyse these data. Ring-recovery data from the EDB contribute to research on many ecological issues, particularly migration and movements, hunting, mortality causes, disease transmission, population dynamics and dispersal. Recent developments will facilitate the incorporation of comprehensive sets of first-encounter records and local recaptures which are essential for robust quantitative studies of population dynamics and movements. Furthermore, the recent inclusion of fields for moult, measurements and weights within the EURING code will facilitate novel research at a European scale. We expect increasing use of the EDB for quantitative studies of avian demography and movements with high applied value. Wherever possible, this research should also incorporate complementary ecological information. EURING’s immediate priority is the production of a European Migration Atlas that would provide an up-to-date synthesis of the movements of European bird populations, with many direct implications for their conservation.

Invertebrate prey availability limits reproductive success but not breeding population size in suburban House Sparrows Passer domesticus

Factors affecting avian demography and abundance in urban landscapes are poorly understood and this hinders attempts to manage urban bird communities. Several recent studies indicate that lack of invertebrate prey in urban landscapes may constrain avian productivity and fitness relative to that in other habitats. House Sparrow Passer domesticus populations have undergone large declines in many European urban centres and inadequate reproductive success linked to invertebrate availability has been postulated as a potential cause of these declines. We conducted a replicated supplementary feeding experiment to test whether the availability of invertebrate prey limits the breeding success and adult abundance (colony size) of House Sparrows in suburban London, where House Sparrow populations declined by 60% during the decade preceding our study. Daily mealworm provision over two successive breeding seasons, sufficient to provide 82% of chick energy requirements of House Sparrow pairs nesting within 50 m of feeders, had a large positive impact on the abundance of recently fledged birds (+62%), but only a small positive impact, limited mainly to small colonies, on the overall abundance of territorial males. Colony growth was only weakly related to fledgling abundance in the previous year and did not appear to be constrained by nest‐site availability. Conservation interventions that enhance invertebrate availability for suburban House Sparrows may increase reproductive success but are unlikely, on their own, to lead to population growth or recovery.

Low nest survival of a breeding shorebird in Bohai Bay, China

Nest survival plays an important role in avian demography because of its influence on both individual fitness and population growth. It is also known to vary within species due to local factors such as climate, predation, substrate, and disturbance, among others. Therefore, an understanding of the relative influence of local factors on nest survival is of critical importance for the formulation of appropriate avian conservation and management policies/programs. Over the past 50 years the Yellow Sea has lost almost 65 % of its original intertidal habitats due to land reclamation and development. There has also been a concomitant and rapid decline in the populations of Kentish plover (Charadrius alexandrinus) in East Asia, but the proximate causes of this decline are poorly understood. To gain a better understanding of this conservation issue, we investigated Kentish plover nest survival in Bohai Bay, China, using Program MARK to model the daily survival rate (DSR) of 417 nests. We found that in terms of nest survival, that for the Kentish plover populations in Bohai Bay [0.925 ± 0.004 (±95 % confidence interval)] is the lowest reported worldwide for this species. The most common cause of nest failure was related to anthropogenic disturbance. We determined that nests occupying salt crystallization habitat had the highest hatching success and that initiation date, nest age, and nest density had quadratic effects on DSR. If low nest survival persists for consecutive years, fecundity will unlikely compensate for adult mortality, resulting in dramatic population declines of plovers in Bohai Bay. We therefore recommend that the Local Authority managers responsible for local environmental management act accordingly to create protected alternative nesting habitat for plovers in this region.

Spring phenology and timing of breeding in short‐distance migrant birds: phenotypic responses and offspring recruitment patterns in common goldeneyes

Understanding how organisms adjust breeding dates to exploit resources that affect fitness can provide insights into impacts of climate change on avian demography. For instance, mismatches have been reported in long‐distance migrant bird species when environmental cues experienced during spring migration are decoupled from conditions on breeding grounds. Short‐distance migrant bird species that store reproductive nutrients prior to breeding may avoid or buffer adverse phenological effects. Furthermore, reduced short‐term reproductive success could be offset by higher future recruitment of surviving offspring. We evaluated whether recruitment of locally‐hatched female offspring was related to hatching date alone or strength of mismatched breeding date for 405 individually‐marked adult female common goldeneyes Bucephala clangula (a short‐distance migrant) and their ducklings from a site in central Finland where ice‐out date has advanced by ∼ 2 weeks over 24 yr. Path analyses revealed that older, early‐nesting females with good body condition and larger broods recruited the most female offspring. Offspring recruitment decreased strongly among females that bred late relative to other females in the population each year; the extent of mismatched breeding date, i.e. hatching date scaled to annual ice‐out date, was less influential. Overall, most females advanced breeding dates when ice‐out occurred earlier in spring, but some females exhibited greater flexibility in response to ice‐out conditions than did others. In general, directional selection favoured early breeding over a wide range of ice‐out dates. Our results seem most consistent with a hypothesis that some short‐distance migrant species like goldeneyes have the capacity to track and respond appropriately to changing environmental conditions prior to onset of breeding.

Results of 20 years of experimental forest management on breeding birds in Ozark forests of Missouri, USA

Understanding the relationship between forest management and bird populations requires understanding the effects of silvicultural practices on avian demography at large spatio-temporal scales. The Missouri Ozark Forest Ecosystem Project (MOFEP) is a long-term, large-scale manipulative experiment testing the effects of even-aged (3–13ha cuts over 10–15% of the site; n=3), uneven-aged (0.03–3.14ha cuts over 57% of the site; n=3), and no harvest forest management on ecosystem level responses. We report on the effects of these management systems on the density and reproductive success of 11 songbird species from 5years of pre-harvest (1991–1995) to 14years of post-harvest (1997–2010). Density of four of the five mature forest species were lower after harvest in all management types and did not return to pre-harvest density, even in no harvest sites. Ovenbird (Seiurus aurocapilla) and Wood Thrush (Hylocichla mustelina) responded most negatively to even-aged management in the early post-harvest period (1997–2003) where density was significantly lower than in no harvest sites. Kentucky Warbler (Geothlypis formosa) density increased on uneven-aged and even-aged sites during early post-harvest, but returned to pre-harvest density on both management types by 14years post-harvest. Acadian Flycatcher (Empidonax virescens) and Worm-eating Warbler (Helmitheros vermivorus) had lower density in all treatments post-harvest. Among the six early-successional species, density of Blue-winged Warbler (Vermivora cyanoptera), Hooded Warbler (Setophaga citrina), Indigo Bunting (Passerina cyanea) White-eyed Vireo (Vireo griseus) and Yellow-breasted Chat (Icteria virens) were significantly higher in even-aged and uneven-aged sites than in no harvest sites after harvest. Density of Prairie Warbler (Setophaga discolor) were significantly higher only in even-aged management after harvest. Prairie Warbler, Hooded Warbler, and White-eyed vireo appeared on the study sites following harvest while Indigo Bunting, Prairie Warbler, and Yellow-breasted Chat were absent from the study area 14years post-harvest. Nest survival of mature forest and early-successional species did not change significantly from the pre- to the late post-harvest period or with forest management. Brood parasitism rates remained low from pre-harvest to late post-harvest, but parasitism rates were higher for early-successional species (4%) than mature forest species (1%). Although forest management had variable effects on species, we suggest a modified version of even-aged management could maximize benefits to early-successional species while minimizing decreases in mature forest bird species in central hardwood forests. Rather than the current prescription to harvest 10–15% of the mature stands every 15years, we recommend harvesting approximately half the number of mature stands with a shorter re-entry period of 8–10years.

Avian demography in a changing world: a report on the BOU's Annual Conference held at the University of Leicester, 1–3 April 2013

Avian demography in a changing world: a report on the BOU's Annual Conference held at the University of Leicester, 1–3 April 2013

Demography and dispersal of juvenile and adult Brown-headed Cowbirds (Molothrus ater) in the eastern Sierra Nevada, California, estimated using multistate models

ABSTRACT. Despite recent progress in avian demography, much still needs to be learned about patterns of survival and dispersal across different ages and sexes of birds. This is expressly true for Brown-headed Cowbirds (Molothrus ater) in light of their vagility and importance in the life history of many other bird species. We used multistate mark—recapture models to estimate annual survival and dispersal rates of Brown-headed Cowbirds across dialect subpopulations in the Sierra Nevada of California across the 1982–1988 breeding seasons. We estimated rates for male and female hatch-year (HY) and after-hatch-year (AHY) birds and found no evidence of differences in capture probabilities across sex and age states. To account for high apparent nonresidency, adult survival was estimated separately for the year following initial capture and for subsequent years. The latter estimates are near the midpoint of those from other studies (AHY females: 56.1%; after-second-year [ASY] males: 63.4%; second-year [SY] males...

African Penguin Populations Reported in a Puzzling Decline

ZOOLOGY![Figure][1] On the move. Some African penguins are establishing mainland colonies such as this one as prey becomes scarcer around their island habitats. CREDIT: R. KOENIG/ SCIENCE PRETORIA, SOUTH AFRICA— African penguin populations, on the upswing since the mid-1990s, appear to have gone into a surprising nosedive. New data indicate that their numbers may have dropped in the past few years by as many as 50,000-40% of the population. And the birds, which normally breed on island colonies, have puzzled scientists by establishing a growing number of new colonies on the mainland. Marine zoologists see this population pattern reflected in several observations. “Every piece of information we have—breeding success, breeding counts, diet sample analysis, and [a bird census during molting season]—all show the same trend and are serious cause for concern,” says Samantha Petersen, who manages BirdLife South Africa's seabird conservation program. Zoologist Rob Crawford, a penguin expert with South Africa's Environmental Affairs Department, agrees that the trend is “quite disturbing.” He believes that the birds' prime food sources—sardines and anchovies—are becoming scarce around established colonies. Although overfishing may be part of the problem, Crawford and South African fish experts also blame “a large eastward shift” in the distribution of the fish. In a recent study, they found that the biomass of those fish species in the region near the penguins' largest breeding islands west of Cape Town fell sharply after 2002. In what Crawford suspects is a “desperation move” to get closer to their fish prey, some penguins—apparently from island colonies—have been moving eastward and settling on the mainland, most recently at the De Hoop Nature Reserve. They normally shun these locations because of predators. Les Underhill, who directs the University of Cape Town's avian demography unit, agrees that the new colonies reflect a trend of penguins moving eastward toward the current fish biomass center, near Mossel Bay. African penguins, called jackass penguins because of their braying, once numbered more than 1.5 million on islands off South Africa's western coast. But guano and egg harvesting a century ago led to a 90% decline in the population; oil spills in 1994 and 2000 also held them back. Even so, South African penguins climbed to about 120,000 earlier in this decade before the most recent downturn. Long-term oceanographic studies are needed to assess whether climate change could be a factor. For now, South Africa's environment department is considering various short-term options to try to protect the penguins, including establishing no-fishing zones around several breeding islands. Underhill contends that “setting up areas around breeding colonies which are closed to fishing is the critical issue facing penguin conservation today.” This month, Crawford's group will begin a new count of nests and, later, of penguins themselves during molting. Researchers are also using satellite-tracking and transponders to analyze the birds' feeding habits. Parallel efforts are ongoing in Namibia, where the number of penguins is now about 24,000, down from 100,000 in 1956. Life is not likely to get easier for South Africa's penguins, threatened by oil spills, predators, and habitat changes. But Underhill says the new colony at De Hoop offers some hope: It shows that the penguins might be safely relocated. If scientists could figure out how to start colonies, “then we could secure a piece of coastline from land predators and get a colony going near the fish.” [1]: pending:yes

Demographic Responses by Birds to Forest Fragmentation

Abstract: Despite intensive recent research on the effects of habitat loss and fragmentation on bird populations, our understanding of underlying demographic causes of population declines is limited. We reviewed avian demography in relation to habitat fragmentation. Then, through a meta‐analysis, we compared specific demographic responses by forest birds to habitat fragmentation, providing a general perspective of factors that make some species and populations more vulnerable to fragmentation than others. We obtained data from the scientific literature on dispersal, survival, fecundity, and nesting success of birds. Birds were divided into subgroups on the basis of region, nest site, biogeographical history, and migration strategy. Species most sensitive to fragmentation were ground‐ or open‐nesters nesting in shrubs or trees. Residents were equally sensitive to fragmentation in the Nearctic and Palearctic regions, but Nearctic migrants were more sensitive than Palearctic migrants. Old World species were less sensitive than New World species, which was predicted based on the history of forest fragmentation on these two continents. Pairing success was the variable most associated with fragmentation, suggesting an important role of dispersal. Fledgling number or condition, timing of nesting, and clutch size were not associated with sensitivity to fragmentation, suggesting that negative fragmentation effects on birds do not generally result from diminished food resources with increasing level of fragmentation. Future studies on demographic responses of birds to habitat fragmentation would be more effective if based on a combination of measures that can distinguish among the demographic mechanisms underlying population changes related to habitat fragmentation.

Factors influencing fecundity in migratory songbirds: is nest predation the most important?

Nest predation often is reported to be the major cause of nest failure in birds and by extension the predominant influence on annual fecundity. However, other factors such as the ability of some individuals to renest after a failed nest or to lay additional clutches after successful ones, both of which may in turn be influenced by food availability, can also contribute to individual fecundity. We evaluated the relative importance of nest predation, multiple brooding, renesting, and food availability on mean annual fecundity of a migratory songbird population, using AICc from multiple regression models. In a 16‐yr study of black‐throated blue warblers Dendroica caerulescens, these four variables combined explained 87% of the annual variance in fecundity. Individually, however, food abundance explained more of the variability (35%) than did predation (29%), double‐brooding (19%), or renesting (15%). These results suggest that measuring food availability may be essential to understanding annual differences in reproductive output. They also demonstrate that failing to account for the contribution of food availability, renesting, and multiple brooding in studies of avian demography can result in serious underestimates of mean annual fecundity, potentially biasing calculations of population growth rate, source‐sink dynamics, habitat quality, or population viability.

Effects of Experimental Forest Management on Density and Nesting Success of Bird Species in Missouri Ozark Forests

Abstract:A critical step in understanding the relationship between forest management and bird populations is conducting studies that employ rigorous experimental designs, relate forest management to avian demography, and explore relationships at expanded temporal and spatial scales. The Missouri Ozark Forest Ecosystem Project ( MOFEP ) is a long‐term, large‐scale manipulative experiment that is testing the effects of even‐aged ( i.e., clearcutting ) and uneven‐aged ( i.e., selection cutting ) forest management on a suite of response variables in Missouri oak‐hickory forests. We report on the short‐term effects of these management systems on the density and reproductive success of birds by evaluating 5 years of preharvest and 3 years of postharvest data from MOFEP. Densities of mature forest species declined 24–69% on the control ( i.e., no timber harvest ) sites during post‐treatment years, confounding interpretation of treatment effects. Densities of both Kentucky Warblers ( Oporornis formosus ) and Worm‐eating Warblers ( Helmitheros vermivorus ) increased in treatment sites relative to control sites. Conversely, even‐aged management negatively affected Ovenbird ( Seiurus aurocapillus ) density. Treatment significantly and positively affected density of four of the six early successional species. Densities of Indigo Bunting ( Passerina cyanea ) and Yellow‐breasted Chat ( Icteria virens ) were significantly higher in both even‐aged and uneven‐aged treatments, whereas densities of Prairie Warbler ( Dendroica discolor ) and White‐eyed Vireo ( Vireo griseus ) were significantly higher in even‐aged treatments than in the controls. Nest success rates averaged 29% for all species and did not change significantly from pre‐ to post‐treatment years. Mature forest bird populations declined as trees were removed, but significant changes in nest predation and brood parasitism did not occur. In addition, openings associated with timber removal provided habitats for early successional species. We suggest that a mixed strategy of timber management may be necessary to support the full range of breeding birds in this region.

Analysis of landscape sources and sinks: the effect of spatial pattern on avian demography

To develop theoretical insights into the relationship between spatial pattern and demography, we coupled a spatially structured demographic model with neutral landscape models to investigate how landscape structure affected population persistence and the source–sink potential of landscapes for a generalized, territorial migratory songbird. Four species-types, with different sensitivities to habitat area and edge effects, were simulated on replicated landscapes across a range of habitat abundance (1–90%) and fragmentation or spatial contagion (random, fractal with minimal contagion, and fractal with maximum contagion). For each species-type in each landscape, the expected number of female offspring produced per female (fecundity, b) was modeled as an explicit function of habitat area and spatial structure (patch edge-to-area ratio). Fecundity estimates (b) were combined with survivorship in a life-table analysis to estimate the net lifetime reproductive output (R0) for the population of each landscape. Landscapes for which R0<1 were identified as population sinks, and as potential population sources when R0>1. As expected, reproductive output (R0) was generally highest on fractal landscapes with maximum clumping (minimum fragmentation) and lowest on random landscapes (maximum fragmentation), especially for species with high edge sensitivity. For species with low edge sensitivity, population persistence was unlikely when the landscape had <40% habitat (i.e. R0<1.0 in these landscapes and were population sinks). Thus, thresholds in population persistence could be identified for different species in these landscapes, but the level of habitat required for persistence ranged widely from 5 to 90% depending upon the species’ response to habitat fragmentation (edge sensitivity) and the specific landscape pattern. Our results caution against the adoption of general rules or ‘cookbook prescriptions’ in conservation regarding the amount of habitat required for population persistence (e.g. the ‘20% rule’). Landscape configuration (the ‘details of how habitat is arranged’) can mitigate the effects of habitat loss and enhance population persistence in fragmented landscapes.

SURVIVAL RATES OF A NEOTROPICAL PARROT: IMPLICATIONS FOR LATITUDINAL COMPARISONS OF AVIAN DEMOGRAPHY

Latitudinal variation in avian demography played an important early role in the development of life history theory, especially in the idea of a cost of reproduction. Recent attempts to determine the survivorship of tropical birds with mark-recapture sta- tistics have proved controversial. Here, we use a small neotropical bird, the Green-rumped Parrotlet (Forpus passerinus), as a model system for investigating sources of heterogeneity that might bias interspecific comparisons. Mark-resighting data were collected on 1334 adult parrotlets over a decade. We expected adult survival to be low because this parrot lays a large clutch (mean 5 7 eggs), is a cavity nester, and breeds in a highly seasonal environment. A two-age-class term in local survival was nonsignificant, indicating that an age or transience effect was unimportant. Local survival of males did not vary annually, but 19.3% of the yearly variation in female survival was explained by rates of nest loss during stages when females were incubating or brooding young. The overall local survival rate of parrotlets ( f5 0.565) was identical to temperate hole-nesting species of the same body size but was lower than that of tropical birds that lay smaller clutches. However, we also detected considerable heterogeneity in parrotlet survival. Females and males that were sighted but did not breed comprised a mean 23.5% and 52.9% of our population, respec- tively. Using multistate models, we found that breeders had significantly higher probabilities of local survival ( f5 0.678 vs. 0.486), of retaining their status as breeders ( c5 0.719 vs. 0.279), and of detection ( p 5 0.997 vs. 0.375) than did nonbreeders. Overall, males and females had comparable local survival rates (breeders f5 0.698 vs. 0.658, nonbreeders f5 0.536 vs. 0.436). Our estimates of local survival could be affected by breeding dispersal, but site fidelity of parrotlets was strong: 95% of adults moved ,500 m in consecutive years. A literature review for tropical birds showed that mark-resighting studies usually report return rates based on resightings of breeding or territorial adults, whereas mist net studies rely on recaptures and pool birds of different age and social status in their calcu- lations of local survival. Future studies should attempt to compare subsets of avian pop- ulations that are similar in demography. Because rates of site fidelity and social system may differ among species, these factors must also be considered in interspecific comparisons of avian life histories.

Survival Rates of a Neotropical Parrot: Implications for Latitudinal Comparisons of Avian Demography

Survival Rates of a Neotropical Parrot: Implications for Latitudinal Comparisons of Avian Demography

Avian demography: statistics and ornithology

Underhill, L.G. 1999. Avian demography: statistics and ornithology. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 70 (1): 61–70. Avian demography is defined here as the study of the statistics of bird populations. Ornithology has generated types of data—or been the first discipline to generate them—which do not lend themselves to standard statistical analyses. In these situations, the role of the avian demographer is to develop custom-built statistical methods—the analysis of primary moult is an example of a type of data needing specialised methods and this is discussed in the final sections of the paper. Data from the Southern African Bird Atlas Project is used to present some insights into the biogeography of southern African birds, and to describe the ‘texture’ of bird distributions. The year 1998 marks, approximately, the centenary of the introduction of the European Starling Sturnus vulgaris to Cape Town and the House Sparrow Passer domesticus to Durban, and the paper notes this event and these species.