A controversy exists in the Pacific Northwest of the United States between logging of old-growth coniferous forests and conservation of Northern Spotted Owl (Strix occidentalis caurina) populations. This species has a strong association with old-growth forests that also have economic value as timber. Research questions relevant to conservation of this species include how temporal trends in Northern Spotted Owl populations are influenced and how spatial configuration of old-growth forests affects these populations. To address these questions, we studied a population of marked Northern Spotted Owls on 95 territories in northwestern California from 1985 through 1994. We examined the mag- nitude of temporal and spatial variation in life history traits (survival, reproductive output, and recruitment), the effects of climate and landscape characteristics on temporal and spatial variation in these traits, respectively, and how this variation affected aspects of population dynamics. We used a components-of-variation analysis to partition sampling from process variation, and a model selection approach to estimate life history traits using capture- recapture and random-effects models. Climate explained most of the temporal variation in life history traits. Annual survival varied the least over time, whereas recruitment rate varied the most, suggesting a ''bet-hedging'' life history strategy for the owl. A forecast of annual rates of population change ( l), estimated from life history traits, suggested that Northern Spotted Owl populations may change solely due to climate influences, even with unchanging habitat conditions. In terms of spatial variation, annual survival on territories was positively associated both with amounts of interior old-growth forest and with length of edge between those forests and other vegetation types. Reproductive output was nega- tively associated with interior forest, but positively associated with edge between mature and old-growth conifer forest and other vegetation types. A gradient existed in territory- specific estimates of fitness derived from these life history estimates. This gradient suggested that a mosaic of older forest interspersed with other vegetation types promoted high fitness in Northern Spotted Owls. Habitat quality, as defined by fitness, appeared to buffer variation in annual survival but did not buffer reproductive output. We postulated that the magnitude of l was determined by habitat quality, whereas variation of l was influenced by recruitment and reproductive output. As habitat quality declines, variation in l should become more pronounced.

/pdf/climate-habitat-quality-and-fitness-in-northern-spotted-owl-53cuekoyx8.pdf

Climate, habitat quality, and fitness in northern spotted owl populations in northwestern california

Why do the young of cooperative breeders--species in which more than two individuals help raise offspring at a single nest--delay dispersal and live in groups? Answering this deceptively simple question involves examining the costs and benefits of three alternative strategies: (1) dispersal and attempting to breed, (2) dispersal and floating, and (3) delayed dispersal and helping. If, all other things being equal, the fitness of individuals that delay dispersal is greater than the fitness of individuals that disperse and breed on their own, intrinsic benefits are paramount to the current maintenance of delayed dispersal. Intrinsic benefits are directly due to living with others and may include enhanced foraging efficiency and reduced susceptibility to predation. However, if individuals that disperse and attempt to breed in high-quality habitat achieve the highest fitness, extrinsic constraints on the ability of offspring to obtain such high-quality breeding opportunities force offspring to either delay dispersal or float. The relevant constraint to independent reproduction has frequently been termed habitat saturation. This concept, of itself, fails to explain the evolution of delayed dispersal. Instead, we propose the delayed-dispersal threshold model as a guide for organizing and evaluating the ecological factors potentially responsible for this phenomenon. We identify five parameters critical to the probability of delayed dispersal: relative population density, the fitness differential between early dispersal/breeding and delayed dispersal, the observed or hypothetical fitness of floaters, the distribution of territory quality, and spatiotemporal environmental variability. A key conclusion from the model is that no one factor by itself causes delayed dispersal and cooperative breeding. However, a difference in the dispersal patterns between two closely related species or populations (or between individuals in the same population in different years) may be attributable to one or a small set of factors. Much remains to be done to pinpoint the relative importance of different ecological factors in promoting delayed dispersal. This is underscored by our current inability to explain satisfactorily several patterns including the relative significance of floating, geographic biases in the incidence of cooperative breeding, sexual asymmetries in delayed dispersal, the relationship between delayed dispersal leading to helping behavior and cooperative polygamy, and the rarity of the co-occurrence of helpers and floaters within the same population. Advances in this field remain to be made along several fronts.(ABSTRACT TRUNCATED AT 400 WORDS)

The Evolution of Delayed Dispersal in Cooperative Breeders

The evolutionary causes of small clutch sizes in tropical and Southern Hemisphere regions are poorly understood. Alexander Skutch proposed 50 years ago that higher nest predation in the south constrains the rate at which parent birds can deliver food to young and thereby constrains clutch size by limiting the number of young that parents can feed. This hypothesis for explaining differences in clutch size and parental behaviors between latitudes has remained untested. Here, a detailed study of bird species in Arizona and Argentina shows that Skutch's hypothesis explains clutch size variation within North and South America. However, neither Skutch's hypothesis nor two major alternatives explain differences between latitudes.

/pdf/parental-care-and-clutch-sizes-in-north-and-south-american-3tnwnwqre2.pdf

Parental care and clutch sizes in North and South American birds.

Why have birds in the woodlands of Southern Australia declined

The factors affecting the population dynamics of seabirds have long intrigued biologists. Current data suggest that density-dependent depletion of prey during the breeding season may regulate population size. However, much of the evidence for this has been circumstantial, and the underlying mechanisms are unclear. Here, we show that the per capita population growth rates of northern gannet Morus bassanus at colonies in Britain and Ireland have declined with increasing population size. Furthermore, direct observations reveal that the mean foraging trip duration of breeding gannets is positively correlated with colony size, both among colonies of different sizes in the same year, and within colonies as they change in size. To understand this phenomenon, we have developed a model which demonstrates that disturbance of fish alone can readily generate conditions under which gannets at larger colonies have to travel further to obtain food.

Evidence of intra-specific competition for food in a pelagic seabird

Ian Newton: Population limitation in birds of prey: a comparative approach Ian Rowley & Eleanor Russell: Demography of Passerines in the temperate Southern Hemisphere Jacques Blondel: Birds in biological isolates J. Clobert & J-D. Lebreton: Estimation of demographic parameters in bird populations J-D. Lebreton & J. Clobert: Bird population dynamics, management, and conservation R.H. McCleery & C.M. Perrins: The effects of predation on the numbers of Great Tits Parus major James N.M. Smith, Peter Arcese & Wesley Hochachka: Social behaviour and population regulation in insular bird populations Fred Cooke & E.G. Gooch: Demographic changes in a snow goose population C.M. Perrins: Constraints on the demographic parameters of bird populations Franz Bairlein: Population studies of White Storks Ciconia ciconia in Europe M.P. Harris & S. Wanless: Population studies and conservation of Puffins Fratercula arctica A.R. Johnson, R.E. Green and G.J.M. Hirons: Survival rates of greater flamingoes in the West Mediterranean region J.P. Croxall & P. Rothery: Population regulation of seabirds Pierre Jouventin & Henri Weimerskirch: Changes in seabird population sizes in French Antarctic territories Carlos Bernstein, John R. Krebs & Alex Kacelnik: Distribution of birds amongst habitats P.R. Evans: Seasonal and annual patterns of mortality in migratory shorebirds G.R. Potts & N.J. Aebischer: Modelling the population dynamics of the Grey Partridge A.P. Dobson & Robert M. May: Parasites, cuckoos and avian population dynamics Peter J. Hudson & A.P. Dobson: Control of parasites in natural populations Peter H. Becker: Population and contamination studies in coastal birds Chris J. Feare: Control of bird pest populations J.C. Coulson: The population dynamics of Herring Gulls and Lesser black-backed Gulls James D. Nichols: Responses of North American duck populations to exploitation Hugh Boyd: Science and craft in waterfowl management in North America Glen Woolfenden & John W. Fitzpatrick: Florida Scrub Jay ecology and conservation Russell Lande: Population dynamics and extinction in heterogenous environments M.R.W. Rands: Conserving threatened birds R.E. Green & G.J.M. Hirons: The relevance of population studies to the conservation of threatened birds.

Bird population studies : relevance to conservation and management

Plate 1 . The fi rst wild photograph of Black-browed Babbler from an ongoing OBC

G. J. M. Hirons

Papers

Bird population studies : relevance to conservation and management

conservation of avifauna and habitat at Gadi-Siraichuli Forest Area,