Geology topics

J. D. Brawn

Publications and source records attributed to J. D. Brawn.

4 recordsLinked to original sources

USGS1999

Demography of forest birds in Panama: How do transients affect estimates of survival rates?

Estimates of annual survival rates of neotropical birds have proven controversial. Traditionally, tropical birds were thought to have high survival rates for their size, but analyses of a multispecies assemblage from Panama by Karr et al. (1990) provided a counterexample to that view. One criticism of that study has been that the estimates were biased by transient birds captured only once as they passed through the area being sampled. New models that formally adjust for transient individuals have been developed since 1990. Preliminary analyses indicate that these models are indeed useful in modelling the data from Panama. Nonetheless, there is considerable interspecific variation and overall estimates of annual survival rates for understorey birds in Panama remain lower than those from other studies in the Neotropics and well below the rates long assumed for tropical birds (i.e. > 0.80). Therefore, tropical birds may not have systematically higher survival rates than temperate-zone species. Variation in survival rates among tropical species suggests that theory based on a simple tradeoff between clutch size and longevity is inadequate. The demographic traits of birds in the tropics (and elsewhere) vary within and among species according to some combination of historical and ongoing ecological factors. Understanding these processes is the challenge for future work.

Book chapter↗

USGS1998

Demography of forest birds in Panama: How do transients affect estimates of survival rates?

Estimates of annual survival rates for a multispecies sample of neotropical birds from Panama have proven controversial. Traditionally, tropical birds were thought to have high survival rates for their size, but analyses by Kart et al. (1990. Am. Nat. 136:277-91) contradicted that view, suggesting tropical birds may not have systematically high survival rates. A persistent criticism of that study has been that the estimates were biased by transient birds captured only once as they passed through the area being sampled. New models that formally adjust for transient individuals have been developed since 1990. Preliminary analyses using these models indicate that, despite some variation among species, overall estimates of survival rates for understory birds in Panama are not strongly affected by adjustments for transients. We also compare estimates of survival rates based on mark-recapture models with observations of colour-marked birds. The demographic traits of birds in the tropics (and elsewhere) vary within and among species according to combinations of historical and ongoing ecological factors. Understanding sources of this variation is the challenge for future work.

Book chapter↗

USGS1995

Demography of birds in a neotropical forest: Effects of allometry, taxonomy, and ecology

Comparative demographic studies of terrestrial vertebrates have included few samples of species from tropical forests. We analyzed 9 yr of mark—recapture data and estimated demographic parameters for 25 species of birds inhabiting lowland forests in central Panama. These species were all songbirds (Order Passeriformes) ranging in mass from 7 to 57 g. Using Jolly—Seber stochastic models for open populations, we estimated annual survival rate, population size, and recruitment between sampling periods for each species. We then explored relationships between these parameters and attributes such as body size, phylogenetic affiliation, foraging guild, and social behavior. Larger birds had comparatively long life—spans and low recruitment, but body size was not associated with population size. After adjusting for effects of body size, we found no association between phylogenetic affiliation and any demographic trait. Ecological attributes, especially foraging guild, were more clearly associated with interspecific variation in all demographic traits. Ant—followers had comparatively long life—spans, but species that participate in flocks did not live longer than solitary species. The allometric associations we observed were consistent with those demonstrated in other studies of vertebrates; thus, these relationships appear to be robust. Our findings that ecological factors were more influential than phylogenetic affiliation contrasts with comparative studies of temperate—zone birds and suggests that the relative importance of environmental vs. historical factors varies geographically.

Ecology↗

USGS1990

Survival rates of birds of tropical and temperate forests: Will the dogma survive?

Survival rates of tropical forest birds are widely assumed to be high relative to the survival rates of temperate forest birds. Much life-history theory is based on this assumption despite the lack of empirical data to support it. We provide the first detailed comparison of survival rates of tropical and temperate forest birds based on extensive data bases and modern capture-recapture models. We find no support for the conventional wisdom. Because clutch size is only one component of reproductive rate, the frequently assumed, simple association between clutch size and adult survival rates should not necessarily be expected. Our results emphasize the need to consider components of fecundity in addition to clutch size when comparing the life histories of tropical and temperate birds and suggest similar considerations in the development of vertebrate life-history theory.

American Naturalist↗