W. Douglas Robinson

Technology on the Move: Recent and Forthcoming Innovations for Tracking Migratory Birds

Basic questions about the life histories of migratory birds have confounded scientists for generations, yet we are nearing an era of historic discovery as new tracking technologies make it possible to determine the timing and routes of an increasing number of bird migrations. Tracking small flying animals as they travel over continental-scale distances is a difficult logistical and engineering challenge. Although no tracking system works well with all species, improvements to traditional technologies, such as satellite tracking, along with innovations related to global positioning systems, cellular networks, solar geolocation, radar, and information technology are improving our understanding of when and where birds go during their annual cycles and informing numerous scientific disciplines, including evolutionary biology, population ecology, and global change. The recent developments described in this article will help us answer many long-standing questions about animal behavior and life histories.

ARTIFICIAL BIRD NESTS, EXTERNAL VALIDITY, AND BIAS IN ECOLOGICAL FIELD STUDIES

We review an increasingly commonly used experimental approach in avian ecological studies that typifies the difficulties of maximizing two major components of experimental design: internal and external validity. Avian ecologists estimate reproductive success by measuring the rate at which nests in specified treatment groups fledge or fail. Because real bird nests are time-consuming and difficult to find and are rarely dispersed across treatments in sufficient numbers for powerful statistical analyses, investigators often utilize artificial nests, which are human-constructed surrogates baited with eggs and distributed in the field across treatments of interest. The primary advantage of artificial nests is that investigators can control the distribution and numbers of nests. The underlying assumption of similar predation rates on artificial and real nests is almost never tested. In the few studies that directly compare predation of artificial and real nests, however, external validity (or the degree to which results of an experiment can be generalized to subjects other than the experimental sample) is poor: predation of artificial nests differs from that of real nests in unpredictable and inconsistent directions. A primary reason for discrepancies may be that real and artificial nests attract different predators. We conclude that past protocols for use of artificial nests should be abandoned unless investigators carefully demonstrate minimal bias between measures of predation on real and artificial nests.

Integrating concepts and technologies to advance the study of bird migration

Recent technological innovation has opened new avenues in migration research - for instance, by allowing individual migratory animals to be followed over great distances and long periods of time, as well as by recording physiological information. Here, we focus on how technology - specifically applied to bird migration - has advanced our knowledge of migratory connectivity, and the behavior, demography, ecology, and physiology of migrants. Anticipating the invention of new and smaller tracking devices, in addition to the ways that technologies may be combined to measure and record the behavior of migratory animals, we also summarize major conceptual questions that can only be addressed once innovative, cutting-edge instrumentation becomes available.

Interspecific associations between circulating antioxidant levels and life-history variation in birds

Antioxidants play an important role in protecting tissues against aging‐associated oxidative damage and are thus prime candidates for relating physiological mechanisms to variation in life histories. We measured total antioxidant capacity, antioxidant response to stress, and levels of uric acid, vitamin E, and four carotenoids in 95 avian species, mostly passerines from Michigan or Panama. We compared antioxidant measures to seven variables related to life histories (clutch size, survival rate, incubation period, nestling period, basal metabolic rate, body mass, and whether the species lived in a tropical or temperate climate). Life‐history‐related traits varied over at least three statistically independent axes. Higher antioxidant levels were generally characteristic of more rapid development, lower survival rate, smaller body size, larger clutch size, and higher mass‐adjusted metabolic rate, but the relationships of particular antioxidants with individual life‐history traits showed considerable complexity. Antioxidant–life history associations differed between tropical and temperate species and varied with respect to taxonomic sampling. Vitamin E showed few relationships with life‐history traits. Overall, our results partly support the hypothesis that antioxidant levels evolve to mirror free radical production. Clearly, however, the complex patterns of physiological diversification observed here result from the interplay of many factors, likely including not just investment in somatic maintenance but also phylogenetic constraint, diet, and other aspects of ecology.

REPRODUCTIVE SEASONALITY OF SEVEN NEOTROPICAL PASSERINE SPECIES

Abstract We investigated gonad sizes, nesting patterns, and plasma hormone concentrations of seven Neotropical passerine species in a lowland tropical rainforest (Panama) at 9°N latitude over at least one annual cycle. All species had entirely regressed gonads for several months of the year (October to January), coinciding with the end of heavy rainfall. Gonadal recrudescence started in the dry season between January and March. Nesting began earlier (March) for open-habitat species than for forest-interior species (April), and lasted until August or December, respectively. Species differed widely in maximal relative testes sizes and reproductive hormone concentrations, but both measures were lower than in most temperate-zone species. Although we cannot provide a phylogenetically controlled comparison, polygynous and group-living species appeared to have larger testes than monogamous species, possibly indicating sperm competition. Testosterone concentrations were highest in polygynous species, intermediate in socially monogamous species, and lowest in a group-living species, indicating that testosterone is more influenced by social systems than by gonad cycles. Tropical birds may generally maintain low testosterone concentrations and relatively small gonads to decrease overall energy expenditure compared to temperate-zone relatives. Estacionalidad Reproductiva de Siete Especies Passeriformes Neotropicales Resumen. Investigamos los tamaños de las gónadas, los patrones de nidificación y las concentraciones hormonales plasmáticas de siete especies de aves paserinas Neotropicales en una selva lluviosa tropical de tierras bajas en Panamá (9°N) durante por lo menos un ciclo anual. Todas las especies tenían gónadas enteramente retraídas por varios meses del año (octubre a enero), coincidiendo con el fin del periodo de precipitación lluviosa pesada. El crecimiento gonadal comenzó en la estación seca, entre enero y marzo. La nidificación comenzó primero en las especies de hábitat abierto (marzo) que en las especies de interior de bosque (abril) y duró hasta agosto y diciembre, respectivamente. Las especies difirieron extensamente en los tamaños relativos máximos de las gónadas y en las concentraciones de la hormona reproductiva, aunque ambas medidas fueron menores que para la mayoría de las especies de la zona templada. Aunque no podemos proveer una comparación controlada por filogenia, las especies poliginias y las especies que viven en grupo tenían testículos más grandes que las especies monógamas, lo que posiblemente podría estar indicando competencia espermática. Las concentraciones de testosterona fueron mayores en las especies poliginias, intermedias en las socialmente monógamas, y menores en las especies que viven en grupo, indicando que la testosterona estaría más influenciada por el sistema social que por los ciclos de las gónadas. Las aves tropicales podrían generalmente mantener niveles bajos de testosterona y tamaños relativamente pequeños de gónadas con el fin de disminuir los gastos energéticos totales en comparación a las especies emparentadas de las zonas templadas.

Gulliver travels to the fragmented tropics: geographic variation in mechanisms of avian extinction

Irrespective of geography, forest destruction and fragmentation lead to lower avian species richness. The underlying mechanisms causing local extirpations have been studied most thoroughly in northern temperate landscapes, where higher levels of brood parasitism, nest predation, and possibly decreased food availability are responsible for the loss of some species. Tropical landscapes are being similarly altered, but studies of responses by tropical birds remain relatively scarce. Predicting how tropical birds respond to habitat loss and fragmentation should not be extended directly from the results of temperate investigations. Tropical birds possess different evolutionary and life histories, which make them vulnerable to a different suite of threats than those normally considered for birds from temperate regions. These same traits, including greater physiological and sensory specialization, reduced dispersal capabilities, and much lower local and regional population densities, indicate that strategies for...

The Status of the Panama Canal Watershed and Its Biodiversity at the Beginning of the 21st Century

P is a small Central American country, but it operates a big canal and the world keeps an eye on developments there. Problems with the canal or ecological disasters in its watershed would attract a lot of attention. As Theodore Roosevelt planned, the Panama Canal is a major shipping corridor, transporting 37 ships a day and providing substantial income to the Panamanian government. Yet as Roosevelt could not have recognized—despite his interest in conservation—the canal sits in the center of one of the world’s most biologically diverse areas (Myers et al. 2000): Southern Central America has more forest bird species than any other region in the world, except Amazonia and the northern and central Andes, each of which is vastly larger than southern Central America (Stotz et al. 1996); and Panama has as many plant species per 10,000 km2 as any region in the world, more than Amazonia or the Malay Peninsula (Barthlott et al. 1996). Roosevelt may have suspected, though, that forests are crucial for protecting the water supply of the Panama Canal and for maintaining the plant and animal communities. Fortunately, the year 2000 still found extensive forests around the canal, protected largely thanks to military and shipping interests, but it also found the watershed adjacent to a large and expanding capital city. Maintaining the ecosystem integrity of the canal will pose a major challenge for conservation in the 21st century. Is urban and economic development compatible with a hydrologically functioning canal and conservation of an extremely diverse flora and fauna?

Serum antioxidant levels in wild birds vary in relation to diet, season, life history strategy, and species

Micronutrient antioxidants are thought to be generally important for health in many animals, but factors determining levels in individuals and species are not well understood. Diet and season are obvious environmental variables that might predict the degree to which species can accumulate such nutrients. We analyzed antioxidant levels [Trolox-equivalent antioxidant capacity (TEAC), uric acid (UA), vitamin E, and four carotenoids] in 95 bird species and compared these to species-level data on diet from the literature. Using compositional principal components analysis, we identified two main axes of diet variation: invertebrate consumption and seed-to-fruit ratio. We then examined associations between diet axes and antioxidant measures, with and without control for life-history variation and phylogeny. We also analyzed a subset of 13 species for which we had data on seasonality of antioxidant levels and diet, assessing the variance in antioxidant levels explained by seasonality, diet, and species. Unsurprisingly, there were strong associations between antioxidant levels and diet. TEAC and UA concentration were consistently positively associated with invertebrate consumption and seed-to-fruit ratio, and carotenoid concentrations (e.g. zeaxanthin and β-carotene) were negatively associated with invertebrate consumption. However, vitamin E was not associated with diet as measured here. Importantly, there is much variation in antioxidants that is not explained by diet, and we are able to identify diet-independent effects of species, season/breeding stage, and life history on antioxidant levels. Circulating antioxidant concentrations within and across species can therefore be viewed as a function of multiple factors, including but not limited to diet, and antioxidant metabolism appears to differ across species and seasons irrespective of diet.

Conifer regeneration in stand-replacement portions of a large mixed-severity wildfire in the Klamath-Siskiyou Mountains

Large-scale wildfires (*10 4 -10 6 ha) have the potential to eliminate seed sources over broad areas and thus may lead to qualitatively different regeneration dynamics than in small burns; however, regeneration after such events has re- ceived little study in temperate forests. Following a 200 000 ha mixed-severity wildfire in Oregon, USA, we quantified (1) conifer and broadleaf regeneration in stand-replacement patches 2 and 4 years postfire; and (2) the relative importance of isolation from seed sources (live trees) versus local site conditions in controlling regeneration. Patch-scale conifer regener- ation density (72%-80% Douglas-fir (Pseudotsuga menziesii (Mirb). Franco)) varied widely, from 127 to 6494 stemsha -1 . Median densities were 1721 and 1603 stemsha -1 2 and 4 years postfire, respectively, i.e., *12 times prefire overstory densities (134 stemsha -1 ). Because of the complex burn mosaic, *58% of stand-replacement area was £200 m from a live-tree edge (seed source), and *81% was £400 m. Median conifer density exceeded 1000 stemsha -1 out to a distance of 400 m from an edge before declining farther away. The strongest controls on regeneration were distance to live trees and soil parent material, with skeletal coarse-grained soils supporting lower densities (133 stemsha -1 ) than fine-grained soils (729-1492 stemsha -1 ). Other site factors (e.g., topography, broadleaf cover) had little association with conifer regen- eration. The mixed-severity fire pattern strongly influenced the regeneration process by providing seed sources throughout much of the burned landscape.

INFLUENCE OF FIRE ON BACHMAN'S SPARROW, AN ENDEMIC NORTH AMERICAN SONGBIRD

Abstract Bachmans sparrow (Aimophila aestivalis), a near endemic songbird of the longleaf pine (Pinus palustris) ecosystem, is known to respond positively to prescribed fires. The influence of season (growing vs. dormant) and frequency (1 to ≥4 yr since burning) of fire on density of Bachmans sparrows, however, is poorly understood. We examined effects of fire on density of Bachmans sparrows in longleaf pine forests at the Conecuh National Forest, Alabama, and Blackwater River State Forest, Florida, USA. Density of Bachmans sparrows was greater the first 3 years after burning than ≥4 years after burning, and season of burning had little effect on the density of Bachmans sparrows. Percent coverage by grass had a greater influence on density of Bachmans sparrows than either season or frequency of burning. Percent canopy cover had a strong negative effect on coverage of grass but had a weaker effect on grass at stands burned frequently during the growing season. Growing-season fires (Apr–Sep) did not adversely affect density of Bachmans sparrows. Results from our study suggest that management and restoration of longleaf pine communities probably can be accomplished best by burning on a 2–3-year rotation during the growing season, when most fires historically occurred. Suppression of fire, or burning at intervals >4–5 years, will greatly reduce or eliminate habitat required by Bachmans sparrows.