Eli S. Bridge

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.

A test of comparative equilibration for determining non-exchangeable stable hydrogen isotope values in complex organic materials

Comparative equilibration has been proposed as a methodological approach for determining the hydrogen isotopic composition (deltaD) of non-exchangeable hydrogen in complex organic materials, from feathers to blood and soils. This method depends on using homogenized standards that have been previously calibrated for their deltaD values of non-exchangeable H, that are compositionally similar to unknown samples, and that span an appropriate isotopic range. Currently no certified organic reference materials with exchangeable H exist, and so isotope laboratories have been required to develop provisional internal calibration standards, such as the keratin standards currently used in animal migration studies. Unfortunately, the isotope ratios of some samples fall outside the range of keratin standards currently used for comparative equilibration. Here we tested a set of five homogenized keratin powders as well as feathers from Painted Buntings and Dark-eyed Juncos to determine the effects of extrapolating comparative equilibration normalization equations outside the isotopic range of keratin standards. We found that (1) comparative equilibration gave precise results within the range of the calibration standards; (2) linear extrapolation of normalization equations produced accurate deltaD results to approximately 40 per thousand outside the range of the keratins standards used (-187 to -108); and (3) for both homogenized keratin powders and heterogeneous unknown samples there was no difference in variance between samples within and outside the range of keratin standards. This suggested that comparative equilibration is a robust and practical method for determining the deltaD of complex organic matrices, although caution is required for samples that fall far outside the calibration range.

Age-related differences in baseline and stress-induced corticosterone in Florida scrub-jays

In physiological studies of free-living species, it is essential to consider the context of the life history stage at which an individual was observed in order to link measures of physiology with ecological parameters. One such measure that is important to consider is the age of an individual. We tested whether baseline or stress-induced corticosterone levels vary with age in free-living Florida scrub-jays (Aphelocoma coerulescens) during the pre-breeding period. Corticosterone (CORT), the primary avian stress hormone, is released in response to stressful stimuli, and stimulates gluconeogenesis; however, it also serves as a chemical messenger that can influence other physiological processes, reproduction, and behavior. We monitored both baseline CORT levels longitudinally throughout a five-year period and stress-induced CORT responses over a shorter two-year period. We predicted that older jays would have lower baseline CORT levels and a dampened stress response compared to younger birds, as has been shown in other avian species. We found no significant differences in baseline CORT levels with age. We found a decrease in total corticosterone responses to a stressor with age, however, the oldest birds in the population showed greater total corticosterone responses to a stressor. These results may be a product of age-related changes in physiological processes related to the stress response or a result of selection acting on the population, resulting in only the most responsive individuals surviving to old age.

A library of georeferenced photos from the field

A picture is worth a thousand of words, and every day hundreds of scientists, students, and environmentally aware citizens are taking field photos to document their observations of rocks, glaciers, soils, forests, wetlands, croplands, rangelands, livestock, and birds and mammals, as well as important events such as droughts, floods, wildfires, insect emergences, and infectious disease outbreaks. Where are those field photos stored? Can they be shared in a timely fashion to support education, research, and the leisure activities of citizens across the world? What are the financial and intellectual costs if those field photos are lost or not shared? Recently, researchers at the University of Oklahoma developed and released the Global Geo-Referenced Field Photo Library (hereinafter referred to as the Field Photo Library; http://www.eomf.ou.edu/photos/), a Web-based data portal designed for researchers and educators who wish to archive and share field photos from across the world, each tagged with exact positioning data (Figure 1). The data portal has a simple user interface that allows people to upload, query, and download georeferenced field photos in the library.

Mind The Gaps: What's Missing in Our Understanding of Feather Molt

1E-mail: ebridge@ou.edu Manuscript received 15 November 2010; accepted 5 January 2011. The life histories of all birds feature two major events, reproduction and molt, and many species of the temperate and polar zones add migration as a third major energetic and scheduling demand to their annual cycle. Although we are far from a complete understand of any of these phenomena, it seems that for a given species basic molt data are most often lacking. Open almost any field guide and you will find information on the timing and location of breeding and migration for almost all species included. For information on molt one must resort to more detailed species descriptions such as the Birds of North America series or the works of Pyle (1997, 2008), and even these species accounts may lack adequate data. If your species of interest is not included in a series that has descriptions of molt, you will probably find that there are no data at all. A few years ago I reviewed molt strategies in seabirds and found no information on molt for 25% of the 314 species I wanted to include in the review (Bridge 2007). The situation for terrestrial species is similar (Ryder and Wolfe 2009). Birds demonstrate a bewildering array of molting strategies. The diversity of avian molts is such that an active debate over molt terminology continues more than 100 years after the first systematic studies of feather replacement (Dwight 1900, Howell et al. 2003, Thompson 2004, Willoughby 2004, Pyle 2005). For almost every rule we try to impose on the patterns or timing of molt, there are invariably several exceptions. However, one rule that does appear to apply to all species is that there is a physiological limit on the rate at which feathers can grow (Rohwer et al. 2009). All of the raw materials used to synthesize a feather must be conducted through a collar of cells that surrounds the base of the feather follicle (Lucas and Stettenheim 1972). Presumably, this bottleneck limits the rate of individual feathers’ growth regardless of their size when fully grown. If we look across species ranging in mass from a few grams to several kilograms, there is little difference in the rate at which feathers are grown (Rohwer et al. 2009). For instance, a 27-g House Sparrow (Passer domesticus) grows its feathers at a rate of 2.7 mm per day, whereas a 9000-g Whooper Swan (Cygnus cygnus) grows 9 mm of feather per day (Prevost 1983). In other words, we see a 300-fold increase in body mass but only a 3-fold increase in feather-growth rate. This constraint on feather growth is evident when birds are pressured to accelerate molt. Across several taxa, speeding up molt results in feathers of low quality according to several metrics, including mass (Dawson 2004), integrity (Dawson et al. 2000, Serra 2001), and coloration (Serra et al. 2007, Griggio et al. 2009). The effect of rapidly grown plumage has been linked to decreased survival (Nilsson and Svensson 1996, Morales et al. 2007). The limit on feather growth gives rise to some interesting life-history tradeoffs as birds balance the various costs of molt and other activities (e.g., breeding and migration). We often see the most complex molt strategies in large birds, in which the time constraints associated with growing large feathers are most pronounced. For large birds like albatrosses that must not only molt long feathers but maintain their ability to fly over vast distances, molt imposes a heavy toll, which is likely to affect other aspects of their annual cycle, such as breeding. More specifically, one might presume that large, aerial foragers should be forced to reduce their molting effort to breed successfully and vice versa. However, it has been difficult to conclusively identify a clear link between variation in molt and breeding success in large birds. The first study to do so appears in this issue of the Condor. Rohwer et al. (2011) found that Black-footed Albatrosses (Phoebastria nigripes) with more worn feathers are more likely to be unsuccessful in their attempts to breed. The problem for the Black-footed Albatross is that there is insufficient time in its annual cycle for it to both breed and molt enough feathers to offset accumulated feather wear, and as a result some individuals accrue a molt debt that eventually erodes their ability to breed successfully. Hence, periodically birds skip breeding altogether to devote more time to growing feathers and paying off their molt debt. In the Black-footed Albatross, the molt debt is usually borne out in primaries 6 and 7 (P6 and P7). These feathers may be molted just prior to the outermost three primaries, or they may remain unmolted for several years, presumably as a means of reducing the time devoted to molt (Langston and Rohwer 1996; Fig. 1). A key finding in Rohwer et al. (2011) is that if P6 and P7 accumulate 2 or more years of wear, the birds are less capable of rearing offspring. Although Rohwer et al. (2011) have made an important contribution to the ornithological literature, their work also makes evident some of the gaps in our knowledge of feather molt. Why should something as obvious as a molt/breeding tradeoff only now find empirical support? One reason is that, for many species, we lack comprehensive information about feather molt. Without documentation of basic molt data such as timing, location, sequence, intensity, completeness (for species with incomplete molts), and degree of individual variation, comparative studies aimed at describing the origin and maintenance of molt strategies The Condor 113(1):1–4 The Cooper Ornithological Society 2011 COMMENTARY

Sexual Dimorphism, Female-Female Pairs, and Test for Assortative Mating in Common Terns

Abstract We trapped 656 Common Terns (Sterna hirundo) and measured five body dimensions and body mass for each bird; 313 birds were of known age, and 229 were sexed by DNA. Males were larger than females in all five dimensions, but were smaller in body mass. Early-nesting birds were larger than late-nesting birds in all five dimensions: at least for wing length, this difference was related to both laying date and age. Head length (from back of skull to tip of bill) was the most useful measure for sexing Common Terns in the field. Discriminant functions indicated that 75.9% of single birds and 84.5% of pairs could be sexed correctly by head length alone. We present rules and nomograms for field sexing of Common Terns; these provide trade-offs between sensitivity (proportion of birds classified) and specificity (proportion of birds correctly sexed). Three of 80 pairs (4%) included two females: these pairs nested early and were at least as successful as male-female pairs. Within pairs, tarsus lengths were negatively correlated; we found no evidence for positive assortative mating by linear dimensions or body mass. This study confirms some previous reports of sexual dimorphism in this species based on less reliable methods of sexing, but fails to confirm other reports of sexual dimorphism and assortative mating.

WING MOLT AND ASSORTATIVE MATING IN COMMON TERNS: A TEST OF THE MOLT-SIGNALING HYPOTHESIS

Abstract Many seabirds are monomorphic and lack obvious ornamentation; thus the mechanisms and signaling systems that mediate mate choice can be elusive. We investigated the possibility that a unique characteristic of wing molt in most species of Sterna terns acts as a sexually selected indicator of fitness. Many terns replace a variable number of primaries and sometimes secondaries twice or occasionally three times each year. Some have suggested that this repeated wing molt may serve as an honest indicator of fitness in mutual mate choice. If this molt-signaling hypothesis is valid, one would expect there to be assortative mating with respect to the extent of repeated wing molt. We tested this prediction by examining 262 breeding pairs of Common Terns (Sterna hirundo) from colonies in Buzzards Bay, Massachusetts. Using banding records and plumage characters, we were often able to distinguish young birds making their first breeding attempt from older birds which may have maintained past pair bonds. We found evidence of assortative mating with respect to repeated wing molt in newly formed pairs, which supports the notion of wing molt as a sexually selected character. Muda del Ala y Apareamento Asociativo en Sterna hirundo: Una Prueba de la Hipótesis de la Muda-Señalización Resumen. Muchas aves marinas son monomórficas y no tienen ornamentaciones obvias; por tanto, los mecanismos y los sistemas de señales que media la selección de pareja son evasivos. Investigamos la posibilidad de que una característica única de la muda en la mayoría de las especies de charranes (Sterna spp.) funcione como un indicador de la aptitud sexualmente seleccionado. Muchos charranes substituyen un número variable de primarias y a veces de secundarias dos veces, u ocasionalmente tres veces, cada año. Algunos autores han sugerido que esta muda repetida podría servir como un indicador honesto de la aptitud durante la selección mutua de las parejas. Si esta hipótesis de la muda-señalización fuera válida, se esperaría que existiera apareamiento asociativo con respecto al grado de la muda repetida de las alas. Para poner a prueba esta predicción, examinamos 262 parejas reproductivas de Sterna hirundo, en colonias en la Bahía Buzzards, Massachusetts. Usando registros de anillamiento y caracteres del plumaje, pudimos distinguir con frecuencia los charranes jóvenes que hacían su primer intento de aparearse de los más viejos que pudieron haber mantenido enlaces con parejas anteriores. Encontramos evidencia de apareamento asociativo con respecto a la muda repetida de las alas en parejas nuevas, lo que apoya la noción de que la muda del ala sería un caracter seleccionado sexualmente.

A hidden Markov model for reconstructing animal paths from solar geolocation loggers using templates for light intensity

BackgroundSolar archival tags (henceforth called geolocators) are tracking devices deployed on animals to reconstruct their long-distance movements on the basis of locations inferred post hoc with reference to the geographical and seasonal variations in the timing and speeds of sunrise and sunset. The increased use of geolocators has created a need for analytical tools to produce accurate and objective estimates of migration routes that are explicit in their uncertainty about the position estimates.ResultsWe developed a hidden Markov chain model for the analysis of geolocator data. This model estimates tracks for animals with complex migratory behaviour by combining: (1) a shading-insensitive, template-fit physical model, (2) an uncorrelated random walk movement model that includes migratory and sedentary behavioural states, and (3) spatially explicit behavioural masks.The model is implemented in a specially developed open source R package FLightR. We used the particle filter (PF) algorithm to provide relatively fast model posterior computation. We illustrate our modelling approach with analysis of simulated data for stationary tags and of real tracks of both a tree swallow Tachycineta bicolor migrating along the east and a golden-crowned sparrow Zonotrichia atricapilla migrating along the west coast of North America.ConclusionsWe provide a model that increases accuracy in analyses of noisy data and movements of animals with complicated migration behaviour. It provides posterior distributions for the positions of animals, their behavioural states (e.g., migrating or sedentary), and distance and direction of movement.Our approach allows biologists to estimate locations of animals with complex migratory behaviour based on raw light data. This model advances the current methods for estimating migration tracks from solar geolocation, and will benefit a fast-growing number of tracking studies with this technology.

Corticosterone administration does not affect timing of breeding in Florida scrub-jays (Aphelocoma coerulescens).

Providing supplemental food to Florida scrub-jays (Aphelocoma coerulescens) causes a reliable advance in clutch initiation of 1 to 2 weeks. In some years, supplemental food appeared to not only advance laying date but also decrease baseline concentrations of corticosterone (CORT) relative to controls. The coincidence of low CORT levels and early breeding led us to hypothesize that CORT serves to communicate information about environmental conditions to the hypothalamic-pituitary-gonadal (HPG) axis, which ultimately influences the timing of breeding. To test this hypothesis, we administered small oral doses of CORT three times each day to female breeders that were provisioned with supplemental food. We compared clutch initiation dates of the CORT-dosed females to females with supplementation but no exogenous CORT and to females with neither CORT nor supplemental food. CORT administration had a strong temporary effect on circulating CORT concentrations but clutch initiation did not differ between the two groups of supplemented birds, both of which laid eggs approximately 10 days earlier than nonsupplemented birds. Furthermore, during the year of our study we found no reduction in baseline CORT concentrations in our undosed supplemental groups, as had been observed in past studies.

Using geologgers to investigate Bimodal isotope patterns in painted BUntings (PASSERINA CIRIS)

ABSTRACT. Painted Buntings (Passerina ciris) that breed in Oklahoma and molt in Sinaloa, Mexico, demonstrate a clear bimodal pattern of stable isotope ratios in their flight feathers. Some birds had a C3 carbon signature in primary 1 (P1, the first feather replaced during wing molt) and a C4 carbon signature in primary 9 (P9, the last primary to molt), whereas other sympatric birds evinced a C4-based diet throughout feather molt. The bimodal pattern of stable isotope ratios in flight feathers suggests that some birds may initiate molt immediately upon arrival in northwestern Mexico (and carry a C3 signature with them from the breeding grounds) whereas others may delay molt (and grow feathers solely from C4 plants of Sinaloa). From 2010 to 2012, we used geologger tags to test whether differences in the timing and route of fall migration movements were related to stable isotope signatures in primary feathers. We analyzed stable isotopes of hydrogen and carbon in P1 and P9 from 25 individuals fitted with geologger tags in two consecutive years. Of these, 60% changed the diet (C3 vs. C4) that was used to grow P1 between years. We also observed variation among individuals in migration routes, wherein birds from the same breeding population differed greatly in their use of molting and wintering locations. However, we did not find a relationship between isotope signatures and the timing or route of fall migration. We speculate that the bimodal isotope signature we observed represents a carryover effect related to local landscapes (grassland or agriculture vs. shrubland) used during the late breeding season and early molting period, and that these effects diminish as molt progresses. If this is the case, there is the potential for breeding-season diet to directly affect plumage quality in this molt migrant.