Seasonal stress physiology and body condition differ among co-occurring tropical finch species

Seasonal changes in avian hormonal stress responses and condition are well known for common species found at temperate and arctic latitudes, but declining and tropical species are poorly studied. This study compares stress and condition measures of co-occurring declining and non-declining tropical grass finch species in Australia. We monitored declining Gouldian finches (Erythrura gouldiae) and non-declining long-tailed and masked finches (Poepila acuticauda and P. personata) during two seasons that are potentially stressful: peak breeding (early dry season when food is plentiful) and moult (late dry to early wet season when food may be scarce). We measured body condition (muscle and fat), haematocrit, and stress response to capture using plasma corticosterone and binding globulin concentrations. All species had higher muscle and lower fat indices during breeding than moult. Haematocrit did not consistently differ between seasons. Long-tailed finches had higher stress responses during breeding than moult, similar to other passerines studied. Masked finches showed no seasonal changes in stress response. Gouldian finches had stress response patterns opposite to those of long-tailed finches, with higher stress responses during moult. However, seasonal trends in Gouldian and long-tailed finch stress responses sometimes differed between years or sites. The differences in stress response patterns between species suggest that the declining Gouldian finch is more sensitive to recent environmental changes which are thought to further reduce grass seed food resources during the late dry to early wet season. Retention of stress responsiveness during a protracted moult could increase the survival potential of Gouldian finches. This study highlights the utility of stress and condition indices to determine the sensitivity of co-occurring species to environmental conditions.     

High rates of extra‐pair paternity in two equatorial populations of rufous‐collared sparrow,Zonotrichia capensis

The latitudinal increase in extra‐pair paternity (EPP) rates in birds suggests broad selective benefits to low EPP rates in the tropics. However, we have few EPP data from tropical birds, particularly from species with close relatives at high latitudes. Here, we report EPP rates in two resident equatorial populations of rufous‐collared sparrow Zonotrichia capensis, a genus well‐represented at high latitudes. We found 64% and 60% of broods contained extra‐pair offspring, and 42% and 52% of all young were extra‐pair. EPP rates were similar in these populations, despite clear differences in elevation, temperature, rainfall, and breeding season length. These findings provide evidence that EPP rates in tropical birds can be as high as those observed in temperate birds, and suggest that the selective pressures acting on EPP rates vary markedly across tropical birds.     

Modulation of androgens in southern hemisphere temperate breeding sparrows (Zonotrichia capensis): an altitudinal comparison

Most work investigating modulation of testosterone (T) levels in birds has focused on northern temperate and Arctic species, and to a lesser degree, tropical species. Studies exploring modulation of T in birds in temperate regions of the Southern Hemisphere are lacking. Here we explore patterns of T secretion across the breeding season in two populations of temperate Zonotrichia capensis in Chile, located only 130 km apart, but separated by 2000 m in elevation. We then compared these T profiles to those of conspecifcs in the tropics and congeners in northern zones. We measured baseline T levels during pre-breeding in lowland Z. c. chilensis, early breeding in highland Z. c. chilensis and mid-breeding in both populations. We also tested for social stimulation of T secretion during mid-breeding in both populations. Lastly, we challenged the hypothalamic-pituitary-gonadal (HPG) axis of the lowland population with gonadotropin-releasing hormone (GnRH) to determine maximum possible T production. We found that the highland population adjusted T secretion across the breeding season like northern species. Neither Z. c. chilensis population modulated T in response to social stimuli, nor the HPG axis of the lowland population was not maximally active during either pre- or mid-breeding. These results suggest that patterns of circulating T in the highland population of Z. c. chilensis in the Southern Hemisphere are similar to congeners in the temperate Northern Hemisphere, but those of the lowland population of Z. c. chilensis are not, and are more similar to conspecifics breeding in the tropics.     

Changes in pituitary and adrenal sensitivities allow the snow bunting (Plectrophenax nivalis), an Arctic-breeding song bird, to modulate corticosterone release seasonally

Several free-living avian species have recently been shown to seasonally modulate corticosterone release in response to capture and restraint. We examined possible mechanisms underlying seasonal adrenocortical modulation in snow buntings (Plectrophenax nivalis), a species that breeds and molts (the energetically costly replacement of feathers) in the Alaskan Arctic. Snow buntings dramatically reduced baseline and maximal corticosterone titers during molt compared to the breeding season. This effect is not explained by changes in either corticosterone binding protein capacity or the overall condition of the bird (assessed by weight and fat storage). Although the adrenal's capacity to secrete corticosterone is reduced during molt, adrenal insensitivity does not fully explain reduced maximal output since exogenous adrenocorticotropic-hormone enhanced corticosterone release during both seasons. In contrast, no exogenous adrenocorticotropic hormone releasing factor (corticotrophin-releasing factor, arginine vasotocin or mesotocin) enhanced corticosterone secretion during molt. This suggests that the pituitary's endogenous adrenocorticotropic secretion was maximal in response to capture and handling, making the pituitary an important site regulating corticosterone levels. Taken together, these results indicate that seasonal modulation of corticosterone release in this species is controlled at both the adrenal and pituitary glands. Accepted: 16 February 1998     

Patterns of seasonal and yearly mass variation in West African tropical savannah birds

Birds in the northern hemisphere usually increase mass reserves in response to seasonal low temperatures and shorter day length that increase foraging unpredictability and so starvation risk. In the lowland tropics, relatively low temperatures and short day lengths are absent and so the risk of starvation may be reduced, leading to much smaller seasonal effects on mass. Nevertheless, other factors such as high temperatures and water and food availability may vary greatly between tropical wet and dry seasons, leading to variable starvation risk and seasonal mass effects. Using data collected from 47 species of birds caught over a 10‐year period in a tropical savannah region in West Africa we tested for seasonal variation in mass in response to a predictable, strongly seasonal tropical climate. Many species (91%) showed seasonal variation in mass, and this was often in a clear annual pattern that was constant across the years. Many species (89%) varied their mass in response to seasonally predictable rainfall. Annual variation in mass was also important (45% of species). Relatively few species (13%) had a seasonal pattern of mass variation that varied between years. Feeding guild or migratory status was not found to affect seasonal or annual mass variation. Seasonal mass change was on average 8.1% across the 21 species with a very large sample size and was comparable with both northern and southern temperate species. Our study showed that biologically significant consistent seasonal mass variation is common in tropical savannah bird species, and this is most likely in response to changing resource availability brought about by seasonal rainfall and the interrupted foraging response due to the constraints of breeding.     

Seasonal variability in survivorship of a cooperatively breeding tropical passerine

Survival of tropical passerines is thought to be higher than those in northern temperate regions, but relatively few tropical studies have addressed this issue, particularly in tropical Asia. We examined factors that may have influenced the survival rate of a cooperatively breeding bird, the puff-throated bulbul (Alophoixus pallidus), in an evergreen forest in northeastern Thailand. These factors included year, season (breeding and non-breeding), sex, and presence of helper(s) in a family group. We present evidence of breeding season-dependent survival in a tropical passerine using an information theoretic approach based on both mark-recapture and resighting data collected during 6 years of study. Based on colour-banded adults the annual survival rate did not vary significantly among years (average = 0.85 ± 0.02 SE). The mean lifespan (MLS) for the population was 6.22 ± 4.38 SE years. Survivorship was lower during the breeding season (0.89 ± 0.02 SE) than during the non-breeding season (0.96 ± 0.02 SE). The MLS of males and females was 6.70 ± 7.73 SE and 5.87 ± 4.88 SE years, respectively. The annual survival rate we observed was high compared to the estimates of other tropical and temperate passerines, possibly due to the relatively stable climatic conditions in tropical latitudes and puff-throated bulbuls being generalists that exploit a wide range of food resources both in space and time.     

Testosterone and corticosterone during the breeding cycle of equatorial and European stonechats (Saxicola torquata axillaris and S. t. rubicola)

Northern-temperate male birds show seasonal changes in testosterone concentrations with a peak during the breeding season. Many tropical birds express much lower concentrations of testosterone with slight elevations during breeding. Here we describe testosterone and corticosterone concentrations of male stonechats from equatorial Kenya during different substages of breeding and molt. This tropical species has a short breeding season of approximately 3 months. We compare their hormone concentrations to previously published data of males of a northern-temperate relative, the European stonechat, also a seasonal breeder but with a breeding season of approximately 5 months. Equatorial stonechats show a pronounced peak of testosterone during the nest-building and laying stage. During all other stages, testosterone concentrations are low, similar to other year-round territorial tropical bird species. Corticosterone concentrations peak also during the nest-building and laying stage suggesting that this period of maximum female fecundity is a demanding period for the male. Equatorial stonechats have significantly lower concentrations of testosterone than European stonechats during all stages, except during the nest-building and laying stage. During this stage of maximum female fertility, testosterone levels tend to be higher in equatorial than in European stonechats. Our results suggest that equatorial stonechats belong to a group of tropical bird species that are characterized by a short breeding season and a brief high peak of testosterone during the female's fertile period. Such brief, but substantial peaks of testosterone may be common in tropical birds, but they may easily be missed if the exact breeding stage of individual birds is not known.     

Monthly survival of African Sylvia warblers in a seasonally arid tropical environment

Latitudinal gradients of life-history traits in animals are thought to be shaped by environmental variables. For example, it has been suggested that the increase in avian clutch size from the tropics towards the northern temperate regions is caused by a reduced survival of adult birds in the north due to increasing environmental seasonality. However, the tropical savannahs of East Africa show pronounced seasonality in resources caused by distinct rainy and dry seasons. This raises the question of whether survival and other life-history traits of birds living in these tropical savannahs are influenced by this seasonality, making them more similar to northern temperate species. We used 2-year monthly resighting data, a multistate modelling approach and the program MARK to test whether survival, transition probabilities between breeding states and other life-history traits of two resident Kenyan Sylvia species (Aves: Passeriformes: Sylviidae) are shaped by seasonality of rainfall in their environment. Contradicting our hypotheses, the two species showed only very slight influence of seasonality of rainfall on their survival. Survival in the dry months was hardly lower than in the rainy months. The species in the more seasonal environment ( S. boehmi , annual survival 71%) survived as well as the one in the more constant environment ( S. lugens , 56%). The observed survival rates correspond well to other life-history traits of the two species and are of similar magnitude to survival rates of other tropical passerines. This implies that either seasonality is not the driving force behind the life-history traits of the two species or the birds do not experience their environment as seasonal, as might be suggested by fluctuations in rainfall.     

Steroid hormone interrelationships with territorial aggression in an Arctic-breeding songbird,Gambel's white-crowned sparrow,Zonotrichia leucophrys gambelii

The breeding season is very brief for arctic-breeding passerines, and any interruptions of parental care by aggressive interactions over territory may reduce reproductive success. We tested both the "testosterone insensitivity" and "corticosterone insensitivity" hypotheses in the arctic-breeding Gambel's white-crowned sparrow, Zonotrichia leucophrys gambelii. Additionally, we tested whether simulated territorial intrusions (STIs), known to stimulate increases in luteinizing hormone (LH) and testosterone (T) in mid-latitude breeding Z. l. pugetensis, would also be effective in either the early or late phases of the brief breeding season of Z. l. gambelii. Plasma levels of T and LH were high early in the breeding season and declined as egg laying began. Exposure of free-living males to 10 min of STI significantly increased LH but not T secretion. Nonetheless, the pituitary-gonadal axis is sensitive as jugular injection of gonadotrophin-releasing hormone increased plasma T at 10 min relative to saline-challenged controls. T implants failed to increase territorial aggression following STI during incubation. These data are consistent with the T insensitivity hypothesis and contrast sharply with the response of the southerly breeding subspecies, Z. l. pugetensis, in which the territorial response to T administration is retained throughout its relatively long breeding season. However, corticosterone implants during the incubation period decreased territorial aggression during STI. This responsiveness to corticosterone is not consistent with the corticosterone insensitivity hypothesis of stress modulation. Z. l. gambelii retain sensitivity to corticosterone levels that may occur naturally in response to environmental perturbations resulting in suppression of territorial behavior.     

The phenology of molting,breeding and their overlap in central Amazonian birds

The energetically challenging periods of molting and breeding are usually temporally separated in temperate birds, but can occur simultaneously in tropical birds, a condition known as molt–breeding overlap. Here, we document great variation in the timing and duration of molting and breeding, and in the extent of molt–breeding overlap, among 87 species of understory passerines in central Amazonia. We analyzed molt and breeding from 26 871 birds captured over a 30‐yr period near Manaus, Brazil. Although most species typically bred during the late dry season (about October through January), many thamnophilids apparently bred year‐round, whereas a few other species from a variety of families bred mainly during the wet season (about January through May). Of all breeding birds with an active brood patch, 12.7% were simultaneously molting. Molt–breeding overlap was more frequently observed among suboscines (13.3%), especially thamnophilids (23.0%), than oscines (6.4%). Some families had <5% molt–breeding overlap frequency, including Tyrannidae (4.4%), Tityridae (0.0%), Pipridae (1.5%), Turdidae (0.0%), and Thraupidae (0.0%), indicating that not all tropical species exhibit molt–breeding overlap. Among 31 well‐sampled species (n ≥15 brood patches), variation in molt–breeding overlap frequency was positively correlated with each species’ average duration of flight feather replacement (range 98–301 d). We also measured feather growth rates of individual birds in nine species; in five of these, slower‐growing feathers increased with an individual's probability of having molt–breeding overlap. Among furnariids, molt–breeding overlap occurred either at the beginning or end of the molt cycle, suggesting that physiological mechanisms typically separate molting from breeding. Thamnophilids showed a much different pattern; molt–breeding overlap occurred at any stage of feather replacement, apparently not regulated to be independent of breeding. These results reveal substantial life‐history variation among Amazonian birds. Future work to resolve the physiological regulation of molting and breeding in tropical birds will greatly contribute to understanding these patterns and their relevance to avian diversity.     

Reproductive asynchrony and population divergence between two tropical bird populations

High-latitude vertebrates generally breed seasonally and synchronouslyas the primary environmental cue used to time seasonal processesis photoperiod. Investigations of tropical vertebrates havealso documented seasonal reproduction, but it is unclear howsynchronous reproduction is, both within and between populations.In this study, we investigated whether seasonal reproductionin a tropical species is synchronous between two populationsin close proximity and, if not, whether asynchrony is correlatedwith genetic and cultural differentiation. We describe two equatorialpopulations of rufous-collared sparrows (Zonotrichia capensis),at the same latitude and separated by 25 km, that each breedseasonally but out of phase with each other. This asynchronousreproductive phenology is associated with local weather patternsand is independent of photoperiod. At a finer scale, reproductivetiming is more highly synchronized within monogamous pairs thanwithin the population as a whole. Associated with the differencein reproductive phenologies, we document that males in eachpopulation sing different song dialects. Using microsatelliteDNA analysis, we found limited gene flow and significant geneticdifferentiation between the two populations. From these resultswe hypothesize that cultural and genetic differentiation betweenpopulations, which can be greater in tropical populations thantemperate ones, can be associated with locally adapted reproductivephenologies.     

Seasonality in Southern Hemisphere freshwater phytoplankton assemblages

Seasonal climatic cycles induce corresponding fluctuations in phytoplankton abundance and productivity at all latitudes, the magnitude of these fluctuations tending to increase with distance from the Equator. In equatorial regions seasonality is dependent on prevailing wind and rainfall patterns while annual temperature fluctuations exert increasing control over seasonal events at higher latitudes. The small annual temperature range of equatorial aquatic systems increases their sensitivity to localized climatic events which can bring about diel changes that exceed normal month-to-month variations. Long-term hydrological cycles with a periodicity greater than one year can also cause dramatic changes in equatorial and tropical aquatic systems leading to greater unpredictability.The factors regulating seasonal patterns of phytoplankton abundance and species composition in equatorial and low-latitude temperate regions of the Southern Hemisphere are examined and compared with similar features in the Northern Hemisphere. Despite the striking diversity of phytoplankton populations and the wide variety of habitats they occupy, seasonal succession follows a common sequence controlled, successively, by physical, chemical and biotic factors. This permits a high degree of predictability in the environmental conditions promoting growth of different taxa.Examination of Southern Hemisphere data indicates that, at class level, phytoplankton successional sequences in Southern Hemisphere aquatic systems are in agreement with the successional paradigm formulated for northern tropical and temperate latitudes. Diatoms characterize early successional episodes and these are followed by chlorophytes, and finally blue-green algae. Extreme habitat modification (e.g. hypertrophy, salinity) tends to lead to dominance of the habitat by a single taxon, often represented by a single species. Predictions of within-taxon species succession in phytoplankton assemblages are far less precise.     

Testosterone and year‐round territoriality in tropical and non‐tropical songbirds

Past studies have suggested a fundamental difference in testosterone concentrations between tropical and northern latitude male birds, with the convention being that males in the tropics express much lower levels of testosterone. However, recent comparative studies have shown that tropical males with a short and synchronous breeding season (i.e. a breeding season typical of northern species) express maximum testosterone levels similar to those of northern latitude birds. Here, we ask the converse: do northern latitude songbirds that express a defining life‐history characteristic typical of the tropics, i.e. year‐round territoriality, have an annual testosterone profile similar to that of tropical songbirds? For the few year‐round territorial species for which data are available, we found that seasonal testosterone profiles and seasonal maxima in plasma testosterone were similar between males of tropical and non‐tropical species. For example, males of both groups expressed seasonal maxima during the period when females were fertile, and testosterone levels at this time were similar. In contrast, this and other studies show that species with seasonal territories typically express maximum testosterone levels earlier in the breeding cycle, when territories are first being established. Taken together, we suggest that specific life‐history traits may play a more important role in determining testosterone profiles of tropical and non‐tropical birds than breeding latitude and encourage further studies to allow for more formal comparisons.     

Seasonal decline in cell-mediated immunity of collared flycatcher Ficedula albicollis nestlings: does the sex of offspring matter?

The seasonal decline in offspring performance is a frequently reported phenomenon in species breeding in a temperate zone, but the potential effect of brood sex ratio on such declines has not been studied. Here, we predicted that this decline may occur if the sex that exhibits the lower immune response or lower survival rate tends to be more frequent among late broods. The seasonal patterns of four performance parameters of collared flycatcher Ficedula albicollis nestlings have been examined during 4 years. Sex was assigned to all studied individuals using molecular techniques. We found significant seasonal decline in cell-mediated immune response, tarsus length and survival of the chicks. The lack of interactions between gender and hatching date revealed that both sexes contributed equally to the observed decline. The brood sex ratio did not vary with the laying date. On the basis of available data, we suggest that the breeding date may only exceptionally induce female-driven sex allocation in species with only slight sexual size dimorphism. In consequence, we suggest that seasonal sex ratio shifts do not account for seasonally declining fitness of nestlings in passerines.     

Diel rhythms of basal and stress-induced corticosterone in a wild, seasonal vertebrate, Gambel's white-crowned sparrow.

Glucocorticoids have a wide array of actions in vertebrates. Daily fluctuations in basal levels of glucocorticoids are thought to regulate homeostatic mechanisms. In contrast, elevated levels secreted in response to stress stimulate changes in physiology and behavior. These changes are thought to aid an animal in avoiding chronic stress or death. Twenty-four-hour rhythms in basal and stress-induced glucocorticoids have been detected in laboratory mammals, but studies in wild, seasonal vertebrates are rare. Identification of plasticity in hormone secretion in wild vertebrates is critical to understanding the effects of hormones on physiology and behavior, and therefore the success of an animal in its natural environment. In the present study, we characterized diel patterns of basal and stress-induced corticosterone (the avian glucocorticoid) under two photoperiods in Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). In contrast to previous findings in the white-crowned sparrow, we demonstrated a robust rhythm in basal corticosterone secretion, in which corticosterone reaches peak levels at the end of the inactive period, and has returned to trough levels just after the active period has begun. We also demonstrated a diel rhythm in secretion of corticosterone in response to a stressor, showing the greatest response at the beginning of the active period. Patterns of CORT secretion were similar under both photoperiods. These patterns show interesting similarities and differences to classical mammalian rhythms.     

Fish larvae assemblages in the Gulf of California

The distributional diversity and assemblages of fish larvae in the Gulf of California indicated two main seasonal stages and two transitional periods: in winter, the tropical water mass is confined to the south‐east portion of the mouth of the Gulf and larval fish assemblages are dominated by subtropical and temperate‐subarctic species; in summer; tropical water invades the Gulf and assemblages are dominated by tropical species. Both seasonal stages are separated by transitional periods coinciding with strong latitudinal temperature gradients. During the autumn and spring transitional periods, the Gulf of California splits into three regions: a northern region where temperate and subarctic species spawn from autumn to spring, a southern region dominated by tropical and subtropical species year round and a central region where tropical and temperate assemblages merge. Seasonal changes in the location of the regions, as well as the borders between them, show expansion and contraction of the northern and southern faunas related to the general oceanic circulation patterns during the year.     

Seasonality of peak metabolic rate in non‐migrant tropical birds

Birds that are year‐round residents of temperate and tropical regions have divergent life histories. Tropical birds have a slower ‘pace of life’, one characteristic of which includes lower peak metabolic rate and daily activity levels. Temperate resident birds are faced with seasonal variation in thermogenic demand. This challenge is met with seasonally increased peak metabolic rate during winter. These thermogenic demands are much lower in birds that are year‐round tropical residents. By measuring peak (summit) metabolic rate in tropical and temperate resident bird species during summer and winter, we asked whether tropical birds exhibit seasonality in peak metabolic rate, and if the direction of seasonality differs between tropical and temperate species. We measured summit metabolism in seven tropical and one temperate species during the winter and during the summer breeding season to test the hypothesis that summit metabolism of tropical residents would change seasonally. We consider whether metabolic seasonality is associated with breeding season for tropical species. We found that summit metabolism was significantly greater during the summer for most tropical residents, while the temperate resident matched several previous reports with higher summit metabolism in winter. We conclude that metabolic seasonality occurs in tropical residents and differs from temperate residents, suggesting that breeding during the summer may be driving relatively higher metabolism as compared to winter thermogenesis in temperate birds.     

Hormonal, behavioral, and thermoregulatory responses to bacterial lipopolysaccharide in captive and free-living white-crowned sparrows (Zonotrichia leucophrys gambelii)

Exposing vertebrates to pathogenic organisms or inflammatory stimuli, such as bacterial lipopolysaccharide (LPS), activates the immune system and triggers the acute phase response. This response involves fever, alterations in neuroendocrine circuits, such as hypothalamo-pituitary-adrenal (HPA) and -gonadal (HPG) axes, and stereotypical sickness behaviors that include lethargy, anorexia, adipsia, and a disinterest in social activities. We investigated the hormonal, behavioral, and thermoregulatory effects of acute LPS treatment in a seasonally breeding songbird, the white-crowned sparrow (Zonotrichia leucophrys gambelii) using laboratory and field experiments. Captive male and female sparrows were housed on short (8L:16D) or long (20L:4D) day lengths and injected subcutaneously with LPS or saline (control). LPS treatment activated the HPA axis, causing a rapid increase in plasma corticosterone titers over 24 h compared to controls. Suppression of the HPG axis occurred in long-day LPS birds as measured by a decline in luteinizing hormone levels. Instead of a rise in body temperature, LPS-injected birds experienced short-term hypothermia compared to controls. Birds treated with LPS decreased activity and reduced food and water intake, resulting in weight loss. LPS males on long days experienced more weight loss than LPS males on short days, but this seasonal effect was not observed in females. These results paralleled seasonal differences in body condition, suggesting that modulation of the acute phase response is linked to energy reserves. In free-living males, LPS treatment decreased song and several measures of territorial aggression. These studies highlight immune-endocrine-behavior interrelationships that may proximately mediate life-history tradeoffs between reproduction and defense against pathogens.     

Comparison of temperate and tropical rainforest tree species: photosynthetic responses to growth temperature

Little is known about the differences in physiology between temperate and tropical trees. Australian rainforests extend from tropical climates in the north to temperate climates in the south over a span of 33° latitude. Therefore, they provide an opportunity to investigate differences in the physiology of temperate and tropical trees within the same vegetation type. This study investigated how the response of net photosynthesis to growth temperature differed between Australian temperate and tropical rainforest trees and how this correlated with differences in their climates. The temperate species showed their maximum rate of net photosynthesis at lower growth temperatures than the tropical species. However, the temperate species showed at least 80% of maximum net photosynthesis over a 12-16°C span of growth temperature, compared with a span of 9-11°C shown by the tropical species. The tropical species showed both larger reductions in maximum net photosynthesis at low growth temperatures and larger reductions in the optimum instantaneous temperature for net photosynthesis with decreasing growth temperature than the temperate species. The ability of the temperate species to maintain maximum net photosynthesis over a greater span of growth temperatures than the tropical species is consistent with the greater seasonal and day-to-day variation in temperature of the temperate climate compared with the tropical climate.     

Using molt cycles to categorize the age of tropical birds: an integrative new system

ABSTRACT Accurately differentiating age classes is essential for the long‐term monitoring of resident New World tropical bird species. Molt and plumage criteria have long been used to accurately age temperate birds, but application of temperate age‐classification models to the Neotropics has been hindered because annual life‐cycle events of tropical birds do not always correspond with temperate age‐classification nomenclature. However, recent studies have shown that similar molt and plumage criteria can be used to categorize tropical birds into age classes. We propose a categorical age‐classification system for tropical birds based on identification of molt cycles and their inserted plumages. This approach allows determination of the age ranges (in months) of birds throughout plumage succession. Although our proposed cycle‐based system is an improvement over temperate calendar‐based models, we believe that combining both systems provides the most accurate means of categorizing age and preserving age‐related data. Our proposed cycle‐based age‐classification system can be used for all birds, including temperate species, and provides a framework for investigating molt and population dynamics that could ultimately influence management decisions.