Julie A. Heath

Estimating the latitudinal origins of migratory birds using hydrogen and sulfur stable isotopes in feathers: influence of marine prey base

Hydrogen stable isotope analysis of feathers is an important tool for estimating the natal or breeding latitudes of nearctic-neotropical migratory birds. This method is based on the latitudinal variation of hydrogen stable isotope ratios in precipitation in North America (δDp) and the inheritance of this variation in newly formed feathers (δDf). We hypothesized that the typically strong relationship between δDp and δDf would be decoupled in birds that forage in marine food webs because marine waters have relatively high δD values compared to δD values for local precipitation. Birds that forage on marine prey bases should also have feathers with high δ34S values, since δ34S values for marine sulfate are generally higher than δ34S values in terrestrial systems. To examine this potential marine effect on feather stable isotope ratios, we measured δD and δ34S in the feathers of nine different species of raptors from both inland and coastal locations across North America. Feathers from coastal bird-eating raptors had consistently higher δD and δ34S values than feathers from inland birds. Birds that had high δ34S values also deviated strongly from the typical relationship between δDp and δDf. We recommend measuring both sulfur and hydrogen stable isotope ratios in feathers when some members of a migrant population could potentially forage in marine habitats. We suggest using a practical cutoff of δ34S >10‰ to remove marine-foraging birds from a migrant sample when using stable isotopes of hydrogen to estimate the latitudinal origins of migrants because high δDf values for marine-foraging birds could potentially distort estimates of origins.

Reproductive failure of a human‐tolerant species, the American kestrel, is associated with stress and human disturbance

Summary 1. The rapid increase of human activity in wild and developed areas presents novel challenges for wildlife. Some species may use human-dominated landscapes because of favourable resources (e.g. high prey availability along roadsides); however, use of these areas may increase exposure to anthropogenic stressors, such as human disturbance or noise, which can negatively affect reproduction or survival. In this case, human-dominated landscapes may act as an ecological trap. 2. We evaluated whether American kestrel Falco sparverius reproductive failure was associated with human disturbance (traffic conditions and land development) or other common predictors of reproductive outcome, such as habitat and clutch initiation date. Also, we examined relationships among human disturbance, corticosterone (CORT) concentrations and nest abandonment to explore potential mechanisms for stress-induced reproductive failure. 3. Twenty-six (36%) of 73 kestrel nesting attempts failed and 88% of failures occurred during incubation. Kestrels nesting in higher disturbance areas were 99 times more likely to fail than kestrels nesting in lower disturbance areas. Habitat and clutch initiation date did not explain reproductive outcome. 4. Females in higher disturbance areas had higher CORT and were more likely to abandon nests than females in lower disturbance areas. There was no relationship between male CORT and disturbance or abandonment. Females spent more time incubating than males and may have had more exposure to anthropogenic stressors. Specifically, traffic noise may affect a cavity-nesting bird’s perception of the outside environment by masking auditory cues. In response, incubating birds may perceive a greater predation risk, increase vigilance behaviour, decrease parental care, or both. 5. Synthesis and applications. Proximity to large, busy roads and developed areas negatively affected kestrel reproduction by causing increased stress hormones that promoted nest abandonment. These results demonstrate that species presence in a human-dominated landscape does not necessarily indicate a tolerance for anthropogenic stressors. Managers should carefully consider or discourage projects that juxtapose favourable habitat conditions with areas of high human activity to decrease risk of ecological traps. Noise mitigation, while locally effective, may not protect widespread populations from the pervasive threat of traffic noise. Innovative engineering that decreases anthropogenic noise at its source is necessary.

Corticosterone levels during nest departure of juvenile American Kestrels

Many avian behavior patterns, such as breeding, migration and territoriality, are correlated with changes in hormone levels. Elevated levels of corticosterone, an adrenal steroid hormone, are associated with increased foraging and increased activity levels in birds. Young birds about to take their first flight may benefit from elevated plasma corticosterone levels that facilitate locomotor activity and foraging behavior while they are developing flight and hunting skills. I examined the relationship between corticosterone levels and the timing of nest departure in nestling American Kestrels (Falco sparverius). American Kestrels are cavity nesters and typically depart from the nest between 25-30 days of age, when they take their first flight. I collected blood from 21-25-day-old kestrels and monitored nest boxes to determine date of nest departure. Consistent with my prediction, as birds prepared to depart from the nest their plasma corticosterone levels increased significantly. The relationship between corticosterone levels and nest departure may be particularly important in those cavity nesters that must make a quick transition from inactivity to full-flight at the time of nest departure.

Accuracy and variation in estimates of large numbers of birds by individual observers using an aerial survey simulator

Abstract The accuracy of aerial estimates of avian aggregation sizes is variable across studies, and the relative importance of techniques and inter-observer error to this variation are poorly understood. Using a scaled physical model of a wading bird colony, we examined accuracy and variation in observer counts of simulated large numbers (200–6000) of densely nesting birds in vegetated situations. Observer estimates averaged 29% less than true numbers (under- and overestimates averaged together), and the mean absolute value of observer errors was 49% of true values. We found no effects of the size of the aggregation, the experience of the observer, the size of the previous aggregations surveyed by observers, the use of corrective lenses, or fatigue on degree of individual error. Over- and underestimates by individuals did not tend to cancel out in estimates by individuals of a “population” of colonies. Photographic counts of the same trials were significantly more accurate than observer estimates. We suggest that many studies using estimates of large numbers of birds may be confounded by similar errors. We urge that researchers use caution in interpreting the results of past surveys and develop ways to minimize, measure, and correct for visual estimation error within individuals and among observers.

A historical record of mercury contamination in Southern Florida (USA) as inferred from avian feather tissue.

During the late 1980s, the upper trophic-level biota of the Everglades (FL, USA) was recognized as being highly contaminated with mercury (Hg). However, the timing and pattern of that increase is poorly known, and no information is available about mercury contamination in Everglades wildlife prior to 1974. We measured methylmercury concentrations in feathers of white ibises (n = 33), great egrets (n = 7), anhingas (n = 21), and great blue herons (n = 12) from museum specimens collected from 1910 through 1980 and combined them with more recent feather samples collected from live birds (1985-2000, n = 98, 37, 49, and 7, respectively). We found no evidence of contamination of museum samples with inorganic mercuric preservatives (0.01-0.28% of total Hg in feathers). All species showed relatively low concentrations of mercury through the 1970s (<5 microl/L dry wt for anhingas, ibises, and egrets, <10 microl/L for herons). Samples from all species taken during the 1990s showed a large and significant increase (4-5X) in MeHg concentration. This evidence suggests that most of the increase in Hg deposition during the 20th century in south Florida occurred during the last two to three decades, which is consistent with information about local source deposition. Contamination levels prior to the 1970s appear to have been associated with normal reproduction in these birds, suggesting partial evidence for a threshold of reproductive impairment.

Regional Distribution Shifts Help Explain Local Changes in Wintering Raptor Abundance: Implications for Interpreting Population Trends

Studies of multiple taxa across broad-scales suggest that species distributions are shifting poleward in response to global climate change. Recognizing the influence of distribution shifts on population indices will be an important part of interpreting trends within management units because current practice often assumes that changes in local populations reflect local habitat conditions. However, the individual- and population-level processes that drive distribution shifts may occur across a large, regional scale and have little to do with the habitats within the management unit. We examined the latitudinal center of abundance for the winter distributions of six western North America raptor species using Christmas Bird Counts from 1975–2011. Also, we considered whether population indices within western North America Bird Conservation Regions (BCRs) were explained by distribution shifts. All six raptors had significant poleward shifts in their wintering distributions over time. Rough-legged Hawks (Buteo lagopus) and Golden Eagles (Aquila chrysaetos) showed the fastest rate of change, with 8.41 km yr−1 and 7.74 km yr−1 shifts, respectively. Raptors may be particularly responsive to warming winters because of variable migration tendencies, intraspecific competition for nesting sites that drives males to winter farther north, or both. Overall, 40% of BCR population trend models were improved by incorporating information about wintering distributions; however, support for the effect of distribution on BCR indices varied by species with Rough-legged Hawks showing the most evidence. These results emphasize the importance of understanding how regional distribution shifts influence local-scale population indices. If global climate change is altering distribution patterns, then trends within some management units may not reflect changes in local habitat conditions. The methods used to monitor and manage bird populations within local BCRs will fundamentally change as species experience changes in distribution in response to climate change.

Reproductive physiology of free-living White Ibises (Eudocimus albus) in the Florida Everglades

We measured plasma concentrations of testosterone, estradiol, progesterone, and corticosterone; and recorded changes in gonad size, body condition, molt, and brood patch development of free-living adult White Ibises (Eudocimus albus) during the breeding season in the Florida Everglades. White Ibises are colonially breeding, long-legged wading birds that inhabit freshwater and estuarine wetlands. They have flexible breeding schedules (nest initiation dates can range from January to September) and onset of nesting is usually associated with increased prey availability caused by concentration of small fish in pools during periods of wetland drying. In this paper, we present the hormonal and physical characteristics of White Ibis reproductive physiology. We classified White Ibis breeding into five stages: pre-breeding, display, copulation/egg production, incubation, and chick rearing. White Ibises showed cyclic gonadal development which corresponded to reproductive stage. Male and female testosterone concentrations increased during the display stage and decreased during copulation, incubation, and chick rearing. Female estradiol concentrations were highest during display and chick rearing and male estradiol concentrations were lowest during copulation. Female progesterone concentrations increased during display and remained high throughout the breeding season. Female ibises had low corticosterone concentrations that increased during incubation and were highest during chick rearing, concomitant with lower body condition and flight muscle-mass scores. Male ibis progesterone and corticosterone concentrations did not show seasonal changes and were more variable than concentrations in female ibises at similar stages. Males and females had elevated body condition scores during the display stage, which were depleted by the onset of incubation. Increased energy stores during display may be used later for fasting in male birds that do not eat during the 10-day copulation/egg production stage, and for egg production in female birds. During incubation, male and female ibises developed brood patches. Ibises molted in all stages of reproduction, indicating that ibis molt and reproductive physiology may not inhibit each other as in most temperate bird species. White Ibises showed similar patterns in reproductive physiology to other monogamous, seasonally breeding bird species in which both sexes incubate and care for the young.

Trapping White Ibises with rocket nets and mist nets in the Florida Everglades

Abstract We lured White Ibises (Eudocimus albus) to trap sites with decoy plastic flamingos and captured them with a rocket net or mist nets. Our ability to attract ibises to a site (and consequently our capture success) was affected by vegetation height and water depth but not by decoy numbers or their arrangement. Both the rocket net (37 ibises) and mist nets (97 ibises) caught birds. The number of birds captured per day was the same for both methods, but the rocket net trap captured more birds per set than did mist nets. Because mist nets were easy to reset we captured 1–2 birds per set multiple times in one day with mist nets. We preferred mist nets over rocket net traps because mist nets allowed for shorter bird processing times, greater ease of set up, and fewer safety considerations for transport and operation of the trap. Also, rocket nets required more equipment and expense. Although we discouraged other species from landing at a trap site, there was evidence that these techniques may also be useful for capturing other wading birds.

Human disturbance and stage‐specific habitat requirements influence snowy plover site occupancy during the breeding season

Habitat use has important consequences for avian reproductive success and survival. In coastal areas with recreational activity, human disturbance may limit use of otherwise suitable habitat. Snowy plovers Charadrius nivosus have a patchy breeding distribution along the coastal areas on the Florida Panhandle, USA. Our goal was to determine the relative effects of seasonal human disturbance and habitat requirements on snowy plover habitat use. We surveyed 303 sites for snowy plovers, human disturbance, and habitat features between January and July 2009 and 2010. We made multiple visits during three different sampling periods that corresponded to snowy plover breeding: pre-breeding, incubation, and brood-rearing and used multi-season occupancy models to examine whether human disturbance, habitat features, or both influenced site occupancy, colonization (probability of transition from an unoccupied site to an occupied site), and extinction (probability of transition from an occupied site to an unoccupied site). Snowy plover site occupancy and colonization was negatively associated with human disturbance and site extinction was positively associated with human disturbance. Interdune vegetation had a negative effect on occupancy and colonization, indicating that plovers were less likely to use areas with uniform, dense vegetation among dunes. Also, dune shape, beach debris, and access to low-energy foraging areas influenced site occupancy, colonization, and extinction. Plovers used habitat based on beach characteristics that provided stage-specific resource needs; however, human disturbance was the strongest predictor of site occupancy. In addition, vegetation plantings used to enhance dune rehabilitation may negatively impact plover site occupancy. Management actions that decrease human disturbance, such as symbolic fencing and signage, may increase the amount of breeding habitat available to snowy plovers on the Florida Panhandle and in other areas with high human activity. The specific areas that require this protection may vary across snowy plover life history stages.

LOCAL RECRUITMENT AND NATAL DISPERSAL DISTANCES OF AMERICAN KESTRELS

Abstract. Variation in recruitment patterns and dispersal behavior can have important consequences for population viability, genetic structure, and rates of evolutionary change. From 1992 to 2006 we studied a marked population of American Kestrels (Falco sparverius) nesting in boxes in southwestern Idaho to identify factors that affect local recruitment and natal dispersal distances. A low proportion (4%) of locally produced kestrels (n = 2180) returned to nest in study area boxes. Offspring of locally produced individuals were 3.1 times more likely to return than offspring of parents that did not hatch in study area boxes and, independent of a parents origin, males were 1.8 times more likely than females to return. Kestrels that hatched earlier in the breeding season and those that hatched immediately prior to a mild winter were more likely to return. Local natal dispersal distances were best explained by sex and parental origin but not by hatching dates. Fifty-four males moved an average of 5.3 km from their natal box to the location of their first breeding in the study area, and 27 females moved an average of 9.8 km. Offspring of locally produced parents dispersed shorter distances within the study area than offspring of other parents, and local natal dispersal distances of locally produced parents correlated with those of their same-sex offspring. Patterns of natal dispersal of American Kestrels in southwestern Idaho appear to be driven by a combination of parental dispersal tendencies and ecological factors. The population consists of a mix of immigrants and philopatric birds.