publications
Permanent URI for this collectionhttps://hdl.handle.net/1969.6/86486
Browse
Browsing publications by Author "Cook, Mark I."
Now showing 1 - 7 of 7
- Results Per Page
- Sort Options
Item Aquatic prey switching and urban foraging by the White Ibis Eudocimus albus are determined by wetland hydrological conditions(The Authors and The British Ornothologists' Union, 2011-02-22) Dorn, Nathan J.; Cook, Mark I.; Herring, Garth; Boyle, Robin A.; Nelson, Jennifer; Gawlik, Dale E.Prey availability is known to limit reproduction of some species of nesting birds, but identifying the primary prey types of a species with a flexible diet can be challenging. For the White Ibis Eudocimus albus, a tactile feeding, medium-sized wading bird, nestling prey composition is suggested to depend on landscape water depths⁄ availability of foraging habitat at the time of nesting and on historical drying events affecting prey production. We collected and compared inter- and intra-annual diet variation of White Ibis chicks reared in the Everglades over two years that were independently identified as being relatively good (2006) and poor (2007) nesting seasons. We collected 127 nestling boluses and analyzed the temporal variation in biomass of eight functional prey groups using multivariate techniques. The boluses from 2006 in the central Everglades were dominated by fish, but in 2007, after fish had been reduced by the previous year of drying, the boluses from the same region were more variable and dominated by garbage (i.e.scavenging). Analysis of five different collections taken from a different colony in the northern Everglades indicated that boluses were characterized by crayfish and had fewer fish or less garbage when landscape water depths were relatively higher and more preferred habitat was available. At lower landscape water depths in 2007 the bolus composition shifted away from crayfish towards small fish and urban food (terrestrial insects and garbage). Our results support the suggestion of depth-dependent diets; prey composition depends on the current landscape water levels around the colonies, and also suggests that previous drying events can lead to increased reliance on alternative food sources. White Ibis partially compensated for unavailable aquatic prey with alternative urban foods, but their nesting success appears to have sufferedItem Dynamic habitat selection by two wading bird species with divergent foraging strategies in a seasonally fluctuating wetland.(The American Ornithologists' Union, 2011-10-01) Beerens, James M.; Gawlik, Dale E.; Herring, Garth; Cook, Mark I.Seasonal and annual variation in food availability during the breeding season plays an influential role in the population dynamics of many avian species. In highly dynamic ecosystems like wetlands, finding and exploiting food resources requires a flexible behavioral response that may produce different population trends that vary with a species’ foraging strategy. We quantified dynamic foraging-habitat selection by breeding and radiotagged White Ibises (Eudocimus albus) and Great Egrets (Ardea alba) in the Florida Everglades, where fluctuation in food resources is pronounced because of seasonal drying and flooding. The White Ibis is a tactile “searcher” species in population decline that specializes on highly concentrated prey, whereas the Great Egret, in a growing population, is a visual “exploiter” species that requires lower prey concentrations. In a year with high food availability, resource-selection functions for both species included variables that changed over multiannual time scales and were associated with increased prey production. In a year with low food availability, resource-selection functions included short-term variables that concentrated prey (e.g., water recession rates and reversals in drying pattern), which suggests an adaptive response to poor foraging conditions. In both years, the White Ibis was more restricted in its use of habitats than the Great Egret. Real-time species–habitat suitability models were developed to monitor and assess the daily availability and quality of spatially explicit habitat resources for both species. The models, evaluated through hindcasting using independent observations, demonstrated that habitat use of the more specialized White Ibis was more accurately predicted than that of the more generalist Great Egret.Item The Effects of Water Depth and Emergent Vegetation on Foraging Success and Habitat Selection of Wading Birds in the Everglades(Waterbirds, 2011-12-01) Lantz, Samantha M.; Gawlik, Dale E.; Cook, Mark I.Wading bird foraging success and habitat preference can be greatly affected by prey availability, which encompasses both prey density and the vulnerability of prey to capture. Two components of prey vulnerability, water depth and emergent vegetation, were manipulated within 10 m × 10 m enclosures to determine the relative effects on foraging habitat preference for eight species of wading birds and foraging success for a subset of four species that strike their prey. All species showed a strong preference for shallow water, and within this water depth showed a preference for the sparse vegetation density treatment. The preference for foraging habitat with a sparse or intermediate vegetation density has been documented in other studies, and may represent a tradeoff between selecting more heavily vegetated areas, which have a higher prey density, and more open areas, where prey are more vulnerable to capture. Almost all foraging occurred in the shallow water treatment, suggesting that preferred water depths constituted high quality habitat for wading birds. The weaker selection for sparse vegetation density and lack of an effect of vegetation density on capture rate and capture efficiency (p>0.05 for all tests, except Snowy Egret (Egretta thula) capture efficiency) suggested that emergent vegetation is of secondary importance to water depth as determinants of wading bird habitat quality.Item The effects of water depth and submerged aquatic vegetation on the selection of foraging habitat and foraging success of wading birds(The Cooper Ornithological Society, 2010-02-12) Lantz, Samantha M.; Gawlik, Dale E.; Cook, Mark I.Successful foraging by avian predators is influenced largely by prey availability, which encompasses not only the density of prey but also its vulnerability to capture. For wading birds (Ciconiiformes), habitat features such as water depth and density of vegetation are thought to affect the vulnerability of their aquatic prey. In January and April 2007 we experimentally manipulated the depth of water and density of submerged aquatic vegetation (SAV) in enclosures (10 10 m) with equal densities of fish to determine their effects on wading birds’ selection of foraging habitat and foraging success. Analysis of the results with Manly’s selection index showed that wading birds preferred habitat with shallow water and SAV. The two habitat components had little effect on the birds’ foraging success, however, as capture rate did not vary with water depth or SAV density. Capture efficiency did not vary by SAV density and was actually lower in shallow water, contrary to our expectations. Our results suggest that birds selected habitat on the basis of environmental cues such as water depth and SAV but that these factors did not affect foraging success strongly. We hypothesize that wading birds were selecting habitat with shallow water and SAV because of an anticipated benefit to foraging through elevated density and vulnerability of prey, but the relatively high and uniform density of prey stocked in the enclosures, as well as the scale of the enclosures, effectively equalized the vulnerability of prey across treatments.Item Food availability is expressed through physiological stress indicators in nestling white ibis: a food supplementation experiment(British Ecological Society, 2010-10-12) Herring, Garth; Cook, Mark I.; Gawlik, Dale E.; Call, Erynn M.Physiological responses to environmental stress such as adrenocortical hormones and cellular stress proteins have recently emerged as potentially powerful tools for investigating physiological effects of avian food limitation. However, little is known about the physiological stress responses of free-living nestling birds to environmental variation in food availability. We experimentally tested how hydrologically mediated changes in food availability affect the physiological stress responses of juvenile white ibises Eudocimus albus in a fluctuating wetland. We provided supplementary food to free-living nestlings during 2 years with contrasting hydrologic and food availability conditions, and used plasma (PCORT) and faecal (FCORT) corticosterone and heat shock proteins (HSP60 and HSP70) from first-hatched (A-nestlings) and second-hatched (B-nestlings) to detect relatively short- to long-term responses to food limitation. Nestling physiological stress responses were relatively low in all treatments during the year with optimal food availability, but PCORT, FCORT and HSP60 levels increased during the poor food year. FCORT and HSP60 responses were clearly due to nutritional condition as elevated concentrations were evident primarily in control nestlings. Significant year by hatch order interactions for both FCORT and HSP60 revealed that these increases were largely incurred by B-nestlings. FCORT and HSP60 responses were also well developed early in neonatal development and remained elevated for the duration of the experiment suggesting a chronic stress response. PCORT and HSP70 were less informative stress responses. The nutritionally mediated increases in FCORT and HSP60 provide compelling evidence that white ibis nestlings can be physiologically affected by environmental food levels. FCORT and HSP60 are effective indicators of nutritional mediated stress for nestling white ibises and potentially for other species prone to capture or handling stress.Item Sensitivity of Nesting Great Egrets (Ardea alba) and White Ibises (Eudocimus albus) to Reduced Prey Availability(The American Ornithologists' Union, 2010-07) Herring, Garth; Gawlik, Dale E.; Cook, Mark I.; Beerens, James M.Life-history theory suggests that long-lived bird species will adjust their nesting effort according to current conditions to balance the costs and benefits of current reproduction with their long-term needs for survival and future reproduction. However, responses to the same habitat conditions may differ between species, even within the same ecosystem, to produce different nesting and population patterns. We examined differences in the nesting ecology of two sympatric wading species, Great Egret (Ardea alba) and White Ibis (Eudocimus albus), between years with high (2006) and below-average (2007) prey availability in the Florida Everglades. Clutch size of White Ibises decreased by ~19% from 2006 to 2007, whereas Great Egret clutch size remained constant. Model selection identified rain, water depth, Julian date, year, and prey biomass as parameters that most influenced daily survival rates (DSR) of White Ibis nests, whereas nest stage, region, Julian date, water depth, and the quadratic form of water recession rate most influenced Great Egret nest DSR. Daily survival for both Great Egret and Whites Ibis nests was higher in 2006 (DSR = 0.992 and 0.999, respectively) than in 2007 (DSR = 0.981 and 0.979). Our results support the hypothesis that prey availability and hydrological factors play crucial roles in regulating populations of wading birds in the Florida Everglades. Results also demonstrated that White Ibis reproduction was more sensitive to changes in hydrological conditions and prey availability than Great Egret reproductionItem The White Ibis and Wood Stork as indicators for restoration of the everglades ecosystem(Elsevier Ltd., 2008-10-24) Frederick, Peter; Gawlik, Dale E.; Ogden, John C.; Cook, Mark I.; Lusk, MichaelLarge numbers of colonially nesting herons, egrets, ibises, storks and spoonbills were one of the defining natural phenomena of the historical Everglades. Reproduction of these species has been tracked over at least a century, and some clear responses to dramatic anthropogenic hydrological alterations have been established. These include a marked decline in nesting populations of several species, and a movement of colonies away from the over drained estuarine region. Ponding in a large portion of the freshwater marsh has favored species that hunt by sight in deep water (egrets, cf. 25–45 cm), while tactile feeders (ibises and storks) that depend on concentrated prey in shallow water (5–25 cm) have become proportionately much less common. There has been a marked increase in the interval between exceptionally large breeding aggregations of White Ibises (Eudocimus albus). Loss of short hydroperiod wetlands on the margins of the Everglades have delayed nest initiations 1–2 months by Wood Storks (Mycteria americana) resulting in poor nesting success. These responses are consistent with mechanisms that involve foraging, and the availability and production of prey animals, and each of the relationships is highly dependent on hydrology. Here, we define a group of characteristics about wading bird dynamics (= indicators) that collectively track the specific ecological relationships that supported ibises and storks in the past. We suggest four metrics as indicators of restoration success: timing of nesting by storks, the ratio of nesting ibises + storks to Great Egrets, the proportion of all nests located in the estuarine/freshwater ecotone, and the interval between years with exceptionally large ibis nestings. Each of these metrics has historical (e.g., predrainage) data upon which to base expectations for restoration, and the metrics have little measurement error relative to the large annual variation in numbers of nests. In addition to the strong scientific basis for the use of these indicators, wading birds are also a powerful tool for public communication because they have strong aesthetic appeal, and their ecological relationships with water are intuitively understandable. In the interests of communicating with the public and decisionmakers, we integrate these metrics into a single-page annual ‘‘traffic-light’’ report card for wading bird responses. Collectively, we believe these metrics offer an excellent chance of detecting restoration of the ecosystem functions that supported historical wading bird nesting patterns.