Dale E. Gawlik

THE EFFECTS OF PREY AVAILABILITY ON THE NUMERICAL RESPONSE OF WADING BIRDS

Reduced prey availability has emerged as a primary hypothesis to explain population constraints on wading birds in numerous wetlands around the world. However, there is almost no understanding of which component of prey availability (i.e., prey density or vulnerability of prey to capture) is affecting populations and whether the relative effects of each component differ among species. In this study, I manipulated prey density and water depth (i.e., prey availability) in 12 0.2-ha ponds to determine their relative effects on the numeric response of eight species of free-ranging wading birds (White Ibis, Eudocimus albus; Wood Stork, Mycteria americana; Snowy Egret, Egretta thula; Glossy Ibis, Plegadis falcinellus; Great Egret, Ardea alba; Tricolored Heron, Egretta tricolor; Great Blue Heron, Ardea herodias; and Little Blue Heron, Egretta caerulea). The experiment was conducted in a constructed wetland adjacent to, and west of, the northern tip of the remnant Everglades, in Palm Beach County, Florida, USA. Each pond was set to a water depth of 10 cm, 19 cm, or 28 cm, and was stocked with golden shiners (Notemigonus crysoleucas) at a density of either 3 fish/m2 or 10 fish/m2. Total bird use (all treatments pooled) increased from day 1 (day after stocking) to day 6, stabilized for several days at ∼280 birds, and then decreased until day 16, when bird use nearly ceased. Fish were depleted most rapidly in the shallow treatment and least rapidly in the deep treatment. The giving-up-density (GUD) of prey increased with increasing water depth. There was no significant difference among species in the slope of that relationship; however, a visual inspection of the data showed that differences in GUDs were becoming more apparent in the deepest treatment. At that depth, the White Ibis, Wood Stork, and Snowy Egret had higher GUDs than did the Glossy Ibis, Great Egret, Tricolored Heron, Great Blue Heron, and Little Blue Heron. Also, the first three species were affected significantly by both prey density and water depth, whereas the latter five species showed a decidedly weaker response to one or the other component of prey availability. The first three species were more abundant in the shallow treatments and the high prey density treatments, and they abandoned the study site before other species reached their maximum density. The feeding strategy of the first group appeared to be one of searching for new high-quality food patches rather than staying and exploiting food patches that were declining in quality. Species that employed a searching strategy also have shown the most severe population declines, suggesting that factors affecting bird density at feeding sites may also have affected population size.

The ecological–societal underpinnings of Everglades restoration

The biotic integrity of the Florida Everglades, a wetland of immense international importance, is threatened as a result of decades of human manipulation for drainage and development. Past management of the system only exacerbated the problems associated with nutrient enrichment and disruption of regional hydrology. The Comprehensive Everglades Restoration Plan (CERP) now being implemented by Federal and State governments is an attempt to strike a balance between the needs of the environment with the complex management of water and the seemingly unbridled economic growth of southern Florida. CERP is expected to reverse negative environmental trends by “getting the water right”, but successful Everglades restoration will require both geochemical and hydrologic intervention on a massive scale. This will produce ecological trade-offs and will require new and innovative scientific measures to (1) reduce total phosphorus concentrations within the remaining marsh to 10 µg/L or lower; (2) quantify and link ecolo...

Maintaining tree islands in the Florida Everglades: nutrient redistribution is the key

The Florida Everglades is an oligotrophic wetland system with tree islands as one of its most prominent landscape features. Total soil phosphorus concentrations on tree islands can be 6 to 100 times greater than phosphorus levels in the surrounding marshes and sloughs, making tree islands nutrient hotspots. Several mechanisms are believed to redistribute phosphorus to tree islands: subsurface water flows generated by evapotranspiration of trees, higher deposition rates of dry fallout, deposition of guano by birds and other animals, groundwater upwelling, and bedrock mineralization by tree exudates. A conceptual model is proposed, in which the focused redistribution of limiting nutrients, especially phosphorus, onto tree islands controls their maintenance and expansion. Because of increased primary production and peat accretion rates, the redistribution of phosphorus can result in an increase in both tree island elevation and size. Human changes to hydrology have greatly decreased the number and size of tr...

Distribution of Wading Birds Relative to Vegetation and Water Depths in the Northern Everglades of Florida, USA

Abstract The response of Great Blue Herons (Ardea herodias), Great Egrets (Casmerodius albus), Wood Storks (Mycteria americana), and White Ibises (Eudocimus albus) to water level (index of depth) and vegetation in the northern Everglades of Florida was studied in two years, each with dissimilar water levels. A regression model was constructed for each species in an average year (1988) and a dry year (1989) to examine the relationship of bird abundance to water depth and area of eight vegetation classes. The analyses showed that bird abundance is related to both water level and the vegetation community, but water level generally had the greatest effect. Models showed that in the average year (1988), there was a water level threshold, above which bird abundance was predicted to decline. The level threshold varied among species and may have reflected species-specific foraging constraints. However, in the dry year (1989), the relationship between bird abundance and water level was positive and linear, indicating that few places had water deeper than the thresholds observed in the average year. Overall, the area of slough vegetation appeared to have the second greatest effect on bird abundance. Generally, all models had low coefficients of determination (R2 range 0.06-0.42) suggesting that factors other than water level and vegetation were important, or birds were responding to variables in the model, but at different spatial scales than that which the data were collected. Models for Great Blue Herons and Great Egrets had higher coefficients of determination than models for Wood Storks and White Ibises. The more solitary feeding behavior of the herons and egrets resulted in a more even distribution across the marsh than for storks and ibises, which were usually found in flocks. Our study suggests that if restoration of the Everglades results in more natural hydrologic cycles, an increase in the amount of slough habitat, and a decrease in the proportion of cattails, foraging conditions for wading birds may improve.

Wading Bird Nesting Effort as an Index to Wetland Ecosystem Integrity

Abstract A comprehensive wading bird nesting database for the Everglades watershed in south Florida was compiled for the time period of 1903 through 2001. The limitations and applications of this database were assessed with the purpose of ultimately strengthening wading bird performance measures used as an index of ecosystem integrity for the Everglades restoration effort. The database is limited for some inferences, such as comparing between individual years, because of a lack of standardization in sampling methodology. As such, it should be used as an index of wading bird numbers rather than as an absolute population measure. However, the database is adequate to understand long-term, system-wide trends and the range of variability in nesting effort by the wading birds. Results from the analyses indicate that trends in numbers of nesting wading birds differ among species. It is estimated that the number of White Ibis (Eudocimus albus) and Wood Stork (Mycteria americana) nests have decreased by 87% and 78%, respectively, in the Everglades since the 1930s. There has also been a significant increase in the time interval between large nesting events for White Ibis and Wood Stork and a decrease in the magnitude of large nesting events for White Ibis over the past 70 years. The number of Great Egret (Ardea alba) nests has increased, the magnitude of large nesting events has increased, and the interval between large nesting events has decreased. There has also been a 61% decrease in the percentage of wading bird nests located in the southern Everglades and a 46% increase in the percentage of nests located in the central northern Everglades since the 1980s. These types of quantitative comparisons can be used to refine and strengthen the proposed wading bird performance measures used to gauge the health of the Everglades for the Everglades restoration effort. The time interval between large nesting events may be appropriate as an additional performance measure because it may capture the variable nature of wading bird numbers in response to hydrologic fluctuations in the Everglades.

LAKE OKEECHOBEE CONCEPTUAL ECOLOGICAL MODEL

With a surface area of nearly 1,800 square kilometers, Lake Okeechobee is a prominent central feature of the South Florida aquatic ecosystem. The lake provides regional flood protection, supports a prized recreational fishery, provides habitat for migratory waterfowl and regional wading bird populations, and is a source of fresh water for irrigation, drinking, and restoration of downstream ecosystems. The main stressors on Lake Okeechobee are (1) large inputs of phosphorus from agricultural and other anthropogenic land uses in the watershed, (2) unnatural variation in water levels due to channelization of inflows and dike containment, and (3) rapid expansion of non-native plants. Ecological effects are complicated due to three distinct in-lake zones with different water chemistry, physical properties, and biota. A central pelagic zone has turbid, nutrient-rich water and phytoplankton dominance; a shallow south and western near-shore zone has submerged plant or phytoplankton dominance (at low vs. high water levels, respectively); and a western littoral zone is dominated by emergent wetland plants. Changes in water level influence the flow of nutrients between zones, thereby creating a synergistic effect between stressors. Under high water conditions, there is considerable advective transport of nutrients from the pelagic zone into the littoral zone. Under low water conditions, the littoral zone is cut off hydrologically and is a rainfall-driven oligotrophic wetland. Low water also facilitates drying and wildfires in the littoral zone, which in turn has an influence on expansion of non-native plants and recovery of native plants from buried seed banks. All of these factors influence fish, wading birds, and other animals, which depend on littoral and near-shore plant communities for nesting and foraging habitat. This paper describes our current knowledge of these complex processes, the lake’s expected responses to ongoing and planned restoration programs, and key areas of uncertainty requiring future research.

Sensitivity of Nesting Great Egrets (Ardea alba) and White Ibises (Eudocimus albus) to Reduced Prey Availability

ABSTRACT. 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 reproduction.

The Role of Stress Proteins in the Study of Allostatic Overload in Birds: Use and Applicability to Current Studies in Avian Ecology

Stress proteins offer a measure of stress in birds at the cellular level that are an alternative to the glucocorticoids. Stress proteins are not biased by handling stress, the increase in stress proteins lasts longer than with other measures (e.g., corticosterone), and, therefore, they may be a more appropriate measure of long-term or chronic stress. However, caution should be practiced when using stress proteins because the level of stress needed to elicit a response may be higher than with corticosterone. Stress proteins have only recently been used to measure the response to competition, food limitation, growth, and parasitism in birds. In other taxa, the stress proteins have been used to measure genetic stress, temperature, toxins, UV radiation, and physical activity. Stress proteins increase the options available to avian ecologists for understanding how avian species respond to changes in the environment.

The use of decoys as a research tool for attracting wading birds

Abstract The ability to attract wading birds to a specific location is an important tool for wading bird research. We examined the effectiveness of three commercially available decoy types in attracting free-ranging wading birds to a precise location. There was a statistically significant effect of decoy type on the abundance of wading birds. Wading birds were more attracted to sites with plastic flamingo and Tyvek® bag decoys than with Texas rag decoys or no decoys (control sites). The flamingo and Tyvek® bag decoys have a rigid three-dimensional shape, which may be a more important characteristic than the detailed features of a decoy in attracting wading birds. White wading birds showed a significant effect of decoy type on abundance, whereas birds with dark plumage did not.

Seasonal habitat use and abundance of loggerhead shrikes in South Carolina

Loss of winter habitat has been implicated in the widespread declines of loggerhead shrike (Lanius ludovicianus) populations; however, our understanding of what represents winter habitat for this species is poor. Thus, we investigated whether shrikes in South Carolina used similar habitats throughout the year. We found that during the breeding season shrikes inhabited areas dominated by short, grassy vegetation, whereas, outside of the breeding season, they decreased (P = 0.047) their use of grassy habitats and increased (P = 0.005) their use of cropland. Declines in shrike populations in the southeastern United States as well as the entire nation, respectively, were correlated (r = 0.83, n = 15, P < 0.001; r=0.34 n = 113, P < 0.001) with a loss of pastureland suggesting that this habitat may be limiting