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Provisioning Rates Suggest Food Limitation for Breeding Bald Eagles in their Southernmost Range
Matthew R. Hanson and John D. Baldwin

Southeastern Naturalist, Volume 15, Issue 2 (2016): 365–381

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Southeastern Naturalist 365 M.R. Hanson and J.D. Baldwin 22001166 SOUTHEASTERN NATURALIST 1V5o(2l.) :1356,5 N–3o8. 12 Provisioning Rates Suggest Food Limitation for Breeding Bald Eagles in their Southernmost Range Matthew R. Hanson1 and John D. Baldwin1,* Abstract - Beginning in the late 1980s, Florida Bay underwent dramatic ecological changes due to altered freshwater inflows from the Everglades. At the same time, the local Bald Eagle population began to decline, a trend that has continued ever since. We documented diet and provisioning rates of eagles to examine the hypothesis that food is a limiting factor to their success. We monitored 4 nests with video cameras in the 2009/2010 and 2010/2011 breeding seasons. We recorded a total of 546 prey deliveries, with 93% determined to class and 46% determined to family. Fish comprised 86% of all deliveries, birds made up 7%, and up 7% were undeterminable items. The mean daily provisioning rates for all nest sites combined were 1.75 deliveries/young/day and 2.64 deliveries/day. These rates significantly declined throughout the breeding season. They are strikingly smaller than those reported for stable Bald Eagle populations and comparable to the rates of another struggling population. The total biomass of prey deliveries/young/day also declined throughout the breeding season. Deliveries were mostly frequently made to the nest during the daily period 3–5 hours after sunrise and then again at a less frequent rate 9–12 hours after sunrise and did not vary between nests or change throughout the breeding season. These results suggest that the Bald Eagle population in Florida Bay is experiencing inadequate prey availability, which may be contributing to their decline. Introduction Inadequate availability of prey can led to changes in diet and food limitation in many predators (Ford et al. 2010, Shine and Madsen 1997). Raptors, both generalist and specialist foragers, are top-level predators in nearly all ecosystems, and their life-history traits, population sizes, and community structure have been affected by limitations of prey availability (Martin 1987, Poole 1982, Rutz and Bijlsma 2006). Prey availability to a predator is determined by the composition, densities, and vulnerability of prey to predation (May and Norton 1996, Orth et al. 1984, Schmidt and Ostfeld 2003, Schneider 2001), which influence a predator’s diet through selection of potential prey species (Bence and Murdoch 1986, Davies 1977, Estabrook and Dunham 1976, Fryxell and Lundberg 1994). For example, the highly endangered Aquila adalberti C.L. Brehm (Spanish Imperial Eagle), whose population has seen significant decline (Gonzalez et al. 1989), has higher reproductive success and occupancy rate in territories in which high densities of its main prey item is found (Ferrer and Bisson 2003, Gonzalez et al. 1990). Aquila chrysaetos (L.) (Golden Eagle) also exhibits higher nesting densities and breeding success where there is high prey availability (Smith and Murphy 1979, Steenhof et al. 1997, Watson et al. 1992). 1Florida Atlantic University, Department of Biological Sciences, 3200 College Avenue, Davie, Florida, 33314. *Corresponding author - Manuscript Editor: Jason Davis Southeastern Naturalist M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 366 Many methods have been used to assess prey use of a raptor in an ecosystem. Prey remains have been collected to observe composition of prey species in diet (Marti et al. 2007, Redpath et al. 2001, Steenhof and Kochert 1985). Direct observations and video monitoring have also been used to measure diet composition as well as provisioning rates during the breeding season (Glass and Watts 2009, Rogers et al. 2005). Measuring provisioning rates of breeding raptors is important, as it can provide a metric of the ability of adults to supply an adequate amount of prey to the young, and has been used to help show or suggest limited prey availability with raptor populations (Dykstra et al. 1998, Gill and Elliott 2003, Warnke et al. 2002). Haliaeetus leucocephalus (L.) (Bald Eagle) is a wide-ranging top-level generalist predator in North America, and as with many other raptor species in North America, suffered dramatically from the effects of pesticides, such as DDT, and human persecution over the past couple hundred years (Buehler 2000). While nearly the entire range of Bald Eagles in the lower 48 states shared this population fluctuation, some local populations, however, did not. Florida Bay, located at the southern tip of Florida, represents the Bald Eagle’s southernmost breeding range. Residing within Everglades National Park since its inception in 1948, this population stayed at what is thought to be carrying capacity up until the late 1980s (Baldwin et al. 2012). Over the past few decades, however, this population has experienced a significant population decline (Baldwin et al. 2012) that coincided with drastic ecological changes to the ecosystem (Fourqurean and Robblee 1999). These ecological changes in Florida Bay, including hypersalinity, seagrass die-offs, and algae blooms, affected the distribution and population of prey communities (Lorenz et al. 2009, Matheson et al. 1999, Powell et al. 1989, Sogard et al. 1989) that are part of the diet of the Bald Eagle. Major dietary components of eagles in Florida Bay shifted from the early 1970s to late 2000s (Hanson 2012), suggesting that prey availability has been altered. In addition, prey availability looks to be limiting another large raptor in this ecosystem. The population of Pandion haliaetus (L.) (Osprey), who share breeding and foraging grounds with Bald Eagles in Florida Bay, has decreased by 58% in Florida Bay from 1973 to 1980 (Kushlan and Bass 1983). Toward the end of this timespan in the same area, Poole (1982) correlated brood reduction with lowered provisioning rates. During 1986–1987, Osprey reproductive success was lower in Florida Bay than nearby sites, foraging trips in Florida Bay were often the least successful, and Ospreys that foraged equally in Florida Bay and other locations provisioned less from Florida Bay (Bowman et al. 1989). It is possible that inadequate prey availability is contributing to the decline of the Florida Bay Bald Eagle population. In this study, we monitored prey deliveries and measured provisioning rates of eagle nests with video monitoring throughout the 2009–2010 (hereafter 2009) and 2010–2011 (hereafter 2010) breeding seasons. If there is adequate prey availability for eagles, then provisioning rates should be high enough to supply young with an adequate amount of food throughout the entirety of the breeding season to meet growing energetic demands of the young. We made comparisons to other Bald Eagle populations from other parts of its range, Southeastern Naturalist 367 M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 healthy/growing and relatively less healthy, to provide insight on the meaning of the provisioning rates from this population in Florida Bay . Methods To determine the nest sites that were included in our study, we first used current and historical productivity trends to determine the nest sites that had the highest likelihood of raising young to fledging age (Baldwin et al. 2012). Next, we determined if these nest sites were accessible and whether video-monitoring equipment could be installed. Certain nest sites were not included because they could not be fitted with video-monitoring equipment. We used video cameras to monitor a total of 3 nest sites in the 2009 breeding season and 1 in 2010. We installed camera equipment before November 1 of the corresponding breeding season, which ensured the eagles had time for acclimation with the equipment before egg laying. We placed full-color video cameras (Supercircuits PC263; 3.5”x0.9”) ~1.5–1.8 m (~5–6 ft) from the center of the nest at an angle greater than parallel. When possible, we attached the camera to a limb on the south side of the nest to limit sun glare. We ran audio, video, and power cords down the nest tree and away from the nest at a distance that helped decrease disturbance to the eagles. The cords were attached to a digital video recorder (DVR) (Secumate MDVR-14; 1.18”x3.58”x5.61”) to record video footage. A 12-volt deep-cycle battery, which was continuously charged with a solar panel (Asunpower KIT-020PS60; 21.7”x13.8”x0.98”), powered the camera and DVR. The DVR recorded all video footage from 0.5 hr before sunrise to 0.5 hr after sunset. All video data was recorded to a removable SD card (Sandisk 32GB SDHC). We placed the equipment (not including solar panel or camera) in a weatherproof box in a location below the nest that was least visible to the adults. We typically visited video-monitoring equipment every week, but no less frequently than every 2 weeks, to replace memory cards. At each visit, we checked equipment for proper function and observed any young eagles in the nest with a portable TV set that connected to the DVR. We limited visitation time to no more than 5 minutes to decrease distraction to the eagles. All footage recorded was transferred to multiple hard drives and digitally stored for future reference. We used VLC multimedia computer software (VideoLan Organization) to analyze all video footage. For each prey delivery, we noted date and time of delivery, lowest-determinable taxonomic classification of prey, overall length of fish (when determinable), and sunrise/sunset times. Time of delivery was represented as length of time after sunrise. We estimated mass of each prey item when possible. For avian deliveries, we used the average mass listed for each species in the CRC Handbook of Avian Body Masses (Dunning 1993). We estimated fish length by visually comparing it to an adult eagle talon length. Measurements were made to the nearest 0.5 talon length (e.g., a fish = 3.5 talon lengths). We then converted the fish length to millimeters by using average Bald Eagle talon-length data from Buehler (2000). We calculated mass by using species-specific length–weight conversions (Appendix A). If the Southeastern Naturalist M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 368 conversion for a certain species was not available, we used the regression of a taxonomically similar species. We averaged the daily provisioning rates (# of deliveries/day) and daily total biomass (total biomass/day) of prey deliveries from the third week after hatching to fledging. To determine how frequently and how long eagles deliver prey to the nest after fledging, we calculated the provisioning rate from the third week after hatching until deliveries were no longer made to the nest for each nest and all nests combined as well. To determine whether eagles changed their rate of delivery and size of deliveries, we regressed the mean daily # of deliveries per week and the mean daily total biomass of deliveries per week, respectively, against the age of the young. The slope was determined to be different from zero if the P-value was less than 0.05. We also used linear regression models to test whether the contribution of each prey group (fish, bird, other) and species, calculated as the proportion of the total amount of deliveries, changed throughout the breeding season. We separated prey deliveries that we could identify to at least family level from beginning to end of the breeding season into 4 quarters, and then used a randomization test of independence to see if the composition was different between these time periods. To determine whether eagles provision at different frequencies throughout the day, we separately summed the number of deliveries for the entire breeding season for each hour after sunrise. All data analyses were performed with SAS v 9.2. Results We recorded a total of 546 prey deliveries over the 2 breeding seasons. We identified 93% (n = 508) to class and 46% (n = 253) to family. Fish comprised 86% of all deliveries (sd = 9.2), birds totaled 7% (sd = 1.1), and prey items that could not be identified to class made up 7% (Table 1). There were no other classes that were distinguishable in the video footage. The mean daily provisioning rates for all 4 nest sites, each successfully fledging 1 or 2 young, combined were 1.75 (sd = 0.31, range = 1.33–2.07) deliveries/young/day and 2.64 (sd = 0.7, range = 1.33–3.71) deliveries/ day (Table 2). These rates were correlated to age of young and significantly declined throughout the breeding season (t = 5.3 r2 = 0.80, P = 0.0012; Fig 1). Each nest site showed declining provisioning rates throughout the progression of the breeding season, although only 2 were statistically significant (1 and 2 young, respectively). Prey was delivered to the nest and eaten by the recent fledglings up to 2 weeks after fledging. As expected, this provisioning rate for all nest sites combined was lower than the breeding-season provisioning rate (1.56 [sd = 0.22, range = 1.33–1.79] prey deliveries/ young/day and 2.31 [sd = 0.92, range = 1.43–3.51] deliveries/day), and declined throughout the period observed (t = 8.1, r2 = 0.88, P < 0.0001). We were able to estimate the biomass of prey deliveries at only 1 nest site. The total biomass of prey deliveries/young/day also declined throughout the breeding season (t = 2.9 r2 = 0.68, P = 0.0066; Fig. 2). The randomization test showed that the compositions of prey deliveries throughout the breeding season were significantly different (P = 0.012), of which there was a slight trend to more fish and fewer birds as the breeding season progressed (t = 4.5, r2 = 0.65, P = 0.0028). Deliveries were Southeastern Naturalist 369 M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 mostly frequently made to the nest duringthe daily period 3–5 hours after sunrise and then again at a less frequent rate 9–12 hours after sunrise (Fig. 3). The timing of when the most deliveries were made did not vary between nests (P = 0.84) or change throughout the breeding season (P = 0.42). Discussion The Bald Eagle is a generalist forager and its diet typically includes a variety of prey types (Buehler 2000). Prey remains collected from eagle nest sites in Florida Table 2. Breeding season provisioning rates of 4 Bald Eagle nests, each successfully fledging 1 or 2 young, during the 2009 and 2010 breeding seasons in Florida Bay . Provisioning rates Breeding season # of young Deliveries/young/day Deliveries/day 2009 1 2.07 2.07 1 1.33 1.33 2 1.72 3.44 2010 2 1.85 3.71 Mean 1.75 2.64 Table 1. Number and percent contribution of prey deliveries to 4 Bald Eagle nests during the 2009 and 2010 breeding seasons in Florida Bay. Prey n % group % total Fish Elops saurus L. (Ladyfish) 30 6.4 5.5 Caranx spp. (jack) 26 5.5 4.8 Family Sciaenidae (drums, croakers, seatrout) 14 3.0 2.6 Cynoscion spp. (seatrout) 11 2.3 2.0 Leiostomus xanthurus Lacépède (Spot) 1 0.2 0.2 Sciaenops ocellatus (L.) (Red Drum) 4 0.9 0.7 Ariopsis felis (L.) (Hardhead Catfish) 13 2.8 2.4 Mugil spp. (mullet) 10 2.1 1.8 Trachinotus spp. (pompanos) 6 1.3 1.1 Sphyraena barracuda (Edwards in Catesby) (Great Barracuda) 3 0.6 0.5 Unknown fish 352 49.1 42.3 Subtotal 470 86.1 Birds Phalacrocorax auritus (Lesson) (Double-Crested Cormorant) 8 21.1 1.5 Larus delawarensis Ord (Ring-billed Gull) 2 5.3 0.4 Ajaia ajaja (Roseate Spoonbill) 1 2.6 0.2 Unknown wading bird 3 7.9 0.5 Unknown bird 24 63.2 4.4 Subtotal 38 7.0 Other 0 0.0 Unknown 38 7.0 Total 546 Southeastern Naturalist M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 370 Bay during the same time period as our study consisted of 33 species from 5 classes (Hanson 2012). While video monitoring tends to show more prey deliveries and prey groups than collecting prey remains (Lewis et al. 2004), we were only able to distinguish 12 species (2 classes) using video data. This difference is due in part to Hanson (2012) having collected prey remains from 13 nest sites, while our video data was collected from only 4 nests; video data collection from additional nests would Figure 1. Linear regression of the mean daily provisioning rate averaged per week for 4 Bald Eagle nests during the 2009 and 2010 breeding seasons in Florid a Bay. Figure 2. Linear regression of the mean daily total biomass for 1 Bald Eagle nest during the 2009 breeding season in Florida Bay. Southeastern Naturalist 371 M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 likely lead to a higher number of species witnessed. In additio n, a number of deliveries were not distinguishable past class level (42.3% fish, 4.9% bird) or not able to be identified at all (7%). While the diet of eagles often consists of multiple taxa, the highest contribution has typically come from fish (Dunstan and Harper 1975, Haywood and Ohmart 1986, McEwan and Hirth 1980, Thompson et al. 2005). Our results confirmed this trend, as fish made up 86.1% of all prey deliveries (92.5% of distinguishable) in video data from Florida Bay. This higher contribution of fish and lower contribution of birds than reported from collection of prey remains (Hanson 2012) supports the general concept of a bias towards larger and heavier-boned species groups in prey remains (Marti et al. 2007, Simmons et al. 1991). Provisioning rates of prey to the nest during the breeding season can be used to show or suggest how population sizes and reproductive parameters of breeding raptors can be limited by prey availability (Amar et al. 2003, Wiehn and Korpimaki 1997). The provisioning rate (1.75 deliveries/young/day, max = 2.07; 2.64 deliveries/ day, max = 3.71) that we witnessed was much lower than expected, even for this dwindled population. Compared to other populations of Bald Eagles, these rates are noticeably lower (Table 3). Eagles from north-central Wisconsin had a mean provisioning rate of 3.0 deliveries/young/day (5.2 deliveries/day; Warnke et al. 2002), almost twice the rates we observed. That population, unlike the population in Florida Bay, had high reproductive rates and had recently increased in numbers. Warnke et al. (2002) suggested that the population in Wisconsin was not limited by prey availability and that these provisioning rates reflected an adequate availability of prey to support a thriving Bald Eagle population. This conclusion was further supported when nests from north-central Wisconsin were compared to relatively near-by nests close to Lake Superior. The nests near Lake Superior had Figure 3. Number of deliveries per hour after sunrise for 4 Bald Eagle nests during the 2009 and 2010 breeding seasons in Florida Bay. Southeastern Naturalist M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 372 a provisioning rate of 1.67 deliveries/young/day (2.16 deliveries/day) versus 3.21 deliveries/young/day (4.87 deliveries/day) with nests at north-central Wisconsin (Dykstra et al. 1998). Lake Superior nests have lower fledging rates (young per breeding attempt) than those in Wisconsin, which was partially attributed to the lower provisioning rates that were presumed to reflect lower prey availability. On Vancouver Island, BC, Canada, Bald Eagles had a mean provisioning rates of 5.4 deliveries/day (Elliott et al. 2005) and 3.02 deliveries/young/day (Gill and Elliott 2003), similar to north-central Wisconsin and again around twice as high as eagles in Florida Bay. In that population, the provisioning rate was positively correlated with nesting success. Clearly, caution should be used when comparing eagles from the Great Lakes and Pacific Northwest regions to eagles in a subtropical mangrove estuary. Prey species, temperature, and weather patterns are drastically different in the regions, and there likely are other factors that influence provisioning rates of these populations. Information on prey size and energetic consumption and demands of growing chicks would help further clarify these comparisons. Unfortunately there are no previous estimates of provisioning rates of eagles in Florida Bay, but these other studies showing higher provisioning rates in healthy populations offer some merit for comparison. Ecological conditions determine how available a prey is to a breeding raptor by influencing their total abundance, where they there are located in relation to a breeding territory, and how vulnerable the prey is to predation (May and Norton 1996, Orth et al. 1984, Schmidt and Ostfeld 2003, Schneider 2001). Deviation from historic ecological conditions can elicit changes in availability of one or many prey communities. In the late 1980s, Florida Bay went through drastic ecological changes believed to be due in part to changes in the hydrology of the Everglades that provides freshwater to Florida Bay (Fourqurean and Robblee 1999). In subsequent years, parameters of water quality, including salinity, showed modifications in level and variability during this time (Fourqurean and Robblee 1999). Although not completely understood, one consequence seemed to be massive die-offs and redistributions of seagrass stands that make up much of the habitat in Florida Bay (Hall et al. 1999, Robblee et al. 1991, Zieman et al. 1988). These changes aided the release of sediments into the water, causing algal blooms and increased turbidity (Boyer et al. 1999, Phlips et al. 1993). Blooms and turbidity were stronger and covered larger areas of Florida Bay during the winter months (Boyer et al. 1999, Butler et al. 1995), which is the time that Bald Eagles are breeding in Florida Bay and requiring a higher food supply (as represented by prey rema ins). Table 3. Provisioning rates at Bald Eagle nest sites reported fr om various areas in its range. Location Deliveries/young/day Deliveries/day Source Florida Bay 1.75 2.64 This study Wisconsin 3.00 5.20 Warnke et al. 2002 Wisconsin 3.21 4.87 Dykstra et al. 1998 Lake Superior 1.67 2.16 Dykstra et al. 1998 Vancouver - 5.40 Elliot et al. 2005 Vancouver 3.02 - Gill and Elliott 2003 Southeastern Naturalist 373 M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 While there is limited knowledge of the mechanisms by which these ecological changes may have affected Bald Eagle prey assemblages in Florida Bay, it is known that they have negatively affected some fish and bird populations, the 2 classes that make up the majority of the diets of eagles in the area (Hanson 2012). Fish communities are dependent on the microhabitat differences that characterize Florida Bay (Sogard et al. 1989), and these fish communities have changed at locations that were affected by the ecological alterations that occurred prior to our monitoring period (Matheson et al. 1999). For instance, mullets, a common prey item, are associated with varying salinity levels in Florida Bay (Sogard et al. 1989). Salinity can also affect the metabolic rate, reproduction, and survival of Mugil cephalus L. (Flathead Grey Mullet), one of the Florida Bay mullet species (Cardona 2000, DeSilva and Perera 1976, Lee and Menu 1981). The distributions, egg survival, and growth of Cynoscion nebulosus (Cuvier in Cuvier and Valenciennes) (Spotted Seatrout), another fish prey, are correlated with the varying salinity levels and seagrass habitats in Florida Bay (Neahr et al. 2010, Powell 2003, Thayer et al. 1999). In the Chesapeake Bay, the provisioning rates of both the Bald Eagle and Osprey have been linked to fish assemblages that are determined by salinity-level differences within the region (Glass and Watts 2009, Markham and Watts 2008). Bird communities have also been affected by ecological changes. Other large-predatory birds in Florida Bay have seen a change in abundance and distribution over time (Powell et al. 1989). The nesting subpopulation of Ajaia ajaja L. (Roseate Spoonbill) in Florida Bay has decreased over the same time frame, which has been attributed to hydrologic conditions and salinity in this ecosystem (Lorenz et al. 2009). Information on the abundance and distributions of other fish and birds in Florida Bay is, however, limited. Seasonal shifts in provisioning rates can occur if prey availabilities change throughout a breeding season, especially for bird species with long fledging periods (Weimerskirch and Lys 2000). This scenario presents a challenge for a pair of breeding birds to properly meet the needs of their growing young for the entirety of the breeding season. Not only do provisioning rates at eagle nests in Florida Bay suggest an overall limit of prey availability, the provisioning rate significantly decreased throughout the breeding season. The correlation of age with the mean provisioning rate of the populations in Wisconsin and Vancouver (Dykstra et al. 1998, Elliott et al. 2005, Gill and Elliott 2003)was not stated, suggesting that it was either not examined or there was neither a noticeable decrease nor increase. It may be logical to think that as young in a nest grow older and become larger, they will require more food intake thereby causing their parents to delivery more prey. A decrease in provisioning rate would not likely be a characteristic of breeding predators with adequate prey availability. A possible explanation of this phenomenon is if parents were able to deliver a constant or increased amount of total biomass of prey to the nest by increasing the size of prey deliveries. Determining length of fish can be a difficult task, as eagles don’t always bring in whole fish, and different parts of the fish are often delivered (e.g., tail vs head). Mean length of individual fish deliveries, corrected for whole length, however, did not seem to change throughout Southeastern Naturalist M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 374 the season, suggesting that a change in the mean length of available fish did not change. However, not every fish has the same length-to-mass ratio (e.g., long and skinny vs. short and fat). Even so, we were able to see a decline in the total biomass of prey deliveries throughout the breeding season in addition to provisioning rates. Although length and biomass data were only collected at 1 nest throughout this study, it indicated a significant decreasing trend. This decrease in provisioning rate and biomass throughout the breeding season is another cause for concern and further suggests inadequate prey availability for these eagles. It is a well-studied aspect of avian ecology that populations of birds on the edge of their range are faced with a different set of external variables, such as weather, temperature, physical barriers compared to those found more centrally within the species’ range (Andrewartha and Birch 1954, Root 1988). The Bald Eagles in Florida Bay are the southernmost breeding population, and they certainly are exposed to different external environments than eagles in other areas of the species’ range (e.g., Alaska, Chesapeake Bay, Wisconsin, etc.). One strikingly different environmental factor is ambient temperature, and it is known that thermal stress can affect the distribution and metabolic rate of avian species, including Bald Eagles (Stalmaster 1983, Stalmaster and Gessaman 1984, Stalmaster and Plettner 1992). While Florida Bay has very different extreme temperatures than the more-northern eagle breeding areas, at the times they are breeding (summer in the north vs winter for the south), the temperatures are not as different. The monthly mean air temperatures experienced by each of the previously mentioned eagle populations were on average only 8–9 °C (range = 4–13 °C) cooler than those in Florida Bay during the respective breeding seasons (historical temperature data retrieved from Of the previous studies on thermal stress in Bald Eagles, only wintering eagles in northern latitudes experience thermal stress to the degree that negatively affects them (Stalmaster and Gessaman 1984). Also, there is a critical temperature of 10.6 °C at which eagles begin to feel thermal stress, and energy intake is not different between 5 and 20 °C (Stalmaster and Gessaman 1984). This finding suggests that even though these eagles experience quite different temperatures than northern eagles, such differences should not influence eagle diet to induce lower energy intake and therefore a significantly lower provisioning rate. In addition to temperature, there are additional factors that could influence this southern population differently. Length of time to fledging stage can be variable in the Bald Eagle (Buehler 2000). Growth rate of Bald Eagles is significantly correlated to the total biomass of prey deliveries (Bortolotti 1989). If this population was under stress from an inadequate supply of food causing the adults to provision a reduced amount of food, the result could be a longer time to fledging, though that was not observed. Fledging time in Florida Bay varied from 11–12 weeks long, which is similar to other populations of Bald Eagles (12 wks in California, 11 in Florida, 11–13 in Maine, and 11–12 in Saskatchewan; Buehler 2000). It is also known that southern Bald Eagles are generally 15–20% smaller than northern Bald Eagles (Buehler 2000). This smaller adult size would require a lower growth rate (and hence lower provisioning rates) if the time to reach fledging stage is constant. Southeastern Naturalist 375 M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 Unfortunately, these parameters are currently not understood in great detail for this population. It is unknown how much of a difference in size there is between the Vancouver and Wisconsin eagles and the Florida Bay eagles, but that is a factor that could potentially play a role in the much lower provisioning rates we observed compared to those for the northern populations. Generalist raptors have the ability to take a wide variety of prey types. This trait should prove useful to them if they were to experience changes in prey availabilities throughout a breeding season. In addition to observing decreasing provisioning rates at Bald Eagle nests, we noted a change in the contribution of prey groups throughout the breeding season. These parameters could be a function of the changing nutritional needs of the young, as stated by Gill (2007), but also as a result of seasonal population fluctuations of both fish and avian prey in Florida Bay and year-to-year variation in the timing of these seasonal population fluctuations. The composition and densities of fish communities within Florida Bay fluctuates with changing seasons and months (Matheson et al. 1999, Thayer et al. 1999). In one example, the Spotted Seatrout, a common prey item of Bald Eagles in Florida Bay, have peak densities and spawning at specific times during the year, and these peaks can occur at different times of the year from year to year (Powell 2003, Powell et al. 2007). Abundance of nearly all wading-bird species of Florida Bay also fluctuate in density seasonally, and peak nesting months very between years (Lorenz et al. 2002, Powell 1987). These annual fluctuations in density of prey species in Florida Bay are connected to the annual seasonal cycle of precipitation in south Florida that represents the wet and dry seasons. This change in the amount of rain directly contributes to seasonal oscillations in salinity levels (Kelble et al. 2007) and water depths (Montague and Ley 1993) of Florida Bay. These ecosystem conditions have a direct affect on the prey composition, abundance, and distribution of prey in Florida Bay (Lorenz et al. 2009, Matheson et al. 1999, Powell et al. 1989, Sogard et al. 1989) and could be leading Bald Eagles to change their food habits throughout the season. Florida Bay has historical regular variability (Hall et al. 1999, Kelble et al. 2007, Lorenz 2014), and these shifts could be a natural occurrence that is unique to this local population. If this were the case, the Bald Eagle population would have evolved within the context of this cyclically fluctuating ecosystem and presumably that variation would not lead to a reduction of the breeding population as we have seen. Further analysis of declining provisioning rates and biomass, and change in composition of prey should be further studied to fully understand how it might affect this population of Bald Eagles. The population fluctuation of Bald Eagles over the past couple centuries in response to pesticides, such as DDT, and human persecution have been documented in great detail (Buehler 2000). Up until the late 1980s, the breeding population in Florida Bay was thought to be at carrying capacity (Baldwin et al. 2012). Residing within the boundaries of Everglades National Park since 1948, this population did not seem to be affected by the same factors that caused declines in other Bald Eagle and raptor populations across the country. However, over the past few decades this Southeastern Naturalist M.R. Hanson and J.D. Baldwin 2016 Vol. 15, No. 2 376 population has seen a significant loss of occupied breeding territories (Baldwin et al. 2012) that coincided with ecological changes from altered hydrology in the Everglades (Lorenz 2014). Since there is no indication that pesticides, habitat loss, persecution, or other potential limiting factors are affecting these eagles, food limitation is a leading hypothesis for this decline. A lack of adequate food supply can affect life-history traits, population sizes, and community structure of raptors (Martin 1987, Poole 1982, Rutz and Bijlsma 2006). Though we were not able to directly correlate provisioning rates to prey availabilities in this study, we believe there is an inadequate food supply for a healthy Bald Eagle population in Florida Bay. This factor could be limiting the breeding population and presents the need for further studies on how provisioning rates and reproductive success are related. With the goal to restore Florida Bay to a historical observed state, management efforts to mitigate environmental impacts may cause prey availabilities to continue to change and thereby alter Bald Eagle food habits. Provisioning rates and diet may serve as a monitoring tool for the condition of prey communities in the future and our findings give merit to their continued examination. Acknowledgments Financial support was received from Everglades National Park, Florida Atlantic University Environmental Sciences Program Everglades Fellowship, and The International Osprey Foundation. The Florida Museum of Natural History offered use of their fish and bird collections for prey-remain reference material. We thank S. Bass, M. Parry, B. Mealey, and L. Oberhofer for their time and input to the project and N. Dorn, C. Hughes, and D. 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List of length–weight equations used and the species that the equation was used for. Equations retrieved from Prey item used for Length–weight equation Species used in equation Barracuda W = 0.0050*L^3.083 (n = 10) Sphyraena barracuda Hardhead Catfish W = 0.0081*L^3.196 (n = 101) Ariopsis felis Ladyfish W = 0.0056*L^3.100 (n = 776) Elops saurus Mullet W = 0.0213*L^2.750 (n = 465) Mugil cephalus Pompano W = 0.0453*L^2.300 (n = 9) Trachinotus carolinus (L.) Redfish W = 0.0077*L^3.098 (n = 41) Sciaenops ocellatus Seatrout, Spotted Seatrout W = 0.0088*L^3.000 (n = 400) Cynoscion nebulosis Spot W = 0.0092*L^3.072 (n = 944) Leiostomus xanthurus