The Condor 99:894-905 0 The Cooper Om~thological Society 1997
HYDROLOGICAL CONSTRAINTS ON TRICOLORED HERON AND SNOWY EGRET RESOURCE USE ’
ALLAN M. STRONG*, G. THOMAS BANCROFT~AND SUSAN D. JEWELL~ National Audubon Society, Tavernier Science Center. 115 Indian Mound Trail, Tavernier, FL 33070
Abstract. We investigatedresource use by breeding Tricolored Herons (Egretta tricolor) and Snowy Egrets (E. rhula) at a regional scale (1,3 12 km*) during three years (1987-1989) with dissimilar water levels. We documented the direction and distance flown from the colony to feeding sites, habitat selection, daily movement patterns, feeding flock size, and diet at the Rodgers River Bay colony in Everglades National Park, Florida. The general pattern of flight direction was similar between the two species;both speciesflew northwest in 1987, a wet year, and northeast in 1988, a year with moderate surface water drying. During drought conditions in 1989, only Tricolored Herons nested successfully.Despite similarities in flight direction, habitat selection differed between the two species.Tricolored Herons were more consistent in habitat use among years, whereas Snowy Egret habitat selection was more variable. However, shifts in habitat selection in response to varying hydrologic conditionsamong years were greaterthan differences in habitat selectionbetween the two species within a year. Prey selection was similar for the two species in 1987, indicating that they procured similar prey from different habitat types. Regardlessof water conditions, Snowy Egrets were significantly more gregariousthan Tricolored Herons, and feeding sites on consecutive days by radio-tagged Snowy Egrets were farther apart than those usedby Tricolored Herons. Our resultssuggest that habitat selectionoccurs at a coarser scale than previously recognized and that studies conductedat single sites over short tem- poral scalesmay be ignoring important aspectsof habitat selection. Key words: Egretta thula, Egretta tricolor, Everglades, habitat selection, hydrologic con- straints, resource use, water level.
INTRODUCTION conditions, but shorter time scales may be ade- Studies of resource use are important in under- quate to examine physiological responses.Sim- standing an animal’s behavior, adaptations to its ilarly, regarding spatial scale, studies of habitat environment, inter- and intraspecific interac- selection should include an organism’s entire tions, and population dynamics. Although re- home range, whereas more local scales may be source use can be investigated across a contin- appropriate for behavioral studies (May 1993). uum of temporal and spatial scales, the scale at Ciconiiformes are readily amenable to studies which the investigation is conducted will have of habitat use, foraging behavior, and prey se- dramatic effects on the observed patterns and lection because they are large, conspicuous,and conclusions about the underlying processes easily observable. Within the Ciconiiformes, Tri- (Wiens 1981, Levin 1992, May 1993). For ex- colored Herons (Egretta tricolor) and Snowy ample, studies of an organisms’ movements in Egrets (E. thulu) are of particular interest be- response to changes in environmental parame- cause they have been consistently characterized ters should be conducted over sufficiently long as one of the most ecologically similar species- time scales to observe a range of environmental pairs based on geographicrange, body size, hab- itat use, and feeding behavior (Kushlan 1978, Recher and Recher 1980). Studies of resource ’ Received 9 January 1997. Accepted 30 July 1997. partitioning between these two species have * Current address: Department of Ecology,‘Evolu- found differences in feeding behavior, prey se- tion, and Oraanismal Bioloav. 310 Dinwiddie Hall, Tulane Univ&ity, New Orl&s, LA 70118, e-mail: lection, and habitat use; however, the segregat- [email protected] ing mechanisms have been inconsistent among 3 Current address: Archbold Biological Station, studies (Willard 1977, Recher and Recher 1980). MacArthur Agro-ecology Research Center, 300 Buck Because most of these studies have been con- Island Ranch Road, Lake Placid, FL 33852. 4 Current address: U.S. Fish and Wildlife Service. ducted at local spatial scales (less than the area Loxahatchee National Wildlife Refuge, 10216 Lee used by an entire colony) and short temporal Road, Boynton Beach, FL 33437-4796. scales (less than one year), they may have been
18941 EGRE77-A RESOURCE USE 895 inadequate to assesshow Tricolored Herons and their nests in mid-March (Bancroft et al. 1990). Snowy Egrets partition resources.In south Flor- Approximately 200 pairs of Tricolored Herons ida, rainfall, and therefore regional wetland wa- began renesting in mid-April, and although nest- ter levels, often varies dramatically between ing success was low, many remained through years (Duever et al. 1994, Fennema et al. 1994). June. Because of this environmental variability and the ability of Tricolored Herons and Snowy HYDROLOGY Egrets to respond to it with a diversity of for- Daily water level data from the ornithology (OT) aging behaviors (Kushlan 1977), quantification gauge, 11.5 km east of the Rodgers River Bay of resource use within a single breeding season colony, provided an indication of water levels in also may result in a simplified view of resource the eastern half of the study area. When water partitioning. levels are at or near ground level at this gauge, We took advantage of large scale interannual substantial shallow pools exist east of the colo- fluctuations in hydrological conditions in south ny; when water levels are high, inland marshes Florida from 1987-1989 to examine the effects are flooded too deeply for efficient foraging. We of water level variation on breeding Tricolored analyzed water levels for January through mid- Heron and Snowy Egret resource use across July, the period of peak nesting activity at the their entire foraging radius (1,3 12 km*) sur- Rodgers River Bay colony (Bancroft et al. rounding the Rodgers River Bay colony in Ev- 1990). To simulate an “average” hydropattem erglades National Park (ENP), Florida. We pres- to compare to the 3 years of our study, we used ent comparative data on distance and direction OT gauge data from 1980-1990 to calculate to foraging sites, habitat selection, daily move- mean daily water levels. No water level data ment patterns, prey use, feeding flock size, and were available for the western coastal areas. The colony-wide responsesto changesin water level. hydrology of the western half of the foraging We use these data to determine the relative area is complex. Mueh of the area is composed strengthsof environmental conditions and intrin- of perched wetlands primarily influenced by lo- sic differences between the species on resource cal rainfall; the remainder is a complex mosaic use patterns. We argue that patterns of resource of ponds and small tidal creeks. Local rainfall, use exhibited at spatial scales less than the area wind driven tides, and lunar tides in that order used by the entire colony and temporal scales appear to be most influential on water depths. less than or equal to one breeding season may be misleading, especially if temporal variation FLIGHT DIRECTION in the environment is extreme. Becausethe relative proportion of habitats avail- able to breeding wading birds varied relative to METHODS direction from the colony, flight-line counts pro- vided a broad assessmentof habitat use patterns STUDY AREA (Erwin 1983). At the Rogers River Bay colony, We studied breeding Tricolored Herons and birds flying east were generally selecting fresh- Snowy Egrets during the 1987-1989 nesting water habitats and birds flying west were se- seasons at the Rodgers River Bay colony lecting marine or brackish habitats. We con- (25”33.28’N, 81”04.28’W) in northwestern ENP. ducted counts approximately weekly when most Since the late 1970s this colony has contained nests contained eggs or nestlings. In 1987, we the largest nesting populations of Tricolored counted the number of Tricolored Herons and Herons and Snowy Egrets in the freshwater por- Snowy Egrets flying into and out of the colony tion of ENP (Ogden 1994). In both 1987 and in four 90% quadrants (northeast, southeast, 1988, approximately 2,000 pairs of wading birds southwest, and northwest) while we circled the nested at Rodgers River Bay colony, about 80% colony for 15 min in a Cessna 172 aircraft at of which consistedof approximately equal num- 143 km hr-l and an altitude of 210 m. In 1987, bers of Tricolored Herons and Snowy Egrets we conducted eight censuses between 12 May (Bancroft et al. 1990). Nesting numbers peaked and 25 June. In 1988, we counted all species in mid-May in 1987 and in mid-April in 1988. flying into and out of the colony from a boat In 1989, Snowy Egrets and Tricolored Herons anchored approximately 150 m from the colony. began nesting in late February, but abandoned Two observers working together censused two 896 ALLAN M. STRONG ET AL.
quadrantssimultaneously for 15 min then moved to breeding Tricolored Herons and Snowy to the opposite side of the colony and censused Egrets. The portion of the circle that enclosed the remaining two quadrants. We conducted 13 the Gulf of Mexico (209 km2) was excluded censuses on consecutive days between 5 April from analyses. We used 13 U.S. Geological Sur- and 30 June. vey 7.5 min orthophotomaps to digitize the boundaries of each of the five habitat types HABITAT AVAILABILITY around Rodgers River Bay colony into a com- We distinguished five habitat types around the puter-compatible format. We surveyed the area colony that correspond roughly to those of pre- in a Cessna 172 aircraft to verify habitat demar- vious habitat classifications (Egler 1952, Duever cations. et al. 1986). From fresh to salt water habitats We calculated the area of each habitat type these are: within the 22-km radius circle (availability) us- Inland marsh. Sawgrass-dominated(Cladium ing Atlas Draw software (Strategic Locations jamaicensis) marsh in which hydrology is influ- Planning, Inc., San Jose, California). Because enced by both upstream freshwater flow and lo- mangrove-inland marsh interface was an eco- cal rainfall. This habitat contains tree islands and tonal habitat, we calculated its area by deline- cypress (Taxodium spp.) domes and is divided ating a 0.5~km wide “corridor” around the dig- by deeper sloughs. itized line separatinginland marsh or slough and Slough. Areas of deeper water (l-2 m) within mangrove-coastal marsh habitats. Of the area the inland marshes provides the majority of contained in the mangrove-inland marsh inter- freshwater reaching the estuaries. Compared to face corridor, 50% was subtractedfrom the &ea inland marsh, sloughs have a lower proportion of mangrove-coastal marsh and 23% and 27% of sawgrassand a higher proportion of submer- were subtractedfrom the area of slough and in- gent vegetation (e.g., Eleocharis sp., Nymphaea land marsh habitat types, respectively. These spp., and Nuphar odorata). values were proportional to the linear distance Mangrove-inland marsh interface. The nar- each habitat extended along the line bisecting row ecotonal area between the mangrove-coast- the mangrove-inland marsh interface corridor. al marsh and the inland marsh and slough hab- The above calculations yielded habitats in the itats. Numerous mangrove “finger” projections following proportions (total area = 1,312 km2): extend into the inland marsh. inland marsh, 0.290; slough, 0.068; mangrove- Mangrove-coastal marsh. A large and het- inland marsh interface, 0.076; mangrove-coastal erogeneous transitional area between the man- marsh, 0.375; mangrove, 0.190. groves and the inland marshes and sloughs. These areas are dominated by emergent grasses, HABITAT SELECTION AND FZOCK SIZE sedges and rushes (e.g., Spartina spp., Juncus To quantify habitat selection, we used following spp., bulrush S&pus spp., cattails Typha sp., flights and radiotelemetry to compare habitat use and sawgrass) and water tolerant trees and patterns versus habitat availability. During fol- shrubs (e.g., willow Salix caroliniana, button- lowing flights, we circled the colony in a Cessna wood Conocarpus erectus, wax myrtle Myrica 172 aircraft and waited until a bird of a prese- cerifera, and red mangrove Rhizophora mangle). lected species flew from the colony. We fol- This habitat also contains many small ponds. lowed the bird, remaining 60-180 m above it, Mangrove. Red and black mangrove (Avicen- until it landed and began feeding or it landed in nia germinans) foreststhat are essentially a con- a flock of other feeding birds. We returned to tinuous cover of woody vegetation. Some areas the colony and circled once before selecting an- of higher elevation harbor less water-tolerant other bird. Following flights were conducted in woody species. both the morning and afternoon during the ,pe- We quantified the available habitat based on riod when most nests contained young (based on the mean foraging flight distances for breeding ground-based nest checks). birds at the Rodgers River Bay colony. As 95% We also fitted nesting adult Snowy Egrets (n of all foraging locations were within 22 km of = 13) and Tricolored Herons (n = 10) with ra- the colony (mean flight distance + 2 SD), we diotransmitters. We caught birds using a walk-in used a 22-km radius circle (1,521 km2) around trap placed above the nest platform (Jewel1 and the colony to define the foraging area available Bancroft 1991) and attached solar/capacitor-as- EGRE77A RESOURCE USE 897 sisted transmitters to them using a backpack- arcsine transformation, and skewness in flight type harness (Dwyer 1972). Telemetered birds distances (t = 8.15, P < 0.001) was reduced (t were then located from a Cessna 172 equipped = 0.80, P > 0.20) through a square root trans- with “side-looking” H-antennae. We could pin- formation (Snedecor and Co&an 1980). point radio-tagged birds by listening to either one antenna alone or both simultaneously using RESULTS a switch-box. Becausenesting wading birds gen- HYDROLOGY erally return to the colony site at least once per Long-term average water levels decreasedfrom day (Hafner et al. 1993), we assumed observa- January though May and increased from June tions separatedby 24-hr intervals were indepen- through July (Fig. 1). During our study, the 1987 dent (Swihart and Slade 1985) and therefore re- hydrograph showed the highest overall water corded a maximum of one feeding location per levels, the most extreme reversals in the spring day per radio-tagged bird. We used data only for drying pattern, and the highest minimum water radio-tagged birds with active nests. If their nest level (10 cm). The 1988 hydrograph was most failed or the young fledged, we excluded sub- similar to the 11-year mean. In 1989, water lev- sequent locations from the analyses. els were nearly always below ground level and We marked each landing site or telemetry lo- exhibited erratic fluctuations. Drought condi- cation on a 7.5 min orthophotomap,and record- tions existed throughout south Florida during ed number and species composition of birds at this period. For the five-month period, Novem- the site. Foraging locations were later converted ber-April, rainfall at 40-mile bend in ENP av- to Universal Transverse Mercator grid coordi- eraged 275 mm for a 32-year period through nates for calculation of flight distances and cat- 1989. Rainfall was 408 mm, 307 mm, and 143 egorization of habitat type. Following flights and mm during November 1986April 1987, No- radio-tagged birds provided similar estimates of vember 1987-April 1988, and November 1988- distance to feeding sites for Tricolored Herons April 1989, respectively. in 1987 and 1988 and Snowy Egrets in 1987 (all t < 1.6, all P > 0.12), but in 1988, radio-tagged FLIGHT DIRECTION Snowy Egrets fed farther from the colony than Because we used different methods in 1987 and the mean following flight distance (t,,, = 3.69, 1988 to evaluate flight direction, we compared P < 0.001). Therefore, we present both values the distribution of landing sites (northeast, for Snowy Egrets in 1988. southeast,southwest, or northwest of the colony) of radio-tagged birds and following flights with FOOD HABITS flight-line count distributions for weeks which To quantify differences in diet between the two we had both types of data. We found no signif- species, we collected regurgitated boluses from icant differences (all Gs < 7.82, P > 0.05) nestling Tricolored Herons and Snowy Egrets among distributions in 5 of 7 weeks and 7 of 9 during routine nest checks in 1987. Boluses weeks for Tricolored Herons in 1987 and 1988, were preserved in 10% formalin, and stored in respectively, and in 5 of 7 and 7 of 12 weeks isopropyl alcohol. The contents of each sample for Snowy Egrets in 1987 and 1988, respective- were identified to species and weighed (wet) to ly. We concluded that counting from the air and the nearest 0.01 g. by boat both produced results generally consis- tent with following flights and telemetry and STATISTICAL ANALYSES therefore differences in flight direction between We used SPSS/PC+ (Norusis 1988) for all sta- years were not an artifact of sampling method. tistics except G-tests (Sokal and Rohlf 1995) and We counted 4,043 Tricolored Herons and confidence intervals around observed propor- 4,762 Snowy Egrets during flight-line counts on tions. Selection or avoidance of a habitat type or 8 days (12 May-25 June) in 1987 and 13 days flight direction was considered significantly dif- (5 April-30 June) in 1988. During 1989, the col- ferent from expected if the expected proportion ony was small and flight-line counts were not fell outside the 95% confidence interval around conducted. During the high water conditions of the observed proportion (Neu et al. 1974). Kur- 1987,44% of the Tricolored Herons and 62% of tosis in flight-line counts (t = 2.68, P < 0.01) the Snowy Egrets flew to the northwest (Table was reduced (t = 1.67, P > 0.05) through an 1). In 1988, when water levels were lower, 49% 898 ALLAN M. STRONG ET AL.
- II-yrmean 41987 -1988 -1989 50
-20
-30
-40 1 Jan 1 Mar 1 May 1 Jul FIGURE 1. Water level (cm) at the OT gauge (located 11.5 km east of the Rodgers River Bay colony) from 1 January to 15 July 1987, 1988, 1989, and the II-year mean hydrograph (1980-1990). The solid horizontal line at 0 representsground level.
of the Tricolored Herons and 45% of the Snowy water level fluctuations than Snowy Egrets. This Egrets flew to the northeast. Although there is well illustrated in 1988 on the 25 April, 2 were differences between Tricolored Herons and May, and 10 May counts, when water level at Snowy Egrets in direction flown to feeding sites the OT gauge increased from -5.49 cm to 4.88 in 1987 (G3 = 93.2, P < 0.005) and 1988 (G3 cm, then decreased to -8.84 cm. During all = 62.8, P < 0.005), the shifts between years three counts, Tricolored Herons selected the were more pronounced (G3 = 513.1, P < 0.005 northeast direction (all P < 0.05). In contrast, for Snowy Egrets; G, = 149.9, P < 0.005 for Snowy Egrets selected the northeast direction Tricolored Herons). during low water levels on 25 April, shifted to The proportion of both Tricolored Herons and the northwest when water levels increased, then Snowy Egrets flying in the two westerly direc- returned to the northeast on 10 May as water tions was positively correlated with water level levels again declined (all P < 0.05). at the OT gauge and the proportion flying nortb- east was negatively correlated with water level MOVEMENT PATTERNS (Table 2). Thus, as water level declined at the Mean (? SD) distance flown to feeding sites by OT gauge (in inland marsh and slough habitats), Tricolored Herons varied among years from 12.8 more birds flew east. -t 5.9 km in 1987 (n = 39, including 31 loca- Tricolored Herons appeared less sensitive to tions of 4 radio-tagged birds) to 8.6 k 4.3 km
TABLE 1. Percentageof Tricolored Herons and Snowy Egrets flying in four directionsfrom the RodgersRiver Bay colony, Everglades National Park, Florida.
DireChIC Total no. birds % NE % SE % SW %NW G Tricolored Heron 1987” 578 24 14- 18- 44+ 113.3** Snowy Egret 1987” 602 6- 12- 21 62+ 418.6** Tricolored Heron 1988” 3,465 49+ 14- 9- 28+ 657.7** Snowy Egret 1988b 4,160 45+ 13- 6- 36+ 952.0**
an = 8 days. bn = 13 days. c “+” indicates selection for that direction, “-” indicates selection against (Bonferroni Z-statistic, P < 0.05). **P < 0.01. EGRETI-A RESOURCE USE 899 in 1988 (n = 91, including 57 locations of 6 TABLE 2. Correlation coefficients (r) and probability radio-tagged birds) to 5.4 2 3.9 km in 1989 (n levels (P) between arc sine percentof birds observedin each flight-line directionand water level at the OT gauge = 135, including 1 location of 1 radio-tagged in 1987 and 1988 (all n = 21). bird) (ANOVA, F2,262= 50.07, P < 0.001, all means significantly different from each other, Snowy Egrets Tricolored Herons
Student-Neuman-Keuls test, all P < 0.05). Dis- Drection I P I P tances flown by Snowy Egrets to feeding sites Northeast -0.71
spectively. Because of the diversity of prey types (> 30 species), we pooled all prey items that contributed < 10 g to the total prey mass into an “other” category. Consequently, the analysis included 7 prey types. For both Tricol- ored Herons and Snowy Egrets, Poecilia latipin- na and Fund&s confluentuswere the two most important speciesbased on prey mass (Table 3). We found no differences between Snowy Egret’s and Tricolored Heron’s consumption of the 7 prey types using prey mass, prey number, or per- cent of bolus mass as dependent variables (re- peated measuresANOVA, all F,,,, < 2.45, all P > 0.12). Thus, although habitat selection dif- fered in 1987, Tricolored Herons and Snowy 100, I Egrets consumed similar prey.
DISCUSSION SPATIAL SCALE AND RESOURCE t PARTITIONING .s tu 60 All previous studies of the foraging ecology of
i Tricolored Herons and Snowy Egrets have de- z termined at least one processby which the two 40 z species partition resources; however, conclu- s sions regarding the methods of partitioning have
20 varied according to the spatial scale of the in- vestigation. Our use of a regional scale (the en- tire foraging area of an active colony) docu- 0 IM SL MI MC MN mented patterns of habitat selection not obvious in previous studies of wading bird ecology. Dur- HABITAT TYPE ing all three years of the study, we found sig- FIGURE 2. Habitat use by Tricolored Herons (solid nificant differences in habitat selection between bars) and Snowy Egrets (hatched bars) breeding at the Tricolored Herons and Snowy Egrets. Although Rodgers River Bav colonv in (A) 1987 and (B) 1988. the specific habitat types selected by Tricolored IM z inland ma&h, SL ‘= sldugh, MI = mi&grove- inland marsh interface, MC = mangrove-coastal Herons and Snowy Egrets will vary acrossstud- marsh, CM = coastal marsh, and MN = mangrove. ies, our results indicate that habitat selection An asteriskindicates a particularhabitat type was used must be quantified across the entire foraging significantly greater or less than expected relative to range of the colony and over more than one the proportion of available habitat (Bonferroni Z-sta- breeding season to accurately address resource tistic, P < 0.05) (Neu et al. 1974). partitioning. Obviously, no single spatial or temporal differences between species appeared unrelated scale will be appropriate for assessing all eco- to environmental conditions. In both years, dif- logical interactions; however, the scale at ferences were greater between the species (both which the investigation is conducted must be G, > 32.8, P < 0.005) than differences within addressed in the interpretation of the results species between years (G, = 26.0, P < 0.005 (Meentemeyer 1989, Levin 1992). If species for Snowy Egrets; G, = 2.4, P > 0.5 for Tri- use several habitats across a heterogeneous colored Herons). landscape, studies confined to a single habitat or single breeding season may produce unre- FOOD HABITS peatable patterns or misleading impressions of In 1987, we collected 27 (total mass = 247.8 g) their ecology (Wiens 1989). For example, and 3 1 (total mass = 392.2 g) boluses from nest- Snowy Egrets tended to use inland or fresh- ling Tricolored Herons and Snowy Egrets, re- water sites more than Tricolored Herons in EGRElTA RESOURCE USE 901
FIGURE 3. Foraging locations of Tricolored Herons (triangles) and Snowy Egrets (circles) breeding at the Rodgers River Bay colony (star) in (A) 1987 (n = 39 and 68, respectively) and (B) 1988 (n = 91 and 156, respectively).
coastal North Carolina (Custer and Osborn deeper water habitats and Snowy Egrets fed 1978a) and in YucatBn, Mexico (Ramo and primarily in open shallow habitats (Jenni Busto 1993). However in coastal New Jersey, 1969). These results emphasize the plasticity no consistent segregation among freshwater in habitat use shown by these two species. and brackish habitats was found (Willard However, in most of these cases, resource par- 1977). In brackish and marine environments of titioning may not have been accurately deter- ENP, Snowy Egrets fed in more open habitats mined because either habitat use was quanti- than Tricolored Herons (Recher and Recher fied in an area less than that used by the entire 1980), whereas in Tampa Bay, Florida, Tricol- colony and/or habitat use was not measured ored Herons used more open water habitats relative to available habitat. Although some and Snowy Egrets fed more in tidal pools questions may be better addressed at a local (Kent 1986). In freshwater habitats in north scale, our results suggest that fine scale habitat Florida, Tricolored Herons used the edges of selection may not address resource partition- 902 ALLAN M. STRONG ET AL
FIGURE! 4. Foraging locations of Tricolored Herons (triangles, n = 13.5)breeding at the Rodgers River Bay colony (star) in 1989. ing in Tricolored Herons and Snowy Egrets mes of which 17 were used by Tricolored Her- because coarser scale segregation has already ons and 20 by Snowy Egrets. The diversity of occurred. foraging behaviors suggests extreme opportun- Similarly, use of foraging behaviors or prey ism (Meyerriecks 1962, Kushlan 1976, 1978), selection may be inadequate axes to examine re- and implies that examination of resource parti- source partitioning in Tricolored Herons and tioning through foraging behavior may be site- Snowy Egrets. Ciconiiformes have evolved a di- specific. For example, in northern Florida, verse array of foraging behaviors that enable Snowy Egrets were more active feeders than Tri- them to take advantage of a variety of environ- colored Herons (Jenm 1969), whereas in Tampa mental conditions. Kushlan (1977) reported 38 Bay (Rodgers 1983, Kent 1986) and Florida Bay different feeding behaviors used by Ciconiifor- (Meyerriecks 1962), Tricolored Herons used more active foraging methods. In New Jersey, Snowy Egrets displayed a larger variety of be- haviors than Tricolored Herons (Willard 1977), but in Tampa Bay, Tricolored Herons used
80 slightly more diverse foraging behaviors (Rodg- ers 1983). Furthermore, differences in prey se- lection also are variable among studies. Kent (1986) found pronounced differences in prey se- lection between Snowy Egrets and Tricolored Herons, whereas others have found only mod- erate (Jenni 1969, Willard 1977) to slight dif- ferences (Ogden 1977, Kushlan 1978, Recher and Recher 1980). In our study, despite differ- ences in habitat selection between the two spe- 1 2-5 8-49 >49 cies in 1987, prey use was similar. Our results Flock size indicate that in the Everglades, foraging behav- FIGURE 5. Percent of breeding Tricolored Herons ior and prey selection may not be the most im- (solid bars, n = 265) and Snowy Egrets (hatchedbars, portant mechanisms of resource partitioning in n = 214) found in four size categories of foraging Tricolored Herons and Snowy Egrets because flocks (1987-1989). Birds were either marked with ra- diotransmitters or followed from the Rodgers River segregationthrough habitat selection has already Bay colony to the feeding site. occurred. EGREiTA RESOURCE USE 903
TABLE 3. Mean (? SE) numberand percentof bolus massfor sevenprey items in the bolusesof nestlingSnowy Egrets (SNEG, n = 31) and TricoloredHerons (TRHE, n = 27) at the RodgersRiver Bay colony, 1987.
Number % bolus mass
Prey item SNEG TRHE SNEG TRHE Poecilia latipinna 7.4 2 1.3 4.7 + 1.1 49.7 2 6.9 36.7 t 7.2 Fundulus conjhentus 1.4 f 0.5 1.2 t 0.3 11.8 5 4.1 17.5 2 4.5 Gambusiaafinis 5.9 2 1.5 5.9 k 1.8 9.0 2 1.9 13.3 2 3.4 Anchoa spp. 3.6 t 1.7 0.6 ? 0.4 13.3 k 6.1 4.6 k 3.2 Cyprinodonvariegatus 0.5 + 0.4 1.4 2 0.6 0.7 2 0.7 13.4 2 5.1 JordanellaJoridae 1.4 5 0.4 1.0 2 0.3 6.8 ? 3.4 6.9 ? 2.2 Other 3.7 5 1.0 3.2 2 0.8 8.6 5 3.2 7.6 2 2.7 Total bolus mass (g) 392.2 248.8
HYDROLOGIC CONSTRAINTS ON HABITAT nesting attempt and did not return to the colony SELECTION site. Although habitat selection varied between the Differences between the species’ responsesto two species,our results suggestthat annual vari- changing hydrologic patterns are most likely re- ation in water levels had a greater effect on hab- lated to differential dependence on group for- itat selection than intrinsic differences between aging (Jenni 1969, Kushlan 1978, Smith 1995). the species. Changes in water levels between Snowy Egrets have greater successfeeding in years were correlated with significant shifts in large flocks than solitarily (Master et al. 1993) direction flown to feeding sites, and consequent- and are core species in the formation of large ly habitat selection. In 1987, when water levels mixed-species feeding aggregations (Kushlan were high in the freshwater marshes east of the 1977, Caldwell 1981). These large feeding ag- colony, both speciesflew west to mangrove and gregations may require higher densities of food mangrove
support large foraging aggregations. Thus, as LITERATURE CITED
food resourcesbecome patchier during drought BANCROFT,G. T., S. D. JEWELL,AND A. M. STRONG. conditions, Tricolored Herons may be able to 1990. Foraging and nesting ecology of herons in nest in conditions that Snowy Egrets may not be the lower Everglades relative to water conditions. able to tolerate. Final report to South Florida Water Management District, West Palm Beach, FL. CALDWELL,G. S. 1981. Attraction to mixed-species CONSERVATION RECOMMENDATIONS heron flocks: proximate mechanism and conse- The availability of food in the Everglades may auences.Behav. Ecol. Sociobiol. 8:99-103. CUSTER,T. W., AND R. G. OSBORN.1978a. Feeding be the single most important variable influencing habitat use by colonially-breeding herons, egrets, nesting success(Frederick and Spalding 1994). and ibises in North Carolina. Auk 95:733-743. The quality, quantity, and availability of food CUSTER,T W., AND R. G. OSBORN.1978b. Feeding- are determined by antecedentand current hydro- site description of three heron speciesnear Beau- fort, North Carolina, p. 355-360. In A. Sprunt IV, logical conditions (Loftus et al. 1990, Loftus and J. C. Ogden, and S. Winckler [eds.], Wading birds. Eklund 1994). The greater shifts in habitat use Natl. Audubon Sot., New York. and abandonment of this area during drought DUEVER,M. J., J. E. CARLSON,J. E MEEDER,L. C. conditions by Snowy Egrets suggest that they DUEVER,L. H. GUNDERSON,L. A. RIOPELLE,T R. ALEXANDER,R. L. MYERS,AND D. F?SPANGLER. may have stricter foraging requirements than 1986. The Big Cypress National Preserve. Natl. Tricolored Herons. This was further supported Audubon Sot. New York. in 1990 (another year of drought conditions) DUEVER,M. J., J. E MEEDER,L. C. MEEDER,AND J. M. when moribund Snowy Egret nestlings were MCCOLLOM.1994. The climate of south Florida and its role in shapingthe EvergladesEcosystem, found at the Rodgers River Bay colony at the p. 225-248. In S. M. Davis and J. C. Ogden [eds.], same time similar age, well-fed Tricolored Her- Everglades: the ecosystem and its restoration.St. ons nestlings were present (Frederick et al. Lucie Press, Delray Beach, FL. 1992). Apparently Tricolored Herons were find- DWYER,T. J. 1972. An adjustable radio-packagefor ducks. Bird-Banding 43:282-284. ing sufficient food to successfully raise young, EGLER,F. E. 1952. Southeastsaline evergladesvege- whereas Snowy Egrets were not. These differ- tation, Florida, and its management.Vegetatio 3: ences in foraging requirements may explain why 213-265. Snowy Egret populations appear to have been ERWIN,R. M. 1983. Feeding habitatsof nesting wad- ing birds: spatial use and social influences. Auk reduced more than populations of Tricolored 100:960-970. Herons by Everglades degradation (Ogden 1994, FENNEMA,R. J., C. J. NEIDRAUER,R. A. JOHNSON,T 1996). Clearly, Everglades restoration will be K. MACVICAR,AND W. A. PERKINS.1994. A com- important for the continuation of these two spe- puter model to simulate natural Everglades hy- cies as breeders in this ecosystem. drology, p. 249-289. In S. M. Davis and J. C. Ogden [eds.], Everglades: the ecosystem and its restoration.St. Lucie Press, Delray Beach, FL. ACKNOWLEDGMENTS FREDERICK.P C.. R. BJORK.G. T BANCROFT.AND G. V. N. PO&L. 1992. Reproductive success of This work was supported by grants from the South three speciesof heronsrelative to habitatin south- Florida Water Management District; we are grateful to em Florida. Colonial Waterbirds 15:192-201. F?David, J. Milleson, V. Osmondson,and the late J. FREDERICK,I? C., AND M. G. SPALDING.1994. Factors W. Dineen of the District for their help and support affecting reproductive success of wading birds throughoutthe project. Salary for AMS was provided (Ciconiiformes) in the Everglades ecosystem, p. by a grant from the John D. and Catherine T Mac- 659-691. In S. M. Davis and J. C. Ogden reds.], Arthur Foundation. R. Bowman, A. Brody, P Cavan- Everglades: the ecosystemand its restoration. St. agh, W. Hoffman, W. McKelvy, R. Sawicki, and C. Lucie Press, Delray Beach, FL. Thompson helped with field work. Discussionswith HAFNER,H., P J. DUGAN,M. KERSTEN, 0. PINEAU,AND W. Dunlop, M. Fleming, J. Ogden, and W. Robertson J. P WALLACE.1993. Flock feeding and food in- improved various aspectsof this study. W. Loftus as- take in Little Egrets (Egrettu garzetta) and their sisted with identification of prey items. D. Sikkema effects on food provisioning and reproductivesuc- and D. Weeks of Everglades National Park provided cess. Ibis 135:25-32. water level and rainfall data. We thank the many pilots JENNI,D. A. 1969. A study of the ecology of four (especially the late A. Turner) who flew us safely species of herons during the breeding season at whenever and wherever we needed to fly. K. Bildstein, Lake Alice, Alachua County, Florida. Ecol. Mon- R. Bjork, D. Combs, T. Custer, P. Frederick, P. Mayer, ogr. 39:245-270. J. Ogden, E Reid, T. Sherry, and two anonymousre- JEWELL,S. D., AND G. T BANCROFT.1991. Effects of viewers provided constructive comments on earlier nest-trapping on nesting successof Egrettu her- versions of the manuscript. ons. J. Field Ornithol. 62:78-82. EGRE77A RESOURCE USE 905
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