diet & prey selectivity of the little blue heron (egretta...
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Diet & prey selectivity of the Little Blue Heron (Egretta caerulea) in coastal South Florida
Emilie R. Kohler*(1), Dale E. Gawlik (1), Marisa M. Martinez (2) & Stephanie S. Romañach (2)
(1)Florida Atlantic University, Environmental Science Program, Charles E. Schmidt College of Science, Boca Raton, FL, USA (2) Florida Atlantic University, Charles E. Schmidt College of Science, Boca Raton, FL, USA (3) USGS, Davie, FL, USA
Contact Emilie Kohler
Florida Atlantic UniversityEmail: [email protected]
Phone: 717.823.0259
• Moderately significant separation between 2017 GWH & ENP colony
(67% dissimilar) due to gulf toadfish (Opsanus beta) & prawns
(Farfantepenaeus spp.)
• Highly selected species in the 2016 & 2017 GWH colonies are mojarra
(Eucinostomus) (CSI = 0.205; 0.196), pipefish (Syngnathidae) (0.191;
0.189), & prawns (0.169; 0.137)
• Herons avoid marine crabs despite their high abundance on tidal flats
• In the 2016 GWH colony, dominant and important prey were fish (53%)
and shrimp (40%), specifically prawns (36%)
• In GWH 2017 shrimp (42%) & terrestrial invertebrates (33%), specifically
prawns (41%) were important and dominant
• The 2017 ENP colony highly selected gulf toadfish (CSI = 0.829)
• Dominant and important prey in the 2017 ENP colony were fish (55%),
specifically gulf toadfish (61%)
• Prawns are not heavily selected, but are largely abundant in heron diet
& thus are more important than highly selected mojarra or pipefish
• Gulf toadfish were dominant and important in the 2017 ENP colony, but
only moderately so in the 2016 GWH colony & not in GWH 2017 colony
• Aquatic species dominate heron diet, but terrestrial prey has a high
occurrence suggesting foraging habitat is not exclusive to tidal flats and
underscoring herons as generalists
• Deviating from typical foraging habitat could be due to oscillating prey
availability on tidal flats
• Herons specialize in fish and shrimp, differing from herons in Brazil
which highly select crabs & shrimp3
Introduction
• Sampled tidal flat prey communities near low tide using a 1-m2 throw
trap at GWH in 2016 (n = 74) & 2017 (n = 123), & ENP in 2017 (n = 86)
• Collected bolus (stomach regurgitate) samples (n = 53) from 1-4 week old heron chicks from 26 nests in 2016 in GWH, 12 nests in 2017 (n = 34), & 10 nests in ENP (n = 29) in 2017
• Investigated prey species selection using Chesson’s index of selectivity
• Diet represented using aggregate percentage approach as (1) average
percent of sample weight & (2) the percent occurrence in nests1,2
• Analyzed regional diet differences using analysis of similarity (ANOSIM)
& similarity percentages (SIMPER) with PRIMER v7
Methods & Materials
• Little Blue Herons (Egretta caerulea, hereafter heron) are reportedly
generalists that eat insects, crustaceans, annelids & fish4, however the
diet of herons in the Florida Bay & Florida Keys is unknown
• Diet composition affects wading bird productivity due to varying
nutritional values of prey species
• Identifying prey species preferences is important for predicting the
effect of environmental stressors on quality foraging habitat
• Investigated prey selection by herons in Great White Heron National
Wildlife Refuge (GWH), in the lower Keys of Florida during the breeding
season in 2016 & 2017, & the Florida Bay in Everglades National Park
(ENP) in 2017
• Hypothesized prey species selection is occurring, therefore no regional
diet differences exist between heron colonies in GWH & ENP
Results and Discussion
In the Florida Bay & Florida Keys, the diet of herons was previously
unknown; therefore testing whether herons specialize on key prey species
is important for conserving this species of concern. Further research is
needed to confirm the importance of gulf toadfish in heron diets. These
findings can also be applied to future work determining whether food
resources could affect heron habitat use & reproductive success.
Future Work
Great White Heron Nat’l Wildlife Refuge Everglades National Park
Ch
esso
n’s
Sele
ctiv
ity
Ind
ex
Prey Species
Selection
Avoidance
Random feeding
Selectivity of herons in Great White Heron National Wildlife Refuge & Everglades National Park
α
ENP Colony
GWH Colony
Study Site
Throw Trap sites 2016
Throw Trap sites 2017
Gulf toadfish
Prawns
Wood louse
Fish
Shrimp
Terrestrial invertebrates
Reptiles0
10
20
30
40
50
60
70
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
% A
bu
nd
ance
% Occurrence
Feeding strategy and prey importance of the 2017 ENP colony
Specialization Dominant
Rare Generalized
Selected References
1. Callaghan, C. T., & Gawlik, D. E. (2016). Diet and selectivity of Porphyrio porphyrio (Purple Swamphen) in Florida. Southeastern Naturalist, 15(sp8), 1-14.
2. Costello, M. J. (1990). Predator feeding strategy and prey importance: a new graphical analysis. Journal of Fish Biology, 36(2), 261-263.
3. Gianuca, D., Gianuca, A. T., & Vooren, C. M. (2012). Abundance, breeding & food of the Little Blue Heron Egretta caerulea (Aves, Ardeidae) in the Patos Lagoon estuary, a recently colonized area in southern Brazil. Iheringia. Série Zoologia, 102(1), 19-25
4. Martínez, C. (2010). Trophic niche breadth and overlap of three egret species in a neotropical mangrove swamp. Waterbirds, 33(3), 285-292.
Gulf toadfish
Prawns
Wood louse
Seahorses and pipefishes
Fish
Shrimp
Crabs
Terrestrial invertebrates
Gulf toadfish
Prawns
Wood louse
Seahorses and pipefishes
Fish
Shrimp
Crabs
Terrestrial invertebrates
Reptiles0
10
20
30
40
50
60
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
% A
bu
nd
ance
% Occurrence
Feeding strategy and prey importance of the 2016-2017 GWH colony
2016 colony2017 colony