Human–Wildlife Interactions 7(1):158–168, Spring 2013

Avian diets in a saline : Great , , USA Anthony J. Roberts, Department of Wildland Resources, Utah State University, Logan, UT 84322, USA [email protected]

Abstract: Saline lakes provide a prey-rich, predator-free environment for to utilize during migration and stopover periods. The (GSL), Utah is the largest salt lake in North America and is utilized by millions of migratory birds. It also is host to multiple commercial endeavors. Proposed expansion of commercial use of the GSL would result in increased impounded area and water extraction for mineral production, which may increase the GSL’s and negatively impact invertebrate abundance. I review previous literature and synthesize diets of avian utilizing the GSL to determine the importance of each invertebrate species, including shrimp (Artemia franciscana) and brine flies (Ephydra spp.), and clarify the anthropogenic impacts on food sources and avian populations. Species considered are eared grebes (Podiceps nigricollis), northern shovelers (Anas clypeata), green- winged teals (Anas crecca), common goldeneyes (Bucephala clangula), American avocets (Recurvirostra americana), black-necked stilts (Himantopus mexicanus), Wilson’s (Phalaropus tricolor), red-necked phalaropes (Phalaropus lobatus), and gulls (Larus californicus). and brine fly adults are consumed by all species considered. Alterations in prey abundance due to increased salinity may alter the ability of the GSL to support large avian populations.

Key words: American avocet, brine shrimp, , commercial harvest, eared grebe, human–wildlife conflicts, mineral extraction, waterfowl, Wilson’s

Anthropogenic impacts on aquatic eco- shrimp cysts removes an average of 3.5 million systems are widespread and include draining, kg of these annually from the GSL to nutrient enrichment, and reduced water quality. provide food for aquaculture facilities around Saline lakes may experience larger effects the world (Stephens and Birdsey 2002). of anthropogenic disturbance due to lower Additionally, long-term mineral extraction species richness and specialization of resident has reduced the surface area of the GSL by organisms (Jellison et al. 2008). Changes impounding previously open water areas and to mineral balance or water levels within concentrating salinity within impoundments saline lakes may greatly reduce productivity to exclusionary levels for brine shrimp and of organisms within the lake and impact brine fly production. Proposed impoundments population levels of organisms that depend on and diversions of water that would otherwise saline lake food sources. The Great Salt Lake enter the GSL exceed the yearly inflow to the (GSL) is a large, saline lake in the of lake (Great Salt Lake Planning Team 2000). the western . Each year, millions of The mineral extraction industry on the GSL waterbirds use the GSL to forage on brine flies is large and growing; for example, a recent (Ephydra hains and E. cinerea) and brine shrimp proposal would increase the area impounded (Artemia franciscana; Aldrich and Paul 2002). for solar evaporation and increase the amount The low gradient bottom of the GSL, along with of water diverted for evaporation and other highly variable water levels, result in expansive industry uses. Principle minerals extracted mudflats and sandbars that create highly from the GSL are (salt), productive habitats where avian species forage. chloride, and sulfate of potassium High exclude many invertebrate food (potash). Current total impounded evaporation sources, thus, creating a simple food web that ponds are >80,000 ha of GSL’s surface area. The may be highly impacted by changes in species salt extraction industry has an annual removal composition (Wollheim and Lovvorn 1995). of >1.8 million metric tons of minerals, and Avian species utilizing the GSL compete the magnesium industry removes >50,000 with multiple industrial uses of the lake and metric tons annually (Gwynn 2002). Proposed may be affected by associated impacts on expansion would increase impounded area in the ecosystem. For example, harvest of brine Gunnison and Bear River bays, expand potash Avian diets • Roberts 159

evaporation ponds by 14,000 ha, and increase yearly water diversion by >185 million cubic meters of water. If the full expansion is carried out, evaporation ponds for potash would cover about 7.4% of the GSL water surface when at the long-term water elevation of 1,280 m above level. Additional evaporation area may reduce GSL levels, resulting in increased salinity. Higher salinity decreases phytoplankton abundance, which, in turn, decreases brine shrimp abundance (Belovsky et al. 2011). The impacts of increased salinity on the GSL can be seen in the current ecology of the separated Gunnison Bay of the GSL (Figure 1). The Southern Pacific Railroad Causeway (SPRC) is a rock-filled levee that was completed in 1959 across the GSL, cutting Gunnison Bay off from the rest of the GSL, except for 1 breach and 2 culverts. Gunnison Bay soon became supersaturated with salt due to small amounts of freshwater inflows (Loving et al. 2002); as a result, the phytoplankton community shifted Figure 1. Map of the Great Salt Lake, Utah, including major to halophytic species. Brine fly and bays, islands, and causeways. brine shrimp populations in Gunnison Bay collapsed due to low food levels Study area and salinity near the saturation point of 30%. The GSL ecosystem covers nearly 780,000 ha Before construction of the SPRC, Gunnison Bay when at a lake elevation of 1,280 m and consists was likely similar to the pelagic areas of Gilbert of saline open water and freshwater wetlands. Bay, which supports abundant populations Brackish and freshwater marshes border the GSL, of eared grebes (Podiceps nigricollis; Figure 2), especially on the east shore at freshwater inflow phalaropes (Phalaropus spp.), and waterfowl. sites of the Bear, Weber, Ogden, and Jordan Aerial surveys from 2006 to 2008 found low rivers. Salinity across the GSL is variable, due avian abundance throughout Gunnison Bay to concentrated areas of freshwater inflow and (Vest 2009), likely a consequence of both reduced anthropogenic alterations of water exchange, food and salinity above the osmoregulatory most notably the SPRC and the Antelope Island capacity of most avian species. Causeway (AIC; Rich 2002). Bear River Bay and In this paper, I review and synthesize Farmington Bay are the least saline due to large published and unpublished reports on the freshwater inflow and low water exchange with diets of avian species utilizing the GSL. I am the main body of the lake caused by the SPRC particularly interested in determining the and AIC, respectively (Gwynn 2002). importance of brine shrimp and brine flies to High salinity in the pelagic areas of the GSL birds foraging within the GSL and the potential support populations of only 2 invertebrates (3 impact of a reduction in these food sources due species), brine shrimp (1 species), and brine to increased GSL salinity. A review of avian flies (2 species). Densities of brine shrimp and diets on the GSL will clarify the effect on avian their cysts vary across the GSL; their numbers populations of management decisions regarding are lowest in areas with less , such the recreational use, commercial harvest, and as Farmington and Bear River bays (Stephens mineral extraction uses of the GSL. and Birdsey 2002), though salinity beyond 160 Human–Wildlife Interactions 7(1)

15% is detrimental to these species. Brine fly The most abundant species utilizing the larvae are found primarily along the substrates GSL and its food resources are eared grebes, of the GSL that are above the anoxic water northern shovelers (Anas clypeata), green- layer; larvae densities are 10 times higher on winged teal (Anas crecca), common goldeneyes bioherms and mud substrates than on sand (Bucephala clangula), American avocets substrates (Collins 1980). In fresh and brackish (Recurvirostra americana; Figure 3), black- water marshes that border the GSL, a variety necked stilts (Himantopus mexicanus), Wilson’s of aquatic invertebrates is present (Cavitt phalaropes (Phalaropus tricolor), red-necked 2006). During wet years and in freshwater phalaropes (Phalaropus lobatus), and California influenced areas, such as Farmington and gulls (Larus californicus). I focused the review Ogden bays, common invertebrates available to on these species. foraging birds include corixids (Corixidae) and chironomids (Chironomidae). Results Eared grebes Methods Eared grebes nest in the marshes surrounding I used on-line search engines and article the GSL, and they also use pelagic regions as databases, particularly Academic Search staging areas during spring and fall migration. Premier, Web of Science, and Google Scholar, The largest concentrations of eared grebes on to search for articles relating to the GSL. All the GSL exist during fall migration, when 1.5 articles found concerning avian diets of the million individuals, or approximately half of relevant species on the GSL are included in this the North American population, stage at the manuscript. Key words used were: Great Basin, GSL. While staging, eared grebes’ flight muscles Great Salt Lake, grebes, , saline atrophy, body weight increases, and digestive lakes, salt lakes, Utah, waterfowl, and all species organs increase in size, resulting in flighlessness names listed below. Unpublished data and (Jehl 1997). Fall staging is also the time when reports were obtained from the Utah Division adult birds molt. Prior to leaving the GSL in of Wildlife Resources or report authors. the fall, eared grebes build up fat reserves, and Early dietary studies often were combined with organ trends reverse. With an increase in flight examination of food items found throughout the muscles, flight capacity is regained (Jehl 1997). birds’ digestive tract. Material collected from the Food habit studies of eared grebes on the proventriculus and ventriculus may represent a GSL have been conducted during migration biased sample toward hard, difficult-to-digest and staging and have reported that adult food items. Recent studies recognize this bias brine shrimp were an important part of eared and use only the esophagus when examining grebe diets (Table 1). During the early fall, food habits. Where the distinction is made in eared grebes consumed both brine shrimp the original publication, I state which part of and brine fly adults (Paul 1996, Conover and the dietary tract food items were sampled. Vest 2009); by late November, they ate brine

Figure 2 . Eared grebe (Podiceps nigricollis; photo Figure 3. American avocet (Recurvirostra ameri- courtesty U.S. Fish and Wildlife Service). cana) and young. Avian diets • Roberts 161 - Area of Great Salt Lake Farmington Bay, south shore Farmington Bay, BRMBR Pelagic areas Pelagic Mud flat NA BRMBR Antelope and Stansbury islands Antelope and Stansbury islands Farmington and Gilbert bays Farmington Bay NA Near Antelope Island Cause Near way Farmington Bay, south shore Farmington Bay, NA Farmington Bay Digestive Digestive organ used Esophagus Esophagus Esophagus Stomach NA Esophagus Esophagus Esophagus Stomach Stomach Stomach Stomach Esophagus NA# Stomach a b ment of diet Method measure - AP AP FO NA FO FO RP RP Count AV AP AV NA 2 51 fly 7.8 (%) 67.6 10.8 Brine larvae larvae 3.7 51.8 79.5 Trace Brine shrimp cysts (%) 5 5 40 70 45 85 10 0.1 100 Trace Brine fly adults (%) Great Salt Lake food items in diet 8 60 88 40 49 90 93 0.2 1.5 100 20.2 adults (%) Brine shrimp 3 N 95 30 17 17 16 16 27 15 45 63 137 241 355 NA NA NA Sept Nov April April Jul–Aug Jul–Nov July–Oct Month(s) Aug–Nov May–Aug March–Oct Nov–March Nov–March Nov–March NA 1989 1984 2006 2006 2007 1972 2006 1925 1992 1996 1966 2004–2006 2004–2006 2004–2006 Year(s) 2011 2011 2009 Vest and Conover and Conover Vest Vest and Conover and Conover Vest Osmundson 1990 Mahoney and Jehl 1985 Wilson 1973 Wilson Wetmore 1925 Wetmore Vest and Conover 2011 and Conover Vest Conover and Vest 2009 and Vest Conover Gaffney 2009 Gaffney 2009 Cullen et al. 1999 Green-winged teal Northern shovler Common goldeneye Conover and Vest Conover American avocet American avocet Paul 1996 Paul Paul 1996 Paul Eared grebe Cullen et al. 1999 Reference . Avian species diet studies done on the Great Salt Lake, Utah, including presence in avian diets of principle macro-invertebrates common to the species diet studies done on the Great Salt Lake, Utah, including presence in avian diets of principle macro-invertebrates Avian 1 . Table Great Salt Lake. 162 Human–Wildlife Interactions 7(1) Area of Great Salt Lake Gunnison Island, various BRMBR, Antelope Island, various Antelope and Iand Hat islands, mineral facility Hat Island, mineral facility Unknown NA NA Mud flat NA Eastern shore BRMBR NA Eastern shore Digestive Digestive organ used Stomach Stomach Esophagus Esophagus Stomach Stomach NA Stomach Stomach Mouth— ventriculus Esophagus Stomach Mouth— ventriculus a ment of diet Method measure - AV AV AP AP FO Count AV FO Count AV AV Count AV fly 7 4 (%) 10 94 19 Brine 46.5 53.7 larvae larvae Brine shrimp cysts (%) 9 19 19 16 5.6 6.1 100 Trace Trace Brine fly 69.6–100 adults (%) Great Salt Lake food items in diet 8 4 72 50 21.1 Trace adults (%) Brine shrimp N 6 3 30 32 16 23 41 24 31 529 NA NA NA NA NA NA July May May Month(s) July–Oct May–Oct May–Sep May–Aug May–Aug March–Aug March–Aug NA 2006 2007 1938 1992 1925 1984 1925 1972 1925 Year(s) 1940–1946 2005–2006 2005–2006 Cavitt 2006 Greenhalgh 1952 et al. 2009 Conover et al. 2009 Conover Cottam and Williams 1939 Cottam and Aldrich and Paul 2002 Aldrich and Paul Wetmore 1925 Wetmore Wetmore 1925 Wetmore Cavitt 2006 Wilson 1973 Wilson Wetmore 1925 Wetmore Colwell and Jehl 1994 Colwell Mahoney and Jehl 1985 Reference NA = the information was not specified. = the information was NA AP = aggregate percentage; AV = aggregate volume; Count = number observed; FO = frequency of occurrence; NA = no information available; RP = relative percentage. = relative RP = no information available; FO = frequency of occurrence; NA Count = number observed; = aggregate volume; AV = aggregate percentage; AP California gull Red-necked phalarope Wilson’s phalarope Wilson’s Black-necked stilt a b Avian diets • Roberts 163 shrimp exclusively (Conover and Vest 2009). northern shoveler diets from October through Only 1 study to date has recorded brine shrimp March (Table 1). Brine fly larvae also were a cysts within eared grebes’ diets. Paul (1996) dietary component for these species (Vest and found trace amounts of cysts in the stomachs Conover 2011). During fall migration in fresh of a few birds collected in the fall. Eared grebes and brackish marshes, waterfowl relied on collected in Farmington Bay consumed corixids wetland plant seeds, such as bulrush and when that prey source was abundant, though widgeon grass, and freshwater invertebrates for eared grebe abundances were lower within this their nutritional needs (Wetmore 1921, Vest and bay (Paul 1996). However, Farmington Bay is less Conover 2011). Northern shovelers and green- saline than the more open waters of Gilbert Bay, winged teal need daily access to freshwater. This allowing corixids to occur where they cannot in restricts their range within the GSL ecosystem other areas, especially during wet years. to freshwater inflow sites, particularly near Eared grebes collected during spring of 2006 the southern end of Gilbert Bay. Freshwater in Gilbert Bay had diets composed of equal parts inflow sites likely have higher populations of brine shrimp and brine fly adults, while birds freshwater invertebrates compared to much of from Farmington Bay during the same time the GSL, increasing the overall percentage of period contained mostly Hemipterans (Gaffney freshwater invertebrates and plant seeds in the 2009). During the spring of 2007, eared grebes diet, even when most freshwater sources have collected from Farmington Bay were consuming frozen over (Vest and Conover 2011). up to 92% brine fly adults, with the remaining diet composed of brine shrimp and Hemipterans American avocets and black-necked (Gaffney 2009). Brine shrimp densities are lower stilts during spring migration, so this food source is American avocets and black-necked stilts are not readily available to eared grebes (Belovsky et two of the most common shorebirds that utilize al. 2011). the GSL during migration. They are most abundant during fall migration, particularly Waterfowl August through October, when adult brine Waterfowl population numbers are highest flies are extremely abundant throughout GSL during fall migration, particularly on impounded marshes and open waters (Stephens 1977). Diets wetlands and brackish marshes. When of American avocets and black-necked stilts on considering open, saline waters of the GSL, the GSL are usually dominated by fresh or salt populations during winter months (December water macro-invertebrates, such as adult brine to February) often are around 300,000 ducks (A. flies (Table 1). Roberts, unpublished data). Primary species Wetmore (1925) examined the stomachs of are northern shoveler, green-winged teal, and American avocets from across the western common goldeneye. Waterfowl diet studies United States and Canada. Though collection within saline areas of the GSL are restricted to sites were not clearly presented, common GSL a single investigation. Vest and Conover (2011) foods found were brine fly larvae and adults, examined diets of northern shovelers, green- along with many corixids. Seeds of many winged teal, and common goldeneye over 2 wetland plants also were observed, including consecutive winters from 2004 to 2006. All dietary sago pondweed (Potamogeton pectinatus), samples were from the esophagus exclusively. widgeon grass, and bulrush species (Scirpus Common goldeneye utilized brine fly larvae for spp.). Wilson (1973) collected staging American ≤77% of their diet throughout the winter, with avocets from the mud flats of BRMBR (Figure brine shrimp cysts, freshwater invertebrates, and 1) to compare diets of healthy and botulism- seeds providing the remainder of the diet (Table 1). infected birds. I restricted this summary to Wetland plant seeds, particularly widgeon grass healthy American avocets and black-necked (Ruppia maritima) and alkali bulrush (Scirpus stilts to represent typical diets. Larval stages maritimus), were consumed more when ice-cover of brine flies were consumed more than retreated from freshwater marshes. Brine shrimp adults, with >72% of adult American avocets’ cysts comprised 80% of the aggregate percentage diet composed of brine fly larvae. Breeding biomass of green-winged teal, and 52% of American avocet diets were examined in 2 164 Human–Wildlife Interactions 7(1) studies, conducted mostly on areas of brackish or (Osmundson 1990, Cavitt 2006). Though results are not typical of open, saline waters, they represent the available food to birds on the GSL margins. Corixids and chironomids comprised most of the diet of breeding American avocets; in particular, chironomid larvae accounted for nearly 80% of the diet. Seeds and sprouts also were present in the diet of breeding American avocets and were represented by numerous taxa, including Typha, Scirpus, and Graminaceae (Osmundson 1990). Black-necked stilts consume both larval and Figure 4. California gull (Larus californicus). adult brine flies on the GSL. An unknown number of black-necked stilts collected from the juveniles collected in this study (n = 7) fed Great Basin, including the GSL, contained both exclusively on adult brine flies, although the adult and larval stages of brine fly (Wetmore authors state that the age difference may be 1925). Black-necked stilts, particularly due to a bias in of their collection habitat (near juveniles, collected from the mud flats on shore) or small sample size (Colwell and Jehl BRMBR fed principally (54% biomass) on brine 1994). fly larvae (Wilson 1973). In the marshes on the Red-necked phalaropes utilize more pelagic eastern shore of the GSL, invertebrates, such areas than do Wilson’s phalaropes (Aldrich as corixids, chironomids, Ephemeroptera, and and Paul 2002) both on the GSL and the Pacific Tendipedidae were the primary food source Ocean, where much of the North American (Cavitt 2006). Differences in the 2 studies are population winters (Rubega et al. 2000). Early likely due to varying salinities. Cavitt (2006) diet studies found brine shrimp and brine collected a variety of birds from breeding areas fly larvae and adults present in red-necked near freshwater marshes. Wilson’s (1973) work phalarope intestines (Wetmore 1925). Adult occurred on mudflats of the BRMBR, an area brine flies accounted for 95 to 100% of red- whose salinities change drastically from year necked phalaropes’ diet on the GSL in early to year as water levels fluctuate; salinity during August 1992 (n = 3), but, by late August (n = the study are not enumerated. 9), brine fly larvae accounted for 60 to 100% of their diet (Aldrich and Paul 2002). In October, Phalaropes their diet consisted entirely of brine fly larvae Wilson’s phalaropes and red-necked and pupae. phalaropes both use the GSL extensively during fall migration, particularly from July California gulls through October. Phalarope species are the California gulls have an omnivorous diet only shorebirds that regularly occur in pelagic and are opportunists in both their food choice areas of the GSL, and the GSL hosts >50% of and feeding styles (Behle 1958; Figure 4). Foods North America’s Wilson’s phalaropes each fall. obtained directly from the GSL include brine Throughout their range, Wilson’s phalaropes shrimp, brine fly adults, and brine fly larvae forage on brine shrimp, as well as brine fly (Table 1). Cottam and Williams (1939) found larvae, pupae, and adults (Table 1). Three birds that brine fly adults, larvae, and pupae made collected from shallow water mud flats of the up 46% of the diet of 6 individuals collected in GSL in 1984 had consumed solely adult brine early July from the GSL vicinity. Carrion and flies (Mahoney and Jehl 1985a). Colwell and various plant matter made up 17% of the diet Jehl (1994) found age difference in foraging of those 6 individuals (Cottam and Williams exists among Wilson’s phalaropes, with adults 1939). A sample of 529 birds collected across 4 consuming a mix of brine shrimp, brine fly years from Antelope Island and surrounding adults, and other aquatic invertebrates, while marshes, including the BRMBR, contained a Avian diets • Roberts 165

variety of food items (Greenhalgh 1952). Brine to around 5%, can support populations of fly adults and other Diptera made up 5%of more invertebrate species (Wurtsbaugh 1991). the diet in this sample. Brine shrimp were also Predatory invertebrates, such as corixids, recovered from California gull stomachs but reduced brine shrimp populations by an order of made up ≤1% of their diet (Greenhalgh 1952). magnitude by consuming brine shrimp nauplii Greenhalgh (1952) found that Orthoptera made (Wurtsbaugh 1991). There is no information on up 53% of California gulls’ diet around the GSL. the ability of avian species to switch from brine During the breeding seasons of 2006 and 2007, shrimp to corixids during these times. Increased Conover et al. (2009) collected 54 California salinity due to diversion of fresh water from the gulls with food in their crops from around the GSL would reduce the primary food of avian GSL near Antelope Island (n = 10), Hat Island (n species utilizing the GSL and result in the = 24), and at a commercial mineral facility along reduction of avian use, as we have seen occur Bear River Bay (n = 20). Most (77%) California in Gunnison Bay in recent years. gulls had eaten brine shrimp, and two had fed The importance of food availability in on brine fly larvae. Garbage, carrion, larval regulating avian populations changes with and adult midges, and corixids made up the salinity levels. In saline lakes in the western remainder of their diets (Conover et al. 2009). United States, the effect of salinity on Brine shrimp cysts have not been found in invertebrates’ food appears to be less important any samples reported, though California gulls to top avian predators than osmoregulation are often seen in pelagic areas of the GSL near (Wollheim and Lovvorn 1995). Wilson’s concentrations of cysts. phalaropes, eared grebes, American avocets, and California gulls all exhibit physiological or Discussion behavioral adaptations to counteract increased For many avian species, the GSL offers a ingestion of salt (Mahoney and Jehl 1985a, predator-free, prey-rich environment during b, c). Waterfowl distributions on the GSL are critical times of the year. All species considered likely determined by access to fresh water. here except American avocets and black-necked Northern shovelers and green-winged teal do stilts, have been shown to consume both brine not have well-developed salt glands and need shrimp and brine flies. Brine flies are present to drink fresh water to aid in osmoregulation. in the diet of all species discussed, but brine When salinity impacts brine shrimp and brine fly abundance, distribution, and population fly populations, avian population levels are fluctuations within the GSL are not well- impacted by food availability, rather than by understood (Belovsky et al. 2011). osmoregulatory capacity. Current GSL salinity is near 9%, but decreased Estimates of food abundance needed for water levels due to increased evaporation or continued avian use of the GSL have been water diversion may quickly increase salinity to demonstrated for 1 species, eared grebes. >14%, which would be detrimental to long-term Conover and Caudell (2008) estimated that populations of both brine shrimp and brine eared grebes needed a minimum adult brine flies. Brine shrimp adults can survive in salinity shrimp density of 0.38 shrimp/L to maintain up to saturation level, but cannot reproduce. body mass. A decrease in densities of brine Newly hatched brine shrimp cannot survive in shrimp below these densities would have large salinity >14% (Stephens and Birdsey 2002), so, consequences on the survival of eared grebes. brine shrimp production is effectively limited Belovsky et al. (2011) hypothesized that a at that salinity. There is no information on the higher density of adult brine shrimp (5.80 adult salt tolerance of brine flies on the GSL, but it brine shrimp/L) was needed to maintain and is likely that osmoregulation in salinity near increase eared grebe body mass for migration saturation is prohibitive to brine fly production. from the GSL. They illustrated a relationship Corixids and other more freshwater-tolerant between per capita eared grebe abundance on aquatic invertebrates are abundant near the GSL and density of brine shrimp during freshwater inflows and low-salinity bays of the the previous year. This implies that these birds GSL. During unusually wet years, the resulting have reduced survival after staging in years decrease in salinity in pelagic areas, from >10% of low brine shrimp abundance or that they 166 Human–Wildlife Interactions 7(1) change migration route the next year, staging at (Young 1952), but they are able to utilize many Mono Lake rather than the GSL. Both estimates anthropogenic food sources that occur on the of brine shrimp abundance needed for eared GSL. Reduced GSL food sources may increase grebes are above the long-term average in the the instances of California gulls raiding other GSL, but Gunnison Bay does not support those avian nests for eggs and young. This activity densities, and eared grebes are not seen there. is seen on the BRMBR (Greenhalgh 1952) and Avian diets on Mono Lake, California, are has been seen in other parts of their breeding similar to avian diets on the GSL and may help range, as well. At Mono Lake, California gulls fill information gaps regarding age- and sex- are predators of snowy plovers’ (Charadrius specific dietary differences on the GSL. Diets alexandrines) nests (Page et al. 1985), and in of eared grebes and other birds on Mono Lake Manitoba, they have been a major predator have been studied more extensively than those of Western grebes’ (Aechmophorus occidentalis) at the GSL. Investigation of eared grebes’ diets nests (Knapton 1988). found that they ate mostly brine shrimp during the fall (Jehl 1988), and that this may account Conclusions for 80% of fall adult brine shrimp depletion Increased salinity due to reduced water through predation (Cooper et al. 1984). Four inflow or increased water evaporation would years of data (1981 to 1984) from Mono Lake be detrimental to brine shrimp and brine flies show that eared grebes fed primarily on the found in the GSL. It is clear that many avian most abundant prey items. Brine fly larvae species rely on these invertebrates on the GSL and pupae dominated diets from mid-winter as a principle food source, and reduction in through May; when brine shrimp became their food availability would result in the loss abundant in June, eared grebes switched to of avian populations. Knowledge of avian diets this resource (Jehl 1988). Large waterfowl is fundamental to management of continued concentrations are not found on Mono Lake, avian populations, but we lack any knowledge although gadwalls (Anas strepera) breed there in beyond basic food habits. Most importantly, small numbers that consume adult brine flies, we do not know if avian species can shift their though survival when eating this food is low diet to another prey species if their preferred (Jehl 2005). food is no longer available due to altered prey Adult female and male Wilson’s phalaropes abundances and increased salinities. on Mono Lake fed on 66% and 38% brine shrimp and 34% and 62% brine flies by volume, Literature cited respectively (Colwell and Jehl 1994). On Aldrich, T. W., and D. S. Paul. 2002. Avian ecol- both the GSL and Mono Lake, juveniles fed ogy of Great Salt Lake. Pages 343–374 in J. W. exclusively on brine fly adults (Colwell and Jehl Gwynn, editor. Great Salt Lake: an overview of 1994). Jehl (1986) found that brine fly adults change. Utah Department of Natural Resourc- were the only food eaten in July and August es and Utah Geological Survey, special publi- by red-necked phalaropes on Mono Lake and cation, Salt Lake City, Utah, USA. made up >90% of the diet during the remainder Behle, W. H. 1958. The life of Great Salt Lake. of the year. Rubega and Inouye (1994) found University of Utah Press, Salt Lake City, Utah, that red-necked phalaropes were unable to USA. switch to diets composed principally of brine Belovsky, G. E., D. Stephens, C. Perschon, P. shrimp. They concluded that birds are unable Birdsey, D. Paul, D. Naftz, R. Baskin, C. Lar- to survive on brine shrimp alone and must son, C. Mellison, J. Luft, R. Mosley, H. Mahon, feed on brine fly adults and larvae to survive J. Van Leeuwen, and D. V. Allen. 2011. The and gain weight while on Mono Lake. The loss Great Salt Lake ecosystem (Utah, USA): long- of brine flies on the GSL may severely impact term data and a structural equation approach. survival of migrating red-necked phalaropes. Ecosphere 2:1–40. The omnivorous nature of California gulls Cavitt, J. F. 2006. Productivity and foraging ecol- likely buffers their population from the loss ogy of two co-existing shorebird species breed- of individual food sources. On Mono Lake, ing at Great Salt Lake, UT: 2005–2006 report. California gulls are likely eating adult brine flies Avian Ecology Laboratory Technical Report Avian diets • Roberts 167

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