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Main Foods Taken
Mostly arthropods by number, but small mammals and birds, rarely reptiles, usually make up bulk of diet. Also occasionally feeds on car-rion and suet or meat at bird feeders and campsites (Bent 1950). Compared to Loggerhead Shrike (Yosef 1996), birds and mammals more important in diet.
Microhabitat For Foraging
Prefers open or semiopen landscapes (see Habitat, above); on breeding grounds, hunts most often from perches in shrubs or trees <1 to >10 m high for prey moving on open ground (tundra or muskeg mat, marshes, grass-sedge meadows, rocky screes, dirt banks, shorelines, tilled ground, roadside verges). Also skulks about in lower branches and exposed roots of shrubs or by hopping over mounded or rocky terrain searching for small arthropods, mam-mals, and fledgling birds. Attacks birds and insects in higher branches and canopy of shrubs and trees, as well as flying in air, often over river or lake. On south-ern wintering grounds, often chooses same micro-habitat used by Loggerhead Shrike and by American Kestrel (Falco sparverius) in breeding season (sometimes coinhabiting with them in winter; see Atkinson and Cade 1993, Yosef 1996): roadside verges with fence posts or utility poles and lines for perches, hedge rows, tree and shrub plantings around abandoned farm-steads, river deltas and estuaries, shrubby hillsides and rimrock formations, mixed woods and farmlands, pastures, orchards, and open suburban (exurban) areas and parks (Atkinson and Cade 1993, Campbell et al. 1997, TJC for New York State).
Food Capture And Consumption
Similar to Loggerhead Shrike (Yosef 1996) and to Palearctic excubitor group of races (Cramp and Perrins 1993). Typically sits and waits on exposed hunting perch to spot prey moving in vulnerable location on ground or in air. Frequently moves from perch to perch depending on frequency of prey exposure. Attacks prey on or near ground by diving down from perch and flying fast and low directly to prey. Also flies out and hovers briefly over ground and cover where prey has been seen but not precisely located; then drops down on prey if located; infrequently hunts systematically over patch of habitat by hovering like kestrel at different locations (McNicholl 1972).
Sometimes actively hunts by hopping about on ground or among branches of bushes and trees, especially for invertebrates. After pursuit and escape of small birds and mammals into cover, sometimes flicks or flashes wings in manner of Northern Mockingbird as it moves about in cover in apparent attempt to flush prey (Cade 1962).
Aerial capture of flying insects frequent—especially bumblebees (Bombus spp.) and other flying Hymenoptera (some flights exceed 100 m); also pursues and catches some birds in flight, including high towering and “ringing” flights up to about 300 m after flocking species such as siskins, goldfinches, and redpolls (Carduelis spp.; Cameron 1908, Munro 1930, TJC). Takes larger birds up to American Robin (Turdus migratorius) size in direct pursuit (Bent 1950, White 1963), perhaps most often by surprise as birds fly to or from nests or feeding locations (e.g.: Bank Swallow [Riparia riparia] on Old Crow River, Yukon Territory [Morlan 1972], and Violet-green Swallow [Tachycineta thalassina] on Yukon River [TJC]), and during courtship or territor-ial display (Cade 1967), but prolonged aerial pursuits also occur (e.g., Rusty Blackbird [Euphagus carolinus] chased for 4.5 km; Jordheim 1975). Ability to catch small sandpipers is remarkable, but method of capture not observed, although seen chasing Spotted Sandpiper (Actitis macularia) directly along edge of river; sandpiper escaped by diving underwater (Osgood 1909).
Has ability to spot motionless birds “frozen” on branches and to capture them before they move (Cade 1967, TJC; see also Yosef 1996 for Loggerhead Shrike). Sometimes sits motionless for many minutes watching mouse holes and runways and birds’ nests (Cade 1967; also see Mester 1965). Has been reported to attack larger birds and mammals >500 g (Cramp and Perrins 1993), whether as prey or enemy not always clear (Cade 1962, 1967), but reliable reports of attacks on female Mallard (Anas platyrhynchos) on Adak I. (Taber 1946), Sharp-tailed Grouse (Tympanuchus phasianellus) in Alaska (White 1963), Rock Ptarmigan (Lagopus muta) in Aleutian Is. (Murie 1959), and said to have captured and killed Rock Dove (Columba livia) in Quebec (Aubry and Bannon 1996).
Catches insects and small mammals, occasionally lizards, by grabbing with bill, but catches birds with bill or feet, most often with feet. Although nails not specialized for predation, foot has powerful grasp (Raikow et al. 1980). Grabs flying birds from above, from side, or from below (Cramp and Perrins 1993).
Foraging success (prey captured per total hunting attempts) not much studied in Nearctic (see Cramp and Perrins 1993 for data from Europe). Winter study of 72 foraging attempts in s. Idaho (Atkinson and Cade 1993) showed 69% overall success but strongly dependent on taxa of prey: 93% for arthropods, mostly insects (40 of 43 attempts), 64% for small mammals (7 of 13), but only 19% for birds (3 of 16). Compares to overall success rates for Loggerhead Shrikes of 28–85% (Yosef 1996).
Kills vertebrates by biting nape and disarticulating cervical vertebrae (Miller 1931, Bent 1950). Bilateral, subterminal tomial teeth on upper bill appear to penetrate through space between adjacent vertebrae, quickly damaging nerve cord, paralyzing, and killing prey (Cade 1967). Invertebrates killed by repeated mandibulations while held in shrike’s bill; insects re-peatedly bitten in thorax; occasionally large insects may be held in foot while being bitten. Seldom holds mammals in feet while killing them, but birds are held in feet while shrike bites neck.
Small insects and invertebrates (<20 mm) usually swallowed whole after mandibulation. Larger insects (and perhaps mammals <10 g) sometimes held in one foot with tarsometatarsus resting on perch and torn with bill into bite-sized pieces. In North America, not observed to hold down prey under foot against perch for tearing apart, but see Cramp and Perrins 1993 for reported cases in Europe.
Large insects and vertebrates carried to special impaling or wedging structures in shrubs and trees (and barbed-wire fences), where they are firmly secured and can be pulled apart with bill. Occasionally impales invertebrate prey while still alive (ECA). Shrikes usually discard wings of large insects such as grasshoppers (Orthoptera) and nip off their spiny tarsi before eating rest of leg, which is thoroughly mandibulated with tomial teeth before swallowing. Before eating them, shrikes sometimes devenomize bumblebees, wasps, and other stinging Hymenoptera by expressing venom onto stinger by biting abdomen around sting gland and wiping off venom onto branch (as do bee-eaters [Meropidae]; Fry 1969), or by impaling bee first and then pulling out stinger and gland with bill (Gwinner 1961, von St. Paul and Gwinner 1962, TJC); but particularly young shrikes often swallow bees and wasps intact without devenomizing them. Hand-reared juvenile that once swallowed intact bumblebee was stung inside its throat, causing it to fall from its perch to ground, where it remained unconscious for about 30 s before recovering (TJC). Sometimes beats large arthropods against perch in manner similar to kingbirds (Tyrannus spp.; EAC).
Shrike typically pulls apart impaled vertebrates starting at head and working posteriorly to hindquarters and tail. Begins by tearing at eyes or mouth and working skin and flesh loose toward neck. Heads of small birds, mammals, and lizards sometimes swallowed whole. Limbs usually pulled off at articulations with pectoral and pelvic girdles, then thoroughly mandibulated with tomial teeth; long bones broken up and then limb swallowed whole. Intestines may or may not be eaten, but all other viscera consumed. Giz-zards of birds sometimes separately impaled; contents of seed-eaters discarded, but invertebrate contents consumed or gizzard swallowed whole if small (TJC).
Can carry prey in flight equal to and exceeding own body weight (e.g., lemmings [Lemmus and Dicrostonyx spp.] and voles [Clethrionomys and Microtus spp.]; Grinnell 1900, Bent 1950, Cade 1967), various birds (e.g., American Robin [Miller 1931], Blue Jay [Cyanocitta cristata; Bent 1950], Pine Grosbeak [Pinicola enucleator; White 1963]). Prey up to about 25 g carried in bill dur-ing flight, but larger prey are shifted from bill to feet as shrike takes off and carried tucked under tail in raptor fashion (Cade 1967, Cramp and Perrins 1993). Large prey usually carried only few meters to nearest impaling location, but longer flights made to bring prey back to vicinity of nest or favorite perch (J. Jehl, Jr., pers. comm., at Churchill, Manitoba, saw shrike carry Stilt Sandpiper [Calidris himantopus], about 55 g, in series of flights, 300 m, 250 m, and 400 m to vicinity of its nest); at Crooked Creek, ID, wintering shrike carried hindquarters and tail of montane vole (Microtus montanus) in its bill upslope from valley to hillside juniper (Juniperus sp.) in continual flight of 1.5 km (TJC); at Peters Lake, AK, male flew upslope to nest about 400 m, carrying male Lapland Longspur (Calcarius lapponicus), about 25 g, in feet (TJC). Reported carrying Rock Dove >50 m before landing to eat it (Aubry and Bannon 1996).
Major Food Items
Difficult to characterize because shrike feeds on such wide range of both invertebrate and vertebrate prey, ranging from tiny insects (mosquitoes; Culicidae) and spiders (Arachnida) to largest grasshoppers and birds and mammals weighting >100 g (Cade 1995, Yosef 1996).
Winter sample of 67 stomachs collected between Oct and Apr in U.S. revealed 28 small birds—American Tree Sparrow (Spizella arborea), Dark-eyed Junco (Junco hyemalis), and House Sparrow (Passer domesticus) identified; “mice” occurred in one-third of stomachs, and insects found in some stomachs during every month of “winter” period. Birds and mice made up 60% of food by volume, insects 40%; grasshoppers 20% of total (Judd 1898). Another winter sample of 140 stomachs confirmed small birds, mice, and insects as principal items (Stegman 1959–1960).
Other birds recorded as winter prey: Mourning Dove (Zenaida macroura); Downy (Picoides pubescens) and Hairy (P. villosus) woodpeckers; Eastern Phoebe (Sayornis phoebe); vireos (Vireo spp.); Blue Jay; Gray Jay (Perisoreus canadensis); Horned Lark (Eremophila alpestris); chickadees (Poecile spp.); titmice (Baeolophus spp.); Bushtit (Psaltriparus minimus); kinglets (Regulus spp.); American Robin; Varied Thrush (Ixoreus naevius); European Starling (Sturnus vulgaris); Bohemian Waxwing (Bombycilla garrulus); Yellow-rumped Warbler (Dendroica coronata); Field (Spizella pusilla), Savannah (Passerculus sandwichensis), Fox (Passerella iliaca), Song (Melospiza melodia), White-throated (Zonotrichia albicollis), and Golden-crowned (Z. atricapilla) sparrows; longspurs; Snow Bunting (Plectrophenax nivalis); Northern Cardinal (Cardinalis cardinalis); Pine Grosbeak; House Finch (C. mexicanus); Red (Loxia curvirostra) and White-winged (L. leucoptera) crossbills; Common (Carduelis flammea) and Hoary (C. hornemanni) redpolls; Pine Siskin (C. pinus); American Goldfinch (Carduelis tristis); and Evening Grosbeak (Coccothraustes vesper-tinus; Bent 1950, Campbell et al. 1997).
Mammals include: voles (e.g., Microtus spp., doubtless other microtines, too), mice (e.g., Peromyscus spp., Reithrodontomys spp., Mus musculus, no doubt also Perognathus spp. and Dipodomys spp.; Miller 1931, Bent 1950), and various shrews (e.g., Sorex spp.).
Winter stomachs revealed surprising variety of insects, including caterpillars (Lepidoptera) and other larvae: mainly orthopterans by bulk but also beetles (Coleoptera; especially Carabidae), ants (Formicidae), wasps, bees, flies (Diptera), but no bugs (Hemiptera). In Judd’s (1898) sample, invertebrates were only 23% of food by volume from Dec to Apr but 73% in Oct–Nov. Northern Shrikes wintering in Idaho preyed heavily on carabid and staphylinid beetles, and acridid grasshoppers; remains of 2 heimpterans also found; some lizards (Sceloporus sp.) taken during winter in suitable microhabitats (Atkinson and Cade 1993).
Infrequently feeds on carrion in winter: F. E. L. Beal saw shrike in Iowa take shreds of flesh from rib of frozen cow carcass in Jan (Judd 1898); one fed on red squirrel (Tamiasciurus hudsonicus) immediately after it had been run over by vehicle on road (James and Harrison 1987); in Dec, adult shrike observed in e. Idaho removing strips of dried flesh and skin from remains of white-tailed jackrabbit (Lepus townsendii) and wedging them among branches of big sage (Artemesia tridentata) bushes; >15 pieces cached within radius of 8 m around carcass (TJC). Also takes meat from campsites: Osgood (1909) mentioned pair that frequently stole meat from camp in Ogilvie Mtns., Yukon Territory, and aggressively drove away Gray Jays. Has been habituated to take dead voles placed conspicuously in winter territories (Olsson 1986, ECA).
Study of 273 winter pellets in Idaho (Atkinson and Cade 1993) revealed 671 indi-vidual prey of 33 taxa (prey groups): arthropods most numerous prey, in 153 pellets (209 individuals in 14 taxa), including grasshoppers 9.8% of all prey and 0.8% of biomass, carabid beetles 22.2% by number and 1.1% of biomass, all beetles 53.5% by number and 4.2% of biomass. Arthropods active throughout winter in sw. Idaho, especially on south-facing slopes and rim-rock outcrops, which shrikes frequent. Vertebrates: 3 lizards 0.45% by number and 0.14% of biomass, 9 species of birds (39 individuals, including 17 Dark-eyed Junco), 5.9% by number and 11.8% of biomass; 8 categories of mammals (200 individuals, minimum of 6 species), 29.8% by number and 83.1% of biomass (Microtus spp. 10.9% by number and 42.4% of biomass; note: analysis of pellets underestimates numbers of small and soft-bodied invertebrates in diet).
(1) From 331 pellets from nests at Peters Lake and environs, 563 individual prey included: 33 small birds (5.9%; American Pipit [Anthus rubescens], redpoll, White-crowned Sparrow, Savannah Sparrow, Lapland Longspur [n = 16], Snow Bunting, and also Gray-crowned Rosy-Finch [Leucosticte tephrocotis] and Smith’s Longspur [Calcarius pictus] subsequently recorded); 121 microtine rodents (21.5%; including tundra [Microtus oeconomus] and singing [M. miurus] voles [n = 91], northern red-backed vole [Clethrionomys rutilus], brown lemming [L. trimucronatus], northern collared lemming [Dicrostonyx groenlandicus]); 187 bumblebees (33%); 55 paper wasps (Vespa spp.; 9.8%); 94 carabid beetles (16.7%); and 72 other beetles (12.8%).
(2) Vertebrate carcasses in larders, n = 119: 35 birds (3 Baird’s Sandpipers [Calidris bairdii], 1 Least Sandpiper [C. minutilla], 1 Semipalmated Sandpiper [C. pusilla], 4 American Pipits, 3 redpolls, 4 White-crowned Spar-rows, 15 Lapland Longspurs, 4 Snow Buntings), and 81 microtine rodents (5 tundra voles, 54 singing voles [46.1% of total], and 22 northern red-backed voles making up 69.1% of total; and 3 cinereus shrews [Sorex cinereus]). In summer diet at Peters Lake, AK, bumblebees were by far most important insects consumed, Lapland Longspur most important bird, and singing vole most important mammal and single most important species.
(3) From 130 pellets at nests along Colville River, n. Alaska, total prey = 246 items: included 38 birds (15.4%; 1 American Robin, 2 Gray-cheeked Thrushes [Catharus minimus], 1 Bluethroat [Luscinia svecica], 3 Arctic Warblers [Phylloscopus borealis], 10 Yellow Wagtails [Motacilla flava], 9 redpolls, 1 Fox Sparrow, 2 White-crowned Sparrows, 7 Lapland Longspurs, and 2 unidentified birds); 8 microtine rodents (3.3%); 170 bumblebees (69.1%); 5 wasps (2.0%); and 25 beetles (10.2%).
(4) Vertebrate carcasses in larders along Colville, total of 41 prey: 37 birds (90.2%; 1 Semipalmated Sandpiper, 2 Bluethroats, 15 Yellow Wagtails [36.6% of total], 4 redpolls, 6 American Tree Sparrows, 1 Fox Sparrow, 8 Lapland Longspurs) and 4 microtines (9.8%). Abundant, riparian brushland birds made up bulk of food in this region. See also Murie (1946) for diet from pellets in Denali National Park, AK.
Composition of winter diet in Idaho was broadly similar to summer diet in Alaska, but standardized diet breadth of Alaska breeders was only 80% of win-ter shrikes, as latter take a wider array of prey, especially arthropods (Atkinson and Cade 1993).
Captive adults (65–75 g) require about 30 g of whole mouse or bird tissues/d at about 20°C; represents min-imum existence estimate for wild shrike (Montagna 1939, Cade 1967, Cramp and Perrins 1993). Based on measured metabolic rates and growth rates, young shrike requires minimum of 463 g gross food intake during 20 d as nestling (Cade 1967). From preincubation to end of fledgling period (60 d), pair and 7 young con-sume minimum of 9 kg food taken from nesting territory. Equals about 75 birds averaging 25 g, plus 222 microtine rodents averaging 30 g, and 394 bumble-bees averaging 0.8 g.
Food consumption in Nearctic winter not measured, but depending on ambient temperature, daily existence metabolism likely approximates 1.5–2.0 times summer value, if heat production below thermal neutrality is similar to that of Loggerhead Shrike (Weathers et al. 1984). Equals 2–3 small birds or mammals/d, including some wastage; see Olsson (1985) for similar estimate in Sweden.
Food Selection And Storage
Although euryphagous and opportunistic, Northern Shrike appears to act as selective predator in some circumstances: (1) more attracted to active than sluggish prey, e.g., tiger beetles (Cicindelidae) and other fast-running beetles; conspicuous and slow-flying insects, e.g., bees, wasps, grasshoppers (Bent 1950, Atkinson and Cade 1993); (2) in mating season, takes disproportionate number of male birds, probably those in territorial or courtship display (Cade 1967); (3) later takes nesting females and young at or near nest (Cade 1967), including downy young ptarmigan (Lagopus sp.; Bent 1950) and female Harris’s Sparrows (Zonotrichia querula) in Thelon Game Sanctuary, NWT (Norment 1992); (4) attracted to conspicuous or odd prey—e.g., Northern Cardinal (Rutter 1958) and errant Vermillion Flycatcher (Pyrocephalus rubinus) wintering in Montana (P. Hendrix pers. comm.); (5) systematically hunts at bee and wasp nests and at mouse holes and runways (Cade 1967); (6) also appears to lure some birds by imitative or acoustically attractive songs (see below).
Impaling prey on thorns and sharp objects or wedging prey in narrow V-shaped forks of branches well-developed behavior summer and winter; involves mainly larger arthropod and vertebrate prey. Insects typically held by thorax in shrike’s bill for impaling; vertebrates held by neck or shoulders. Emplacement on spike or in fork performed by jerking, pulling movements of head and neck; action continues until prey holds fast and further tugging does not dislodge it. Shrike may then start tearing off bites to eat or leave prey in storage (further details in Cramp and Perrins 1993). May spend considerable time (up to 45 min) wedging vertebrate prey, then removing and re-wedging in dif-ferent locations within same shrub (ECA).
In arctic Alaska, where there are no thorny shrubs, 114 vertebrate prey hung at heights ranging from 0 cm (in decumbent willow) to 360 cm above ground, 63 (55%) between 90 and 120 cm. Generally hung by neck, head (mostly birds) or shoulders or forelegs through fork of branch (mammals). See Cade 1967 for details and Olsson 1985 for comparisons during winter in Sweden.
In arctic Alaska, shrikes often—not always—maintain defined larders in their nesting areas. Up to 12 vertebrate carcasses hung at one time in clump of willow bushes over about 200- to 300-m2area. Closest items to nest usually >30 m, farthest about 180 m; if carcass experimentally moved to site <30 m from nest, shrikes take it away, apparent antipredator response (Cade 1967).
Frequent reports in older natural-history literature of shrikes killing in excess of food requirements (“wanton killing”); most instances involve special circumstances; e.g., shrike inside bird-bander’s trap kills all birds before trying to escape, or enters barn and kills all sparrows or mice inside (see Bent 1950 for examples). Shrike seems genetically programmed to kill in excess of immediate needs and to larder surplus as adaptive compensation for absence of crop and reduced anatomical storage capacity and because optimal-sized prey are spatially and temporally variable in occurrence. Owing to limited capacity of alimentary system, shrike may emerge from winter night with short energy reserves, making cached food supply advantageous to survival (Cade 1995).
On winter territories, usually hangs large prey close to site of capture; site-specific larders not often maintained, but W. S. Strode (in Bent 1950) described instance in which shrike cached 9 house mice in honey locust across river from cornfield where mice were caught running out of disturbed shocks. In Palearctic winter, caches reported more frequently (Cramp and Perrins 1993); in Sweden, cached prey often left untouched even in severe winters (perhaps frozen too hard to eat), and up to 50% taken by predators (Olsson 1985). In Nearctic, shrikes consume most stored food, unless too desiccated or frozen hard.
Nutrition And Energetics
No specific laboratory measurements, but for estimates of daily food requirements, see Diet, above.
Metabolism And Temperature Regulation
Few data, none for adults. At Ta 35°C, young shrikes increased basal metabolic rate (BMR) from 0.202 kcal/g body weight (BW)/d (1.1 ml CO2/g/h) at 1-d-old (BW = 5.0 g) to 0.328 kcal/g BW/d (2.1 ml CO2/g/h) at 6 d old, after which BMR remained constant through day 12 (BW = 56.5 g). At day 8 (BW = 37.5 g), young could maintain Tb at 40–41°C when Ta = 30°C (Cade 1967). If Northern Shrike has same low mass-specific BMR reported for Loggerhead Shrike (Cunningham 1979 cited in Yosef 1993, Weathers et al. 1984), it would benefit from significant energy savings when at rest, perched, or roosting.
Drinking, Pellet-Casting, And Defecation
Infrequently observed to drink in nature, and probably not required except under unusual conditions; but captives drink regularly (TJC).
Since Northern Shrike lacks crop and has small-capacity gizzard, pellet formation and egestion occur several times daily, probably as soon as 30–60 min following meal (intervals not recorded for Nearctic populations of Northern Shrike; see Olsson (1985) and Cramp and Perrins (1993) for Palearctic and Yosef (1996) for Loggerhead Shrike. Size of pellet varies with size and composition of meal—generally smallest for pure invertebrate meal (shape often more spherical, also more friable), larger, more elongate, and more compact for mammals, and largest for birds. Size generally in range of 15–35 mm long and 10–12 mm diameter (Bent 1950; see also Cramp and Perrins 1993). Winter sample of 185 pellets from Idaho ranged in length from 10.0 to 37.1 mm (mean 22.93 ± 5.71 SD) and 7.4 to 14.5 mm diameter (mean 11.18 ± 1.07 SD); on average, each contained 2.8 prey items (Atkinson and Cade 1993).
Adult shrike voids semi-liquid droppings consisting of chalky white urates surrounding blackish fecal matter by raising tail slightly and ejecting material as globular mass that falls straight down in typical passerine fashion (not squirted as raptors do). Nestlings void droppings in fecal sac until near fledging (about 18 d); transition from fecal sac to mature dropping occurs quickly (24–48 h); mechanism for change unknown. Nestling signals urge to defecate by elevating rear and vigorously wagging tail from side to side, usually after swallowing food (TJC).