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Antipredator Behavior of American Avoget and Black-Necked Stilt Chicks

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Antipredator Behavior of American Avoget and Black-Necked Stilt Chicks j. Field Ormthol., 53(4):315-325 ANTIPREDATOR BEHAVIOR OF AMERICAN AVOGET AND BLACK-NECKED STILT CHICKS By T•:x A. SORDAHL American Avocet (Recurvirostraamericana) and Black-necked Stilt (Hi- mantopusmexicanus) chicks are precocial,leave the nest within 24 h after hatching,and are flightlessfor their first 4-5 weeks.During this period they occur in open environments,where they are exposedto a variety of predators.The most important antipredatorstrategy of young avo- cetsand stiltsis to seek cover and hide or simply crouch (posturede- scribedby Hamilton 1975:88)in the openwhen dangerthreatens, and then rely on the aggressiveand/or diversionarybehavior of their parents. Chicks and especiallytheir parents are highly vigilant; this is probablyessential, in view of the large number of potentialchick pred- ators and the inability of the parents to actually rout most of them (Sordahl 1981). Gibson(1971:452) noted that avocetchicks are very difficult to find when hiding, and that "hiding behaviorlasted until at leastthe third week,after which theyjust ran." In this paper I examine the antipredator behavior of young recurvirostridsand show that it differs betweenavocets and stiltsand varieswith age and contextin an apparently adaptivemanner. STUDY AREA AND METHODS During the summersof 1977 and 1978, I studiedavocets and stiltsat 2 sitesin northern Utah: the BarrensCompany Hunting Club in Cache Countyand the Bear River Migratory Bird Refuge in Box Elder County. Both areas are managedby dredging, diking, and regulationof water flow. Their vegetationand topography are essentiallyidentical. The general aspectof these marshesis one of broad ponds <1 m deep, usuallybordered by dikes seldom• 1 m high. Vehiclescan be driven on the main dikes, which are paralleled by "borrow" channelsthat are often relativelydeep. The big pondslie on salt flats that are frequently exposed by fluctuating water levels. The flats become parched and crackedby June, water levelsdropping as the seasonprogresses. Vegetationis absentin many places.Salicornia spp. are the only plants growing on most of the salt flats. The Salicorniamay grow as a sparse coverof sprigs5-10 cm high or as widely-spacedsturdy bushes of 20- 40 cm height and 40-70 cm diameter.On someof the drier areasa few greasewood(Sarcobatus vermiculatus) shrubs occur. The dominant plant along channelsand other relatively stablewater boundariesis desert saltgrass(Distichlis stricta), which growsin densemats and reaches15- 40 cm in height. In parts of the Bear River Refuge and at the edge of the Barrens,more typical"marsh" vegetation occurs--e.g., cattails (Ty- pha spp.), rushes(Scirpus spp.), sedges (Carex spp.), and reeds(Phragmites communis).However, neither adult recurvirostridsnor their young reg- ularly use suchhabitats. 315 316] T.A. Sordahl j. k•elclo, ni,lml. Auttlnln 1982 I banded172 young avocets and 120young stilts. Of these,49 avocets (33 nestlings,16 older chicks)and 51 stilts(18 nestlings,33 older chicks) were captured at the Barrens; captureswere made opportunistically during daily field work and during 42 h of nightlightingon 17 nights. The remaining 123 avocetand 69 stilt chickswere capturedat the Bear RiverRefuge. Weekly trips were madeto the refuge oncehatching had begun.I searchedfor chicksby drivingthe dikesin an automobile;an assistantwatched the broodswith binocularsor spottingscope while I approachedthem, then directedme to their hiding placeswith hand signals.A smallboat was used to negotiatethe deeperwaterways. Each chick received a U.S. Fish and Wildlife Service band (on the tibio-fibula),which quickly became dull-colored and difficult to seein the field. Sincecolor bands might reduce the ability of chicksto hide from predators,only 2 avocetand 6 stiltchicks that werecaptured near fiedgingwere color-banded.Eighteen avocet and 15 stilt chickswere recapturedprior to fiedging. I attemptedto recordthe followingdata for eachyoung bird cap- tured: (1) speciesand estimatedage; (2) time of capture;(3) what sub- strateit wascaptured on; (4) whetherit tried to hide whenapproached or simplyran; (5) whetheror not it calledwhen captured.I measured the wing chord of each chick to the nearestmillimeter with vernier calipers.Age estimateswere basedon personalexperience and the knowledgethat fiedging normally occursat ca. 28 days.Since I never misjudgedthe ageof a recaptureby > 1 day (mostrecaptures had been bandedoriginally during their hatchingday and weretherefore of known age), I believethe age estimatesare accurateto +2 days. Wheneverwater of sufficientdepth waslocated near a capturesite, I testedthe underwaterswimming ability of chicks.Each bird wasplaced in the water and givena standard"scare treatment" that consistedof my wadingafter and reachingfor it; it wasthen classifiedas either a diver or nondiver. Dive durations were timed to the nearest. 1 sec with a stopwatch.I pacedthe distancebetween submergence and emergence pointsalong the chicks'underwater paths, and convertedthe pacesto meters(I had earlier calibratedmyself for accuracyand consistencyby pacinga footballfield while wearinghip boots;during 12 trials I paced the 100 yd in 123 to 125 paces,so 124 pacesper 100 yd wasused as a standard).Most chickswere given 3 diving trials, in rapid succession, and the bestperformance was used for calculations;distance and du- ration usuallydecreased on consecutivedives. RESULTS AND DISGUSSION Diurnal rs. nocturnalescape behavior.--The habitats where avocetand stiltchicks were captured in the daytimediffered from thosewhere they werecaptured at night (Fig. 1; althoughthe data for both speciesyield highly significantX 2 values,the statisticaltest for avocetsmay not be validbecause of the low expectedfrequencies at night).During the day I usuallycaught chicks hiding in grassor, lesscommonly, crouching on Vol.•, •,) 4 Avocetand Stilt Chick Behavior [317 DAY NIGHT 1oo • so o avocets stilts avocets stilts n: 116 n=89 n=? n=35 HABITAT FIGURE1. Percentof AmericanAvocet and Black-neckedStilt chickscaptured in 3 hab- itatsduring the day and at night. Open barsrepresent open water, shadedbars mud flats,and blackbars vegetation. open mud fiats;a smallpercentage, usually very youngchicks, crouched in shallowexpanses of open water, and a few older chickstook refuge in deep water (seesection on aquaticescape behavior below).At night, on the other hand, chickswere caughtalmost exclusively in shallowopen water. This pattern couldbe explainedby the fact that chickshiding in the grasswould be extremelydifficult to find at night (despitethe fact that they are alsomore difficult to find in grassthan on mud or water in the daytime). However, it seemedpeculiar when a 1-day-oldavocet brood that I caughtcrouching in the water at night repeatedlyscrambled out to lie fiat in the water each time I placedthem in the grass.My subse- quent capture of so many chicksat night in open water led me to con- sider severalfactors which would promote this behavior (I doubt that chicksroost in the water, as someadults do, at leastuntil they are near fiedging). The overriding factor must be that chicksare lessinhibited by the danger of visualpredators to move about or crouchin the open at night. Twenty-fivepercent (30/118) of avocetchicks captured in the daytime and 43% (3/7) of those capturedat night tried to escapeby running. Eleven percent(10/91) of stilt chickscaptured in the daytime 318] T.A. Sordahl j. FieldOrnithol. Autumn 1982 and 70% (23/33) of thosecaptured at night tried to escapeby running (X2 = 39.79, 1 dr, P < .001). Probably chickswere both more likely to run and lesslikely to be discoveredif hiding at night. The increased tendency to run at night could be explained by the chicks'reduced abilityto seethe predator,if this is the stimulusrequired to causechicks to hide. A proximate explanationof the behavior leading to a large number of capturesin the water at night might be that, under cool nighttime air temperatures,chicks seek the water for warmth when disturbances preventtheir parentsfrom broodingthem. Low temperaturesat night tended to increase chick movement. Small chicks became chilled and chasedtheir parentsin order to be brooded;activity may alsoserve to generatebody heat. Possiblythe youngdo somefeeding in the water at night, but their apparentinability to withstandnighttime temperatures without frequent brooding makesthis an unlikely explanationfor at least the first 2 weeks. Nocturnal releasefrom danger by visualpredators permits chicks to activelyescape from a disturbancecenter. Thus an ultimateexplanation, in viewof the muchgreater importance of olfactorypredators at night, is that the behaviorcould be adaptivebecause: (1) water would not hold the chicks'scent and would therefore be a safer hiding placeor escape route than solid ground or vegetation;(2) water-coveredareas have fewerobstructions than vegetatedareas and thusprovide a fasterescape route. The beststrategy to elude olfactorypredators searching an area at night may be to move to water and crouch(young chicks)or escape to another area (older chicks).Auditory cuesare probablyreduced by the loud callingand splashingof the parents. The loud behaviorof parentswas much more localizedaround their chicksat night than during the day. This and the chicks'greater activity enabledme, as a visualpredator with a flashlight,to find chicksquite readily at night. Avocetvs. stilt escape behavior.--Avocet and stilt chicksare quite similar at hatching,but a stilthatchling is somewhatsmaller, has relatively long- er legs,unwebbed feet, and slightlydarker down
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