Differences in Susceptibility to Predation During Nesting Between Pied and Black Stilts (Himantopus Spp.)
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DIFFERENCES IN SUSCEPTIBILITY TO PREDATION DURING NESTING BETWEEN PIED AND BLACK STILTS (HIMANTOPUS SPP.) RAYMOND J. PIERCE1 Departmentof Zoology,University of Otago,Dunedin, New Zealand ABSTRACT.--Thenesting success of the Pied Stilt (Himantopushimantopus leucocephalus) and the endangeredBlack Stilt (H. novaezealandiae)was studiedfor three consecutivebreeding seasons(1977-1979) in New Zealand. Black Stilts had a breeding successof lessthan t%, comparedwith over 8% for Pied Stilts. Predation by fetal mammalswas the main causeof nest failure for both species,but the impact was greater on Black Stilts. Direct and indirect evidence (including trapping predators)suggests that these introduced ground predators kill manychicks. Several factors made Black Stilts vulnerable, particularly their nestingalong streambanks that were often frequentedby predators;Pied Stiltsnested in swampswhere predatorswere few. BlackStilt chickstook up to 2 weekslonger to fledgeand their foraging patternsmade them more vulnerablethan Pied Stilt chicks.Other factorsthat possibly increasedpredation risk of BlackStilts include nestingat times of high predatoractivity, solitarynesting, high site fidelity, ineffectivedistraction displays, and lack of a disruptive camouflagepattern in adults.Introduced ground predators probably have contributed great- ly to the declineof BlackStilts. Pied Stiltshave not beenaffected similarly, because their evolutionarypast has included long exposureto groundpredators. Received 27 February1984, acceptedI July 1985. Two speciesof stilts (Recurvirostridae) occur data for most speciesare lacking. In this study in New Zealand: the endemic Black Stilt (Hi- of stilt ecology in South Canterbury, I exam- mantopusnovaezealandiae) and the Pied Stilt (H. ined the nesting successof Pied and Black stilts himantopusleucocephalus), which is the Austral- and comparedtheir susceptibilityto predation. asianrace of the widespreadBlack-winged Stilt. The BlackStilt probably has occurredin New STUDY AREA AND METHODS Zealand for many thousandsof years,but the Pied Stilt is a more recentimmigrant, probably Mostdata were collectedduring three field seasons having arrived in the late 18th or early 19th (1977-1978, 1978-1979, and 1979-1980) in the lower century (Fleming 1962, Pierce 1984a).During 15 km of the CassRiver Valley, LakeTekapo at 710- the 19th century Black Stilts were widespread 850 m (Fig. t). in New Zealand riverbeds and nested at least The study site is a glaciated valley flanked by as far north as central North Island, but they mountains,with tussocks(Festuca spp., Poaspp., and declined rapidly. Since the 1950's a small Chionochloaspp.) and other grassesand small shrubs breeding population has remained only in cen- on the lower slopes.Stilts of both speciesnested in four habitats: riverbed, side streams, ponds, and tral South Island (Pierce 1984a). By contrast, swamps.The riverbed consistedof bare shingle and Pied Stiltshave expandedrecently and are now had braided channelswith widely varying flows common in most types of wetland throughout (mean annual flow was about t0 m•/s). The side New Zealand. streamshad relatively constantflows, and a variety Stilts are ground nesters and are therefore of grassesand other small plants grew on theirbanks. vulnerable to predation by mammals. The in- Muddy ponds were surroundedby grasslandand troduction of carnivorous mammals in the 19th usually containedwater all year. The swampswere century often is claimed to have had a delete- small (<t0 ha), and the dominant plants were Juncus rious effect on New Zealand wildlife, particu- spp. and Carex spp. I previously described some larly forestbirds (e.g. see Moors 1983), but good physicaland biologicalaspects of thesehabitats and the seasonaluse of eachhabitat by stilts and other charadriiformspecies (Pierce 1983). Supplementary data on Black Stilts were collected in the Cass River aPresent address: P.O. Box69, Lake Tekapo,South Valley from 1970 to 1982 and in the neighboring Island, New Zealand. GodleyRiver Valley from 1977 to 1979. 273 The Auk 103:273-280. April 1986 274 RAYMONDJ. PIERCE [Auk,Vol. 103 TABLE1. Breeding successof Pied and Black stilts. Black Stilt Unpro- Pro- Pied Stilt tected tected Total nests 125 27 23 Nest days (N) 1,374 305.5 353 aodley % Total failed nests(F) 49 19 12 Total eggshatched a 260 33 40 CassLake • Total fledged young 69 2 13 RiverTekapo Probability of nest surviving to hatchingb (A) 0.349 0.155 0.367 Probability of egg hatching in a sur- viving nest• (B) 0.903 0.971 0.909 Probability of chick surviving to fledg- ing (C) 0.265 0.061 0.325 Breeding successd 0.084 0.009 0.108 Wamonc ' Mean clutch size of Pied Stilts was 3.8 (range 3- 4); mean clutchsize of BlackStilts was 4.0 (range 3- 6). b(1 - F/N) 29.The duration of egg-layingand in- cubationwas 29 days for both species. "'•• cThe remaining eggswere infertile or addled. a Probabilityof egg producingflying young (= A x Fig. 1. Location of study areas. The main study BxC). area was in the CassRiver Valley. On every sixth day of the breeding seasonI walked Wildlife Service. I found no evidence of mammals the wetland nesting grounds to check whether in- scavengingon desertedstilt eggs.Deserted eggs of dividual pairs were laying, incubating, or guarding stilts (3 clutches)and of other charadriiform species chicks. Other data collected were habitat, substrate, were usually still intact two weeks after having been distanceto water (all after the eggshad hatched),and abandoned. proximityof other nestingstilts. Inevitably somenests To test the hypothesisthat breeding successwould would have been abandoned before I found them, so increaseif predator numberswere reducednear nests, I used the Mayfield (1975) method for calculating I set traps around 23 randomly selectedBlack Stilt breeding success.Because nest checkswere frequent, territories. Gin or Fenn traps were set at intervals of I used the midpoint assumptionrather than the May- 40-90 m (average 60 m) and no closer than 30 m to field -40% method (Miller and Johnson 1978, John- nests.Each trap was placed under a Discariabush or son 1979). Failed nests were examined for evidence covered with a wooden tunnel 30-45 cm long x 20- of the causeof failure. I identified nest predators as 24 cm wide x 16-20 cm high. The traps were baited (1) ferret (Mustelafuro), 13 nestsin which the eggs with rabbit flesh and checkedevery secondday. The were taken whole (no eggshellleft), 10 of which had successof these "protected" Black Stilt nests was ferret footprints leading to or from the nest and 4 of comparedwith 27 Black Stilt nests without traps which had ferret faeces at the nest; (2) feral cat (Felis ("unprotected").In casepredators followed my scent catus),11 nests in which 4-6 large and many small to nests (Bart 1977), I visited only those nests that eggshell fragments were in or near the nest, 6 of could be approached through water. When I was which had catfootprints and 5 of which had catfaeces within 10 m of these nests I recorded the reactions nearby;(3) Norway rat (Rattusnorvegicus), 14 nestsin of the adult birds to my presence,as well as their which very small shell fragmentswere scatteredin nesting stage. Nocturnal observationsof breeding and around the nest, 3 of which had rat footprints behavior were made using an NVC night vision sys- and 2 of which had rat faecesnearby; or (4) Austra- tem with 500-mm lens from a hide or tent 60-200 m lasian Harrier (Circusapproximans), 2 nests in which away. more than half of each eggshellremained in or near To establish chick survival, observations of brood the nest, eachshell with one or two puncturemarks; size were made with a telescopeabout every 10 days 1 nest also had a harrier pellet present. I identified for 9 protectedBlack Stilt broods'and 6 unprotected mammalsigns from the guide of Lawrenceand Brown Black Stilt nests over 3 yr. I abandonedsimilar at- (1973) or with help from staff of the New Zealand temptswith Pied Stilt chicks,because family groups April1986] PredationonStilts 275 were usually closely spacedand therefore easy to T^BLE 2. Cause of nest failure of Pied and Black stilts. confuse. It was not until juvenile Pied Stilts were flying (and lesswary) that family sizescould be es- Side tablished. Adult Pied Stilts were individually rec- stream River Pond Swamp Total ognizable all seasonby their plumage markings Pied Stilt (Pierce 1984b),and many were also banded with in- Total nests 16 24 15 70 125 dividual color combinations. Family parties with Preyedon 5 2 6 11 24 flying youngusually stayed near the nestingarea for Flooded 0 13 0 3 16 a few days at least. To be certain that the correct Other a 3 2 0 4 9 number of young was established,however, I made Black Stilt unprotected more visits than usual to pairs about to fledgeyoung. Total nests 15 2 3 7 27 Preyedon 7 0 3 1 11 Flooded 1 0 0 0 1 Other a 4 1 0 2 7 RESULTS Black Stilt protected BREEDING SUCCESS AND CAUSES OF FAILURE Total nests 13 0 6 4 23 Preyed on 3 0 2 0 5 The breeding successof BlackStilts was much Flooded 3 0 0 0 3 Other a 2 0 1 1 4 lower than that of Pied Stilts (Table I). The probability that an egg present at the begin- a"Other" includesdesertion, damage by stockand ning of incubation would produce a fledgling wind, and unknown causes, some of which could have been predation. was 0.084 for Pied Stilts and 0.009 for unpro- tectedBlack Stilts. Eggsin protectedBlack Stilt nests had a much higher probability of pro- and chick mortality. I had no direct evidence ducing a fledgling (0.108) than those in unpro- of predatorstaking Pied Stilt chicksbut found tected nests. two cases in which Black Stilt chicks were tak- Predation by mammalsand birds accounted en by a ferret and a cat. for 49% of Pied Stilt failures and 64% of Black Indirect evidence of the impact of predators Stilt failures for which the cause was estab- on BlackStilt chickscame from using two pred- lished (Table 2).