Bird Feeding and Irruptions of Northern Finches: Are Migrations Short-Stopped?
W. Herbert Wilson, Jr. Department of Biology Colby College Waterville, ME 04901 [email protected]
ABSTRACT
The provisionof seed at feeders improvesthe and Canada(http://www.wbu.com/). Similarly, the survivorshipof birds,facilitates range expansions, firstWild Bird Center opened in 1985and currently and influences habitat choice. In this paper, I there are 100 stores in North America (http:// examine the notion that bird feeding in northern www.wildbirdcenter.com/).Wild Bird Marketplace partsof NorthAmerica can short-stopthe irruptive has increasedfrom a singlestore in 1990 to 31 in migrationsof fringillids.Indirect data to supportan 1999 (http://wbm-bird.com/). increase in bird feeding over the past 25 years are provided. Using an area bounded by the The increase in birderscertainly translates into an Appalachian Mountains to the west and the increasein the numberof peoplefeeding birds. A Atlantic Ocean to the east, I examined band re- U.S. Fishand WildlifeService survey (Caudill and encounterdata for sixfringillids and foundthat the Laughland1998) conducted in 1996,found that 63 studyarea seemsto be neithera sinknor a source million U.S. citizens are wildlife watchers. The of irruptivebirds. Using Christmas Bird Count data amountof moneyspent on birdseed, binoculars, for the studyarea (1975-1994),I showthat greater and other equipmentin 1996 is 21% higherthan irruptionstend to occurearlier in the studyperiod. the correspondingamount in 1991. Dataon the However,the data show no statisticallysignificant amount of sunflower seeds sold as bird feed over increasein birdswintering in northernlatitudes as the past25 yearsare notavailable as the end-use predicted if migrationsare short-stopped.The of sunflower seeds is typically not reported. analysisdoes not provide compelling evidence of a However, even between 1995 and 1996, the short-stoppingeffect. amount of bird seed purchased in the United States increased by 14% (Wild Bird Feeding INTRODUCTION Institute,pers. comm.). I believeit is reasonableto believe that there has been a steady increase in NorthAmerican interest in birdshas grownsteadily birdfeeding over the past 25 years. over the past 25 years. Several lines of evidence can be used to bolsterthis assertion. Subscriptions The impactsof supplementalfood on birds are to birdingmagazines have grown significantly. The manifold.Supplemental food is knownto increase AmericanBirding Association began in 1969 witha survivorshipin Black-cappedChickadees (Poecile handful of members and now boasts over 20,000 atricapillus)(Desrochers et al. 1988; Egan and members.The ABA's magazine Birdinghas been Brittingham1994; Brittinghamand Temple1988, joined by other birding publications(Wild Bird, 1992). Grubband Cimprich(1990) demonstrated Birder's World and Bird Watchers' Digest), each withptilochronology that the physiological status of with at least 80,000 paid subscribers. Stores birdsis improvedwith supplemental food. Wilson devoted to wild birds continue to increase. In 1981, (1994)showed that the habitatpreference of some the first Wild Birds Unlimited store opened; now winteringbirds can be changed if supplemental there are more than 240 stores in the United States feedersare placedin lesspreferred habitats.
Oct. - Dec. 1999 North AmericanBird Bander Page 113 On a broaderscale, the provisionof food has been Physicalbarriers on the east (AtlanticOcean) and implicatedin the range expansionof birds.Tufted west (Appalachian Mountains) determined the Titmice (Baeolophusbicolor) have extended their study area. The southern limit is the extreme wintering range into New England in the past 20 southernlimit of finch irruptions.The northernlimit years (Kricher 1981). The northward range was determined by the latitude of the most extension of Northern Cardinals (Cardinalis northerlyChristmas Bird Counts in eastern North cardinalis) and eastern House Finches America. (Carpodacusmexicanus) may have been facili- tated by the provisionof seed at bird feeders. Before gathering Christmas Bird Count data, I analyzedbanding re-encounter data, providedby A possible,but unstudied,impact of supplemental the BirdBanding Laboratory, to identifypatterns of food may be a short-stoppingof the winter irruption.Re-encounters are captures of banded migrationsof irruptivenorthern finches. Common birds outside the 10-minute block in which they Redpolls(Carduelis flammea) only migratesouth were banded. Restrictingthe analysisto eastern from their high-latitudebreeding grounds when North America can be problematicif there is a their food suppliesfail (Bock and Lepthien1976). tendencyfor eastern NorthAmerica to be either a Irruptivebehavior is also drivenby foodavailability sinkor a sourcefor irruptivebirds, compared to the in Pine Siskins (Carduelis pinus), Evening rest of NorthAmerica. I analyzedall of the banding Grosbeaks (Coccothraustesvespertinus), Pine re-encounterdata for Common Redpoll, Purple Grosbeaks (Pinicola enucleator), and Purple Finch, Pine Siskin and Evening Grosbeak. The Finches (Carpodacus purpureus) (Bock and data were separatedinto recaptureswithin a year Lepthien 1976). If supplementalfood at feeding of bandingand recapturesmore than a year after stations has increased over the past 25 years, banding. Recaptures (repeats) by the same these irruptivefinches may not have needed to bander in the same locationin the same year were migrate as far south in recent years to find excluded from the analysis. Each recapture was sufficient food. cross-classifiedaccording to area of banding (easternor western NorthAmerica where western To addressthis possibility,I analyzed data from Americais the entirearea west of the Appalachian Christmas Bird Counts from an area bounded chain) and area of recapture (eastern or western latitudinallyby southern Georgia (30øN) and NorthAmerica). Contingency analysis was usedto centralNew Brunswick(48øN). The studyarea was determine if each species was more likely to boundedon the east by the AtlanticOcean and on disperse east or west. the west by the physiographicbarrier of the AppalachianMountains. If supplementalfood is To measurethe irruptionsof the six finchesas well producinga short-stoppingeffect, the incursionof as the dynamics of the two woodpeckers, I finch irruptionsshould not penetrateas far southin gatheredChristmas Bird Count data for the study recent years. area. I divided the study area into one degree blocks of latitude. Within each latitude block, I METHODS chose at least 30 Christmas Bird Counts to analyze.These were chosen primarily on the basis Six speciesof irruptivefinches were analyzed in of continuouscoverage from 1975 through 1994. this study: Evening Grosbeak, Pine Siskin, For each count,the numbersof each specieswere Common Redpoll, Purple Finch, Pine Grosbeak normalizedby dividingeach speciestotal by the and White-wingedCrossbill. The firstfour species numberof party-hoursto removethe problemof visit bird feeders regularlyin the winter. I also differentialeffort among counts. analyzedtwo sedentaryspecies--Downy Wood- pecker (Picoides pubescens) and Hairy Wood- To analyzethe distributionof birds, I dividedthe pecker (P. villosus)--thatoccur throughoutthe 20-year intervalfrom 1975 to 1994 into four five- geographic area studied. Insufficientdata pre- year intervals (1975-1979; 1980-1984; 1985- cluded analysis of Hoary Redpoll (Carduelis 1989; 1990-1994). For each latitudinalblock, I hornemann/). averagedthe numberof birdsseen/party-hour for Page 1 14 North American Bird Bander Vol. 24 No. 4 I all of the Christmas Bird Counts available in a banded in the east) compromisesthe power of the givenyear. A grand mean withineach latitudewas analysis, reducingconfidence in the results. obtainedby averagingfive years of data for each five-year interval. For Pine Siskins banded withina year of banding, 95% of birdsbanded in the eastern regionwere re- To test statisticallyfor differencesin the limitsof encountered in the east compared to 93% of southern irruptionsthrough time, I tested for the western birds banded in the west. For siskins re- numbersof individuals/party-hourof each species encountered over a year after banding, 69% of in the area between 30øand38 ø N. A single-factor eastern birds were re-encountered in the east ANOVA was used with the four five-year time while 72% of birds banded in the west were re- intervalsserving as the levels of the factor of time. encountered in the west. Contingency analysis If the ANOVAindicated a significantdifference (p < reveals these proportionsare not significantly 0.05), then.pair-wise Bonferroni contrasts were differentin both cases (p > 0.05). used to find where significantdifferences lay. In like fashion, I tested for differences in abundance Finally, for Evening Grosbeaks re-encountered for each species in the area between 40 ø and withina year of banding,89% of eastern birdswere 48ON. recaptured in the east and 89% of western birds were recapturedin the west. For recapturesmore RESULTS than a year after banding, 75% of re-encountersof eastern birds were in the east and 77% of Table 1 providesa compilationof bandingre- recaptures of western birds were in the west. encounter data for four of the fringillidsstudied. These pairs of percentages do not differ Too few data for analysiswere availablefor Pine statisticallyby contingencyanalysis (p > 0.05). Grosbeakand White-wingedCrossbill. Figures 1 and 2 present data for the abundance, Contingencyanalysis of the data in Table 1 reveals measured as numbers of birds/party-houron a remarkable concordance between sides of the Ch'ristmasBird Counts, of eightbird species. I first continent.For Purple Finches recapturedwithin a analyzed two sedentary species, Downy Wood- year of banding, 89% of birds banded in the east pecker and Hairy Woodpecker, predicting little were recaptured in the east while 91% of birds difference in abundance among time intervalsat bandedin the west were recapturedin the west. each latitude.Visual inspection of the dynamicsof Similar results were obtained for recaptures these birds (Fig. 1) revealssimilar patterns for all greater than a year after banding with 84% of four intervalsas expected. Pine Grosbeak(Fig. 1) eastern birds banded in the east and 83% of shows some variation among intervals but it is western birds banded in the west. Continency evident that members of this species only rarely analysis reveals that the two proportionsfor the venture below 43ø N. Purple Finch (Fig. 1) shows two recaptureintervals are notstatistically different complex dynamics. For latitudesbelow 41 øN, the (p > 0.05). fewest numbers of Purple Finches are seen in the most recent time interval. The lowest number of re-encounters for these four species was for Common Redpoll. For birds re- White-wingedCrossbills (Fig. 2) typicallyare not encounteredwithin a year of banding,there is no found south of 36øN and, except for widespread significantdifference in the proportionsof birds occurrences in 1975-1979, most of the birds are banded,with 91% of eastern birdsrecaptured in found north of 43øN. Common Redpolls (Fig. 2) the east and 99% of western birds recapturedin show high abundance in 1990-1994 at high the west.However, there is a significantdifference latitudes. Typically this species is not found in for recapturesover a year after banding.Only 46% significantnumbers south of 42øN. Pine Siskins of eastern birds were banded in the east while 79% (Fig. 2) show an unexpectedbimodal distribution of western birds were re-encountered in the west with the higher mode being in the north and (p < 0.01 in contingencyanalysis). However, the relativelylow abundance in the middle latitudes. small number of re-encounters (only 28 birds
Oct. - Dec. 1999 North American Bird Bander Page 115 The highest incursionof siskins into the south Table1. Analysisof bandre-encounter data for occurred during the 1985-1989 interval. Finally, four fringillid species.Data are divided into Evening Grosbeaks (Fig. 2) are relatively recapturesless than or equalto one year after uncommonsouth of 41ø N. Highestnumbers in the bandingand recaptures more than one year after south occurredduring the 1980-1984 interval. banding.The East is definedas the portionof easternNorth America east of the Appalachian Table 2 presentsthe resultsof a more quantitative mountain chain. West is all of North America analysis of the changes in abundance through westof the Appalachians. time. The mean numbersof birds/party-hourare analyzed for areas south of 38øN in Table 2(a). BandingRecaptures Recaptures Downy Woodpeckers show a significantincrease Region: in both 1980-1984 and 1990-1994 compared to Recapture<1year after >1year after 1975-1979. Hairy Woodpecker shows no signifi- Region banding banding cant changes in abundance. Pine Grosbeaks, rare south of 38øN, do not differ in abundance across a. Pine Siskin the 20-yr period.The ANOVA for Purple Finches showed many significantchanges; five of the six East: East 643 1 O0 possible pair-wise combinationswere statistically East: West ß35 44 different.The highestabundance was in the 1980- 1984 interval. West: West 654 128 White-wingedCrossbills rarely occurred in these West: East 49 49 southern areas and showed no significant differences through time. Similarly, Common b. CommonRedpoll Redpoll is quite rare south of 38øN and no significant differences among intervals were East: East 75 13 found.Pine Siskin was significantlymore common in 1985-1989 comparedto each of the remaining East: West 7 15 three intervals. Evening Grosbeaks were signifi- cantly more abundant in 1980-1984 comparedto West: West 248 44 both 1985-1989 and 1990-1994.
West: East 3 12 Some of these results are consistent with the c. EveningGrosbeak prediction that irruptions are weaker into the southernpart of the studyarea. Highestincursions East: East 939 982 of Purple Finches, Pine Siskins, and Evening Grosbeaks occurred either in 1980-1984 or 1985- East: West 119 324 1989.
West: West 445 559 Examinationof the numbersof these speciesnorth of 40øNdid not mirrorthese changes.Analysis of West: East 53 170 variance revealedthat none of the speciesshows any changein abundancein the northernlatitudes d. PurpleFinch from year to year (Table 2). East: East 337 413 DISCUSSION East: West 43 77 In this study, I address the possibilitythat the West: West 523 534 provision of food at northern latitudes causes irruptivefinches to overwinterat higher latitudes West: East 51 107 than they would withoutthe largessefrom bird
Page 116 North AmericanBird Bander Vol.24 No. 4 feeding stations. I begin with the claim that bird establishment of Project FeederWatch by the feeding has increased significantlyover the past Cornell Laboratoryof Ornithologyin 1988 are all 25 years. Unfortunately,it is impossibleto collect indicativeof growth in interest in birds by the irrefutableconfirmation of this assertion because general populace. of the way that sunflowerseed sales are reported. Nevertheless,as detailed in the Introduction,the Elucidationof populationdynamics of migratory proliferationof wild bird stores,the rapidgrowth of birds requires the concerted work of many the subscriptionsto birding magazines and the individuals.For this study, I used the substantial
Table 2. Abundance of species (total/party hour) in the southern and northern portion of the study region. Data reported are means and standard errors in parentheses. The results of a one-way ANOVA to test for differences among intervals are given in the right-hand columrL Bonferroni contrasts were used to identify significantpair-wise differences. These differences are keyedto lettersbelow each interval. a. Latitudes 30øN to 38øN
1975-1979 1980-1984 1985-1989 1990-1994 Results of (A) (B) (C) (D) ANOVA
0.36 0.48 0.44 0.46 DownyWoodpecker (0.030) (0.038) (0.205) (0.027) A,B,A
b. Latitudes between 40øN and 48øN
0.46 0.63 0.49 0.54 Downy Wood pecker NS (0.48) (0.093) (0.046) (0.044) 0.20 0.22 0.20 0.21 HairyWoodpecker (0.016) (0.012) (0.012) (0.017) NS 0.36 0.33 0.34 0.25 Pine Grosbeak NS (0.069) (0.051 ) (0.071 ) (0.074) 0.38 0.25 0.11 0.26 PurpleFinch (0.108) (0.054) (0.015) (0.082) NS 0.19 0.0 0.0 0.0 White-wingedCrossbill (0.146) (0.0) (0.0) (0.0) NS 0.56 1.18 1.02 2.45 CommonRedpoll (0.201) (0.372) (0.204) (1.350) NS 0130 0.32 0.59 0.61 Pine Siskin NS (0.087) (0.071 ) (0.147) (0.193) 3.22 3.30 2.56 2.47 EveningGros beak (0.342) (0.359) (0.298) (0.502) NS Oct. - Dec. 1999 NorthAmerican Bird Bander Page 117 Page 118 North American Bird Bander Vol. 24 No. 4 Oct. - Dec. 1999 North AmericanBird Bander Page 119 database from the Christmas Bird Count to Levine et al. 1998), these woodpeckersare one of examine how the populationsof several finches the few sedentary species whose range includes have changed since 1975. The data providedon all of the study area. The abundance data as a the Christmas Count summaries were normalized function of degree of latitude for Downy by dividingthe number of birds of each species Woodpeckerand HairyWoodpecker show largely observed by the total party-hoursfor a particular concordant of patterns across the four 5-yr count. Reporting birds/party-hours has the intervals(Fig. 1) althoughsome variation is evident advantage of being independent of area. Two in that HairyWoodpecker abundance varies more regionsof differentareas can be compared using at highlatitudes than at moresoutherly areas. Both the numbersof birds/party-hour. woodpeckers show a local peak at 32øN in the 1980-1984 interval. Nevertheless, the general For this study, I chose to focus on the eastern pattern is remarkable congruityamong five-year seaboard of North America to limit the data that intervals in woodpecker abundance. These two would need to be analyzed and to focus on the species have had stable populationnumbers from area of the United States where bird feeding is 1966-1996 as judged from the Breeding Bird most prevalent because of the high human Surveydata (http://www.mbr-pwrc.usgs.gov/bbs). population densities. The area was delimited These two speciesprovide a test of the efficacyof physiographicallyto the west by the Appalachian using Christmas Bird Count data to test for Mountainchain, to the east by the AtlanticOcean, population changes. The consistency of the to the north by the northernlimit of ChristmasBird distributionsof both woodpeckers(Fig. 1) agrees Counts and to the south by 30øN, below which with independentevidence that these two species irruptivefinches are extremely rare. have stable populationsizes.
A possiblebias in the ChristmasBird Count data is Pine Grosbeak(Fig. 1) and White-wingedCrossbill that incursionsof irruptivefinches into the study (Fig. 2) were analyzed to provide examples of area may come from areas west of the studyarea. dynamics of irruptive birds which rarely come to Alternatively, the eastern seaboard could be a bird feeders. Both species infrequentlyoccur at sourceof irruptivefinches in the westernportion of more southerly latitudesand both show striking the United States and southern Canada. To test for variabilityin abundance as expected for irruptive this bias, I analyzed the band re-encounters for species. four species of irruptivefinches (Purple Finch, Common Redpoll. Pine Siskin, and Evening For finchesthat frequent feeders, I examinedthe Grosbeak). As shown in Table 1, no such bias data for evidence of a short-stoppingeffect (fewer exists.The percentageof birdsbanded in the east birds occurring in the southern latitudes) during and recapturedin the east is statisticallyidentical irruptionsin the latter part of the study interval. to the percentageof birdsbanded in the west and Inspectionof the dynamics for Common Redpoll recaptured in the west with the exception of (Fig. 2) does not suggest any short-stopping Common Redpollscaptured over a year after effect. However, the remaining three irruptive banding.The majorityof theseresults indicate that finches show patterns that suggest that short- analyzing Christmas Bird Count data from the stoppingmay be occurring.Purple Finch (Fig. 1) easternseaboard is not burdenedby confounding and Evening Grosbeak (Fig. 2) show lowest net gains/lossesfrom areas west of the defined incursions in the south in the most recent time study area. interval.Pine Siskinhad greatestabundance in the south in the 1985-1989 intervaland generallythe As a test of the precisionof the CBC data, I chose lowest abundance in 1990-1994. two sedentary species, Downy Woodpeckerand Hairy Woodpecker, whose population abun- However, statistical analysis of these patterns danceswere analyzedthroughout the studyarea. gives equivocal results. If short-stopping is Althoughsome individualsof both species are occurring, one predicts lower abundance in the known to undertake periodic fall movements, southern part of the region in more recent years. particularlyin northernpopulations (McPeek 1994, Analyzingareas south of 38øN, four of the eight Page 120 North American Bird Bander Vol. 24 No. 4 species showed significant changes. Downy analysisof the ChristmasBird Count data. Mary Woodpeckers have increased since 1975-1979 Gustarsonfrom the Bird Banding Lab kindlysent (Table 2). Pine Siskinswere significantlymore all the bandingdata for the finchesanalyzed in this abundant in 1985-1989 and Evening Grosbeaks study. Thanks to the thousandsof observerswho were significantlymore abundant in 1980-1984. have contributed to the Christmas Bird Count PurpleFinches were leastabundant in 1990-1994. database and the banderswhose many thousands of birds contributedto the banding data used If decreaseddensities of Purple Finches,Pine herein. Siskins,and EveningGrosbeaks in recentyears are real, then one would predict concomitant LITERATURE CITED increases at northern latitudes; northern winter populationswould be swelledby individualswhich Bock,C. E. and L. W. Lepthien.1976. Syn- formerlymigrated to the south of a given area. chronous eruptions of boreal seed- Analyzingthe regionnorth of 40 øN, such increases eating birds. Am. Nat. 110:559-571. are not evident.None of the eightspecies showed Brittingham,M. C. and S. A. Temple. 1988. Im- significantdifferences across the 20-yrperiod. It is pacts of supplementalfeeding on sur- possible that the much higher abundances of vival rates of Black-cappedChickadees. Evening Grosbeaks and Pine Siskins, north of Ecology69:581-589. 40øN might absorb the short-stoppedindividuals Brittingham,M. C. and S. A. Temple. 1992. Use of withoutshowing a statisticalincrease, given the winter bird feeders by Black-capped levelof accuracyof the ChristmasBird Count data. Chickadees.J. Wildl.Manage. 56:103-110. The lack of increase for Purple Finch is more Caudill,J. and A. Laughland.1998.1996 National difficult to explain as the abundance in the and state economicimpacts of wildlife southernpart of the regionis at leastas highas that watching.U.S. Dept. of the Interior,U.S. in the northernpart. Fish and WildlifeService Report, 11 pp. Desrochers, A., S. J. Hannon, and K. E. Nordin. Froma graphicalanalysis (Figs. 1 and 2), there is 1988. Winter survivaland territoryacquisi- some evidenceto suggestthat short-stoppingof tion in a northernpopulation of Black- irruptivefinches is occurring.However, the lackof capped Chickadees. Auk 105:727-736. an increase in irruptivefinch abundance in the Egan, E. S. and M. C. Brittingham.1994. Winter northern part of the region counters this survivalrates of a southernpopulation of suggestion.Continued analysis of ChristmasBird Black-cappedChickadees. Wilson Bull. Count data will falsify the short-stopping 106:514-521. hypothesisif significantirruptions occur in the Grubb, Jr., T. C. and D. A. Cimprich. 1990. southernpart of the regionin the future.Data from Supplementaryfood improvesthe nutri- ProjectFeederWatch will also be importantin this tionalcondition of winteringwoodland regard. One drawbackof the ChristmasBird Count birds:evidence from ptilochronology.Ornis is that the counts occur early in the winter when $cand. 21:277-281. many irruptions are just beginning. Project Kricher,J. C. 1981. Range expansionof the Tufted FeederWatch participants record data from Titmouse (Parus bicolor)in Massachu- Novemberthrough March, so better resolutionof setts. Amer. Birds 35:750-753. the dynamics of irruptivespecies is possible. Levine, E., B. B. Lincoln,and S. R. Lincoln(eds.). Nevertheless,we cannotwith certaintyadd short- 1998. Bull's birds of New York state. stoppedmigrations to the impactsthat supplemen- ComstockPublishing Associates, Ithaca, tal food has on wintering birds with present NY. information. McPeek, G. (ed.). 1994. The birdsof Michigan. Indiana UniversityPress, Bloomington. ACKNOWLEDGMENTS Wilson, Jr., W. H. 1994. The distribution of winteringbirds in central Maine: the I am gratefulto Julie Moran, Rachel Zierzow, and interactiveeffects of landscapeand bird Ann Savage for assistancewith the tabulationand feeders. J. Field Ornith. 65:512-519. Oct. - Dec. 1999 NorthAmerican Bird Bander Page 121