POPULATION ECOLOGY Gone forever Passenger Pigeon (Ectopistes migratorius)HREEHUNDRED wasYEARS the AGO, world'sTHE mostabundant land bird. Althoughfound only in easternNorth America, it num- beredthree to five billion,accounting for about a quarter of all North American landbirds. Passenger Pigeons traveled in flocks of hundredsof millions, at times obscuringthe sun. More than a century haspassed since the lastgigantic nesting colonies,and over seventyyears since the deathof the last PassengerPigeon, Martha,in the CincinnatiZoo. Although therehas been much speculationabout the extinctionof the PassengerPigeon sincethat time, mostof the proposedex- planationsare inadequate. The PassengerPigeon was never seri- ouslystudied while it still existedand the publishedaccounts are incompleteand oftencontradictory. Most of ourinforma- tion comesfrom reportsin the scientific and popular literature of the late nine- teenthand early twentiethcenturies. Lit- tle hasbeen written about the Passenger Pigeon'srole asan importantcomponent of the easterndeciduous forest ecosys- tem. Its reproductivebehavior exploited the mastfruiting of thesetrees, which in Photo/TheBell Museumof Natural History. turnsupported the tremendousPassenger Pigeonpopulations. One of the keys to the bird's successlay in its ability to nu- mericallyoverwhelm its predators. The precipitousdecline of the Passen- A contemporary look ger Pigeonfrom 1871 to 1880, and the birds' subsequentextinction, was an in- escapabledemographic consequence of at the extinction the relentlessdisruption of the nesting colonies,which resulted in repeatednest- ing failures.This was facilitatedby the developmentof the easternrailroad net- work andthe telegraph,which madeev- of the Passenger Pigeon ery colonyaccessible to marketharvest- ing. Overhuntingdid not exterminatethe PassengerPigeon, as is commonlybe- lieved, rather, over two decades,cohorts died withoutthe opportunityto replace themselves. Was overhunting the causeof the Passenger Pigeon'sdemise or was the bird an early victim of technology? tinctionNORDER of TO theUNDERSTAND Passenger Pigeon, THEEX-its ecologyand incredibleabundance must first be considered. Nomadism and colo- nial behavior were critical to the Passen- David E. Blockstein and Harrison B. Tordoff ger Pigeon'ssuccess. The birdsnested in coloniescontaining millions of pairs, Volume 39, Number 5 845 traveledtogether to feed, and roostedto- autumnremained on the ground Because getherat night. During springmigration thelocations of largemast crops changed andthe breeding season, the entirepopu- fromyear to year, PassengePigeons rare- lation consistedof an amazingly small ly nestedin the sameplace in two con- numberof groups,probably fewer thana secutiveyears. After certainpoor mast dozen.These groups were stupendous, years,there was probably no majornest- oftenfilling the sky and taking days to ing attempt.In most years, at leastone passover an area. The descriptionsof enormouscolony nested in the east(usu- •these flights--"indescribable multi- ally in the forestsof New York or Penn- tudes,""so thick that they obscurethe sylvania)and one in the midwest.During light"--would be unbelievablewere they the 1800s, the midwestcolony usually not repeatedby so many authors.The nestedin Michigan,where it fed onbeech largestflight ever recorded was estimated mast, and in alternateyears, in Wiscon- to contain2,230,270,000 birds (Wilson sin, where the birds fed on acorns 1832). By travelingand nestingin such A nestingusually consisted of several Beech leaves and mast largegroups each pigeon was essentially coloniesin adjacentcounties. The largest shieldedfrom predators,a concept nestingever recordedcarefully, in W•s- known as predatorsatiation. Wherever consinin 1871, covered850 squaremiles the pigeonswent there were not enough andcomprised at least135 millionadults local predatorsto seriouslydetract from (Schorger1937). Every suitable tree had their numbers. nests,sometimes fifty or more, eachcon- How could so many pigeons find tainingonly one egg. Nestingwithin a enoughfood? Before settlement by Euro- colony was highly synchronous,w•th peans,eastern North America was almost mostof the eggslaid on the sameday completelycovered by deciduousforest. Therewere so many nests that nonhuman Severalof the forest trees--beech, oaks, predatorscould hardly affect nesting suc- and chestnut--producedlarge nutritious cess. nuts that were the main food of the Pas- Certain aspects of their biology, sengerPigeons. Every few yearsin any sharedwith otherdoves and pigeons (Co- given area, these trees producedenor- lumbiformes),allowed Pasenger Pigeons mouscrops of nuts.This patternof "mast to form thesegigantic nesting colomes fruiting" at unpredictabletimes enabled Their tremendousmobility, granivorous x....-•..•_.• the treesto satiatetheir local seedpreda- diet, andthe productionof pigeonmilk* Oakieaves and mast tors--as colonial behavior enabled the in the cropsof breedingadults, allowed pigeonsto satiatetheir predators--and the birdsto providetheir nestlingsw•th thusincrease their reproductivesuccess. food gatheredfar from the nestingcolo- Flocksof PassengerPigeons roamed the ny. Like othercolumbids, the parentsex- forest, often flying hundredsof miles a changednest-tending duties twice dally, day, untilthey found an areawith a heavy with the male on the nest for about s•x mast crop. They remained there until hoursat middayand the femaleon for the they depletedthe food supply. So effi- remainder of the time. This allowed each ciently did they scour the forests that parenttime to travel far enoughto get hogssometimes starved in their wake. In foodfor itself and its nestling.The d•ff•- summer,when nuts weren't ayailable, culty of providingenough food for two the PassengerPigeons wandered to the nestlingsand pressuresto keepthe nest- northern Great Lakes states and Canada ing periodas shortas possiblemay have to feed on a wide varietyof berries.Be- beenfactors working to limit the clutch causethe pigeons dispersed after nesting, size to a single egg. the summer and fall flocks were smaller The nestingseason usually began m in comparisonwith the springflights, but April andconcluded in May or June Th•s still huge. In late fall and winter the onlyallowed time for one or two nestings flocks continued their nomadic wander- a year, but thereis no evidencethat any ing throughthe southernUnited States, individualattempted a secondbrood after where acorns were the main food. Here a successfulnesting. It is known that the pigeonsformed tremendousroosts, pairsreplaced clutches, especially those Chestnut leaves and mast oftenbreaking limbs and destroying trees destroyedby late snowstorms.The nest- by theircollective weight. Someof these ing seasonseems to havebeen controlled PassengerPigeons often disgorgedthe con- fall and winter roosts were at traditional tentsof their cropsin order to consumea more by the availabilityof nuts; the supply favoredfood. Theypreferred beechmast over sites,occupied whenever there was a suf- acorns, and buckwheat more than wheat. Au- ficientfood supply. * a curd-likesubstance produced by both sexes at hatch- ingtime to feed the young. Though it containsno sugar, dubon estimated that 1,115.136,000 birds Nestingcolonies formed in the spring it resemblesmammalian milk in thatit consistsof salts, consumed8,712,000 bushelsof mast daily. where large cropsof nuts from the past fats and proteins, 846 American Birds, Winter 1985 .\ ( 'dDl.I xl I1.\ ßII, zl,.Z.I•:•//,•/•..". Volrune39, Nmnber5 847 diminishedrapidly each spring as a result themand enjoy their shareof the spoil." subjectof gmat speculation:unknown of germinationand consumption not only diseases,"loss of vigor," mass drown- by pigeons,but alsoby insects,microor- Y ALL ACCOUNTS,THE FINALDE- ings, emigration to South America or ganisms,mammals, and other birds. Al- cline of the PassengerPigeon was Australia. Yet, at the time, some orni- thoughPassenger Pigeons ate a varietyof rapid. It is impossibleto know whether thologistsunderstood quite well "that all fruits and seeds,the mastcrops were es- populationlevels in the mid-! 800s were other theories... to account for the de- sentialfor nesting. comparablewith those under primitive structionof the birdsby othercauses than The localimpact of a PassengerPigeon conditions. Pigeon numbers declined man'sagency are absolutelyinadequate" flock was devastating.Schultz (in Mc- first in the East, perhapsas early as the (Forbush 1913). The most obvious hu- Kinley 1960)describes a roostingarea in late eighteenthcentury (Schorger 1955), man impact on pigeonswas the direct Missouriwhere a forty acrearea "had the then in the Midwest. Several writers have killingof adultsand young. This wasac- appearanceof having sufferedfrom a suggestedthat this was owing to defores- companiedby habitatdestruction through hurricaneor tornado." A great deal of tation for humanagriculture. deforestationthat might, in time, have timberwas destroyedat theseroost sites. There were still hundreds of millions beenfatal to the species.However, nei- Dung accumulatedbeneath the roost of PassengerPigeons left in the early ther overharvestingnor habitat destruc- trees to a thickness of several inches or 1870s, when they were still regardedas tion by themselvesare sufficientcauses more,killing the vegetation,although ul- pests by Michigan farmers (Barrows for the extinction of the Passenger timately enrichingthe soil. Passenger (1912:246). The 1870s was a decadeof Pigeon. Pigeonsmay have been important disper- catastrophicdecline