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Survey of a MajorSwallow Roostin Pembroke

byR.K. Ross, W.R. Clark and J.M. Bouvier

After the breeding period, manyspecies of swallows typicallycongregate each night incommunal roosts. These gather­ings may be very large in size andcontain several species. Suchroosting aggregations can occur onthe wintering grounds; a singleroost of over one million swallowswas noted on a reed marsh in theTransvaal (Ingram 1974). Roostsare also found along migrationroutes and Bent (1942) hasdocumented sizeable flocks ofmost North American swallows,particularly the Tree Swallow(Taehyeineta bieolor), BankSwallow (Riparia riparia) andPurple Martin (Progne subis). Amajor roost for migrants is locatedat the confluence of the Muskratand Ottawa Rivers in the City ofPembroke. Much concern has beenraised over the conservation of thissite (Hackman 1983; Clark 1984).Information on swallow roosts issparse, so we are providing somedetails of the Pembroke roost withemphasis on results from a surveyto determine the number ofswallows present during the period

of peak usage in August. This wasa joint effort of the CanadianWildlife Service and localnaturalists.

The roosting site is on a pointwhich becomes a small islandduring high water, extending offthe north shore of the MuskratRiver mouth (Fig. 1). It has asandy substrate which supports astand of Black Willow (Salixnigra) that is about 10 to 15 m inheight and 0.5 ha in area; the birdsroost in the canopy. Many of thetrees bear scars from ice scouringand there is relatively littleunderstory. The roosting swallowscan be best observed from thenlarina breakwater on the oppositebank of the river (Fig. 1).

Local residents rememberswallows nesting in former boathouses along the Pembrokewaterfront since the early 1960s.The present roost was occupiedwithin the last 15 years (J.Murphy, pers. comm.) andnumbers of birds have apparentlynever been larger than in 1983.Each year the swallows startarriving in early July and the flock

RK. Ross, Canadian Wildlife Service, 1725 Woodward Drive, Ottawa,Ontario KIA OE7W.R. Clark, 206 Alfred Street, Pembroke, Ontario K8A 3A3J.M. Bouvier, 100 Eddy Street, Pembroke, Ontario K8A 7X3

ONTARIO BIRDS APRIL 1984

is thought to peak in size duringthe second week of August.Numbers decline thereafter,dropping with the passage of eachsuccessive weather front until theroost is abandoned in earlySeptember.

Our survey was carried outduring the evening roosting periodof 10 August 1983. The methodused was 1) to determine thevarious flight corridors followed bythe birds in either approaching ordeparting from the roost; 2) to taketimed counts of the birds crossingvisual transects across thesecorridors; and 3) to extrapolatefrom these samples to get an

SWALLOWROOST .....

OTTAWARIVER

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overall estimate of the number ofbirds using the roost. Viewingtransects were established per­pendicular to the flight directions(Fig. 1). During the observationperiod, counts were made for oneminute every five minutes. Thenumber of birds using each routewas calculated by averaging resultsfrom each successive pair ofcounts, multiplying by the numberof minutes separating the two, andsumming over the entire period ofthe survey. In some cases, the 1­minute counts were made moreoften than every 5 minutes. Theevening flight was counted usingfive teams of two people (transects

Figure 1. Map of Pembroke Showing Location of Swallow Roost andViewing Transects

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.....

Swallows returning to roost, Pembroke, Ontario.

1, 2, 3, 5, 6) and a single observer(transect 4) covering all knownflight routes. The observationswere made from 2000 to 2100 halthough a few birds had arrivedbefore 2000 h. Viewing conditionswere good as weather was partiallyovercast, mild (20°C), and calm.The following morning, a similarsurvey was made of the departingbirds. This count, however, wasnot considered accurate as manybirds were seen leaving the roostand crossing one transect, only toreverse their directions and movedownstream past another transectwhere they were counted again. Itwas also hoped to quantify theproportion of the various swallowspecies using the roost. Thisproved impossible as differentspecies flew to the roost atdifferent altitudes; a representativesample could not be obtainedunder these conditions.

ONTARIO BIRDS APRIL 1984

During the evening flight,swallows approached the roostprimarily along the Ottawa River,with the highest numbers comingfrom the east (69%); few birdscame overland. Approximately115,000 swallows were observedentering the roost. This total wasmore than double our casualestimate of the flock and points tothe difficulties of estimatingnumbers of small birds that passby steadily. There are apparentlyno published records of accuratelycensused swallow/roosts of thissize in Canada, although there areseveral undocumented reports. C.Goodwin (pers. comm.) noted thatan estimated 250,000 TreeSwallows (mostly) were recordedin Matchedash Bay in GeorgianBay and that smaller flocks ofapproximately 20,000 individualswere noted on Lake Chemung nearPeterborough and in Holland

Marsh. G. Bennett (pers. comm.)was also aware of a record of morethan 100,000 swallows in theChignecto National Wildlife areanear Amherst, Nova Scotia.

Behaviour of the birds aroundthe roost differed between theevening and early morning periods.In the evening, the birds arrived atthe roost over a period in excess ofone hour, with the highest rate ofarrival occurring 10 to 20 minutesafter sunset (2019 h). Theincoming birds formed a largedense swarm over the roost andrestricted their movements to thearea around the roost delineatedby the viewing transects. The birdsflew about excitedly, often chasingeach other and constantlyvocalizing; feeding activity wasrelatively rarely observed. Justafter sunset as the rate of incomingbirds reached its peak, the flockgradually started to settle in theroosting trees. For a while, birdswere constantly landing in andtaking off from the roost. Onlywith approaching darkness did theflock descend rapidly into thetrees. In the morning, the birdsinitially left the roost in smallgroups to forage over the water justas the day began to brighten. Asthe sun rose (0557 h) the swallowsmoved off the roost in spectacularlarge waves. The exodus from theroost was mostly completed 'over ahalf-hour period (0545 to 0615 h).

Although the proportion of thevarious species could not bedetermined, it was evident thatTree Swallows vastly predomi­nated; Barn Swallows (Hirundorustica), Purple Martins and theoccasional Bank Swallow werealso noted. This contrasts to a

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previous visit (20 July 1983) whenBarn Swallows and BankSwallows were the most commonspecies. Clearly, usage of the roostshifts with time according to themigrational phenologies of thevarious species. Moreover, allindividuals of a given species ar.;likely not present at one time andthe total number using this siteeach summer is probably muchlarger than the peak numbersrecorded on any given day. Thesize and diversity of species of thePembroke flock make it uniqueamong recorded swallow roostsand provides one of the majorspectacles of swallow migration inCanada.

Weare very grateful for theassistance of the following peoplein the survey: H. Boyd, J. & M.Bryant, C. Clark, S. Gray, J.Kearney, A. Keith, M. Kirk, I.Price and G. Tessier. S. Tinkerprepared the map.

Literature CitedBent, A. C. 1942. Life Histories ofNorth American Flycatchers,Larks, Swallows, and their Allies.U.S. Natl. Mus. Bull. 179.555 pp.

Clark, WR. 1984. Canada'sCapistrano. Nature Canada 13(1): 14-15.

Hackman, A. 1983. Swallowsflock to Pembroke in Earth watch.Seasons 23 (4): 5-9.

Ingram, C. 1974. The Migrationof the Swallow. H.F. & G.Witherby Ltd. London. 86 pp.

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