Mist Netting Success in Relation to Weather by Susane

Mist netting success in relation to weather By SusanE. Quinlan and RogerL. Boyd

Introduction located 9.6 km northeast of the research site. Positionsof pressuresystems and weatherfronts Mist nets have often been used to quantify were obtained from the Daily Weather Maps avian populations;however, as Ralph (1976)has published by the National Oceanic and At- indicated,there are possiblevariables that need to mosphericAdministration. be correctedfor. Ralph (1976)provided a method to adjust for unequal net-hoursof operation to Number of birds captured during the morning equalizecomparisons between banding operations hours were comparedto the weather variables at andproposed the useof correctionvalues for vary- 0700 hours using a simple Pearson correlation ing lengthsof time for which the nets were run. statisticaltest. Weather variables include temper- These correctionvalues are basedon the assump- ature,barometric pressure, cloud cover, humidity tion that capture rates do not vary significantly and wind speed. over time.The numberof birds actuallycaptured, Since mostpasserines migrate at night (Lack1960, however, depends at least partially on two Kendeighet al. 1960,Nisbet and Drury 1968,and variables: the amount of bird activity and the Able 1973),the numbersof birdsmigrating through visibility of the nets.If birds are extremelyactive an area would be affectedby the weather during more birds will be captured.Or, if high humidity the night. To determine if capture successwas makes nets more visible, as suggestedby Stewart related to migrationmovements, the numbers of (1971),fewer birds will be caught.Stewart found birds mist netted on a given day was comparedto that peak mist net captureson cloudy days are pressuresystem and weather front locations and to significantlylater than on sunnydays. This implies individual weather variables at 0100 hours on the that weather affects bird activity or mist net previousnight. visibility.The purposeof thisstudy is to determine if mist nettingsuccess is affectedby the weather. Results and discussion Temperature, barometric pressure, and cloud Method coverwere associatedsignificantly (AOV, P•1.05) Researchwas conductedin SpringCreek Canyon, with the number of birds captured.Wind speed a small area of cottonwood habitat, 9.6 km and humidity did not show any correlationabove southwest of Fort Collins, Colorado. Black, 12 m- the .2 confidencelevel, again using analysisof long mist nets with a mesh size of 3.75 cm were variance. operatedover two periodsof ten consecutivedays Temperatureindicated a strongnegative correla- in the springsof 1973and 1975. In all, over450 birds tion with netting successat the 0.044 confidence of 42species were banded.Nets were operatedun- level. Thus, high temperatureswere associated til 0930hours each morning,and again from 1800 with low captures.Since the visibility of nets hoursuntil after dark each evening.Time of cap- would probably be unaffectedby temperature ture, species, age and sex classification,when alone, we assumed that temperature might be possible,were recordedbefore the bird was band- affectingthe activityperiod of birds.Eyster (1954) ed andreleased. Eleven nets were operatedin 1973 studiedthe activityperiod of captivesongbirds and and six in 1975. The six nets used in 1975 were in found that temperatureplays a significantrole in the same locations as six of the nets in 1973. The regulatingthe beginningand end of a bird's daily largernumber of netsused in 1973reflected utiliza- activityperiod. Althoughhe observedsome variation of a largerportion of the SpringCreek Canyon tion amongspecies, Eyster found that an increase and did not indicate a greater number of nets in in temperaturebeyond a given point (normally the same area. 25øC)was generally associated with the end of the Most weather data were obtained from the morningactivity period. If thesesame associations Colorauo State University Experiment Station occurin wild birds, the end of the morningactivity

Page 168 North American Bird Bander Vol. 1, No. 4 periodwould occurat an earlier time on mornings Accordingto many researchers(Kendeigh et al. with temperaturesabove 25øC than on cooler 1960,Lack 1960,Bagg 1965, Richardson 1971, Able mornings.Since the netswere operateduntil the 1973, and Emlen 1975) migration movementsof samelocal standardtime eachmorning, a variation birds are affected by weather variables. The in length of bird activity periods would be ex- possibilitywas consideredthat the resultsin this pectedto causea changein numberof birds cap- studymight only reflect the number of migrantsin tured. As might be expectedbased on Eyster's the studyarea. Thus, the numberof birdscaptured observations,low temperaturescorrelated with was comparedwith weather variables on the high capture success,and high temperaturescor- previousnight when passerinemigration would related with low capture success.This appearsto have occurred.As shownin Table I, no significant be a plausible explanation for the correlation correlations were observed at the 0.10 confidence observedbetween temperatureand captures. level. Barometric pressure was positively correlated with the numbers of birds captured at a TableI. Correlationstatistics resulting from comparison between the significancelevel of 0.037.The reasonfor this cor- numbers of mist netted birds and the weather variables at 0100 hours. relationis not apparent.Barometric pressure does not affect the visibility of the nets, so again, the Statistics Weathervariables possibilityof correlationswith the activity period TemperatureWind BarometricCloud Relative speed pressure cover humidity was investigated.There was no reference found in the literaturedirectly relating barometric pressure Correlation -I-.0570 -I-.0350 -+-.0980 -I-.1980 -I-.0234 to activityperiods of birds;however, one possibili- Coefficient ty is that periodsof high barometricpressure are Significance .717 .850 .560 .219 .890 alsoperiods of unfavorableweather conditionsfor Level migration (e.g., winds from the wrong direction [Richardson1971]). An increasednumber of captures during these periods of unfavorable migrating conditions could be due to the birds Conclusions "holding over" in the study area until more favorable conditionsfor migration occur. More The results of this study show that mist netting researchis needed to verify or refute the existence successis affectedby weatherconditions along the of suchphenomena along the easternslope of the easternslope Colorado foothills. Thus, mist nets do ColoradoRocky Mountains. not givean accurateestimate of the relativedaily abundance of small land birds, as assumed by Cloud cover was also positivelycorrelated with Ralph1976, Baird et al. 1958,and Baird et al. 1959. mist nettingssuccess at a 0.065significance level. However, if used over a longerperiod of time, as Stewart (1971)found that cloud cover causedan Weise (1971)did, the variationsin nettingsuccess hour delay in the peak of mist net captures. due to weathermight be overcome.Also, it should However, since the nets were operated past the be pointedout that the useof mistnets in calcula- times of peak capture reported by Stewart, this tion of a Lincoln-Petersonindex to populationsize, relationshipshould not affect the results. Most as suggestedby MacArthur and MacArthur (1974), probablyvisibility of netsis affectedby the amount wouldbe severelylimited by the effectsof weather of sunshine. Reflection of light, and thus net on mist netting success. visibility, will increase with the amount of sunlight.Thus, nets would be lessvisible on cloudy Summary days, and capture successwould be greater than on sunnydays. This would explain the positivecor- This studycompares number of mist nettedbirds relation observedbetween nettingsuccess and the with weather conditions. Simple correlation amount of cloud cover. statistics show associations between capture successand temperature, barometric pressure, No significantcorrelations between wind speedor and cloudcover. These correlations are explained humidity,0.314 and 0.358respectively, and capture success were observed. Both of these variables by the effectsof theseweather variables on net visibility and bird activity. No significantcor- would be expectedto increasethe visibility of nets relationswere foundbetween capture success and at somehigh level (i.e. 32 km per hour winds, or humidity and wind speed. relative humidity approaching 100 percent). However, suchextremes were not observedduring The possibilitythat thesecorrelations might result this study. from variations in numbers of migrants arriving

Oct.-Dec. 1976 North American Bird Bander Page 169 with certain weather conditions is explored. Ralph, C.J. 1976.Standardization of mist net cap- However, no significantcorrelations were found tures for quantification of avian migration. between the number of mist netted birds and Bird-Banding47:44-47. weather variables during the night, the time at Richardson, W.J. 1971. Spring migration and which mostpasserine migration occurs. weather: a radar studyin easternCanada. Am. Birds 25:684-690. Stewart, B. 1971.Netting successin relation to time Acknowledgements of day and sunny versus overcast skies. W. Bird Bander 46:16-17. The authorsare gratefulto Jenifer S. Slaterfor her Weise, C.M. 1971. Relative abundance of small assistancein operating the mist nets in 1975. land birds in southeastWisconsin. Pass. Pigeon Further,we gratefullyacknowledge the assistance 33:173-188. of the ColoradoState University ExperimentSta- tion personneland Dr. John F. Benci for their Departmentof Fishery and Wildlife Biologyand assistancein collectingand interpreting weather Department of Zoologyand Entomology,Golorado data. Personnelof the StatisticsLibrary were also State University, Fort Gollins, Golorado 80523. helpful in retrievingcomputer subroutines for use (Present address of Quinlan: Department of in data analysis.Dr. Ronald A. Ryder provided Wildlife Biology,University of Alaska,Fairbanks, valuableassistance in reviewingthe manuscript. AK 99701•. (Presentaddress of Boyd:Department of Biology, Baker University,Baldwin Gity, KS 66006.) Literature cited

Able, K.P. 1973. The role of weather variables and flight directionin determiningthe magnitude of nocturnalbird migration.Ecology 54:1031- 1041. Bagg,A.M. 1965. The changingseasons -- spring migration:the few and the many. Aud. Field Notes 19:438--446. Baird,J., C.S. Robbins, A.M. Bagg,and J.V.Dennis. 1958.Operation recovery:the Atlantic coastal nettingproject. Bird-Bonding 29:137-168. __, A.M. Bagg, I.C.T. Nisbet and C.S. Robbins. ß Phlto.n 1959. Operation recovery: report on mist- Available m netting along the Atlantic coastin 1958.Bird- Banding30:143-171. Emlen, S.T. 1975. Migration: orientation and MICROFORM navigation. in "Avian Biology" (D.S. Farner and J.A. King, eds.), Vol. 5, pp. 129-219. Academic Press, New York. Eyster, M.B. 1954. Quantitative measurementof the influenceof photoperiod,temperature, and season on the activity of captive songbirds. Eco/. Monog.24:1-28. Kendeigh,S.C., G.C. West and G.W. Cox. 1960.An- nual stimulus for spring migration in birds. Anita. Behav. 8:180-183. Xerox University Microfilms Lack, D. 1960. The influence of weather on 300 North Zeeb Road, passerinemigration; a review. Auk 77:171-209. Ann Arbor, Michigan 48106 MacArthur, R. and A.J. MacArthur. 1974.On the (313) 761-4700 use of mistnetsfor populationstudies of birds. Proc. Nat. Acad. Sci. 71:3230-3233. PLEASE WRITE FOR Nisbet, I.C.T. and W.H. Drury. 1968. Short-term COMPLETE INFORMATION effectsof weather on bird migration:a field study using multi-variate statistics.Anita. Behav. 16:496-530.

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