Habitat use by different-aged duck broods and juvenileducks

Authors: Nummi, Petri, and Pöysä, Hannu

Source: Wildlife Biology, 1(1) : 181-187

Published By: Nordic Board for Wildlife Research

URL: https://doi.org/10.2981/wlb.1995.017

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SHORTCOMMUNICATION

Short communica tio n art icles are short sc ientific enti ties often dealing withmeth odologic al probl em s or with byp roduct s of larger research projects.Th e style is the same as in origi na l articles.

Habitat use by different-aged duck broods and juvenile ducks

Petri Nummi & Hannu Poysa

Nummi, P. & Poysa, H. 1995: Habitat use by different-aged duck broods and juvenileducks. - Wildl. BioI. 1: 181-187.

Habitat use by different-aged broods andjuveniles of teal Anas crecca, mallard A. pla­tyrhynchos and goldeneye Bucephala clangula was investigated in southern Finlandduring 1988-1993. The study focused on within-lake habitat use and the use of flood­ed wetlands. Downy ducklings of all three species showed significant preference forCm'ex-stands. As the dabbling ducks grew older, their habitat use diversified, juvenilesin particular also made considerable use of floating vegetation. Conversely, habitat useby goldeneye became more uniform: goldeneyejuveniles were almost exclusively seenin open-water and floating vegetation habitats. All three species, but especially teal,used flooded areas intensively. Two thirds of teal downy broods were seen along flood­ed shores which comprised only seven percent of all shore habitats. Preliminary datasuggested that the preferred habitat types, Carex and flooded shores, harboured morenektonic invertebrates and emerging insects than did the other shore types.

Petri Nummi , Department ofApplied Zoology, P.O.Box 27 (Viikki C), FlN-OOO14 Uni­versity ofHelsinki, FinlandHannu Piiysd, Finnish Game and Fisheries Research Institute, Evo Game ResearchStation, Kaitalammin tie 75, FlN-16970 Eva, Finland

Received 21 March 1995, accepte d 4 August 1995

Associate Editor: Sigurour Snorrason

Studies of habi tat use by bird s have often identified dif­ferences between the sexes of a give n spec ies (e.g. Se­land er 1966, Co dy 1985a, W inkl er & Le isler 1985). Age­rela ted differences hav e se ldo m been co nside red; In Co ­dy (I 985 b), fo r example, they are only me ntioned once(Morse 1985). Ho wever, due to di fferences in diet or mo­bil ity, a species may dep end on several dist inctly diffe­rent habit ats during its life history (see W ien s 1989).

In stud ies of duck hab itat use during the breeding sea­son three age-cate gories have usually been con sid ered :pai rs, broods, and juveniles (Evan s & Black 195 6, Patter ­son 1976 , Peh rsson 1984 , Nummi & Poysa 1993), and rel ­ative ass oc iatio n wit h vegeta tio n co ver has been con sid ­ered for brood s of di fferent age (Bengtso n 1971 ). It isknown that, along wi th growth and plumage develop­ment, the diet of duckl ings also changes (Chura 1961 ,

© W ILDLIFE BIOLOGY

W ILDLIFE BIO LOGY · 1:3 ( 1995)

Perret 1962, Sugde n 1973). This change partl y reflectsdifferen ces in effec tive ness of subsurface feeding byduc kl ings of various ages (Pe hrsson 1979). Fur thermore,as ducklings grow, the unspeciali sed bill , typical for new­ly-hatched du ckl ings, sta rts to sho w spec ies -spec ific fea ­tures (Goodma n & Fisher 1962, Sugde n 1973 ). Consid­eri ng these age-related changes in diet , fo ragi ng beh av­iou r and bill morph ology, we may also expec t changes inhabitat requirem ent s. Ha bitat requiremen ts of downyducklings are especially interestin g because most duck­ling mortali ty takes place at thi s stage (e .g . Ba ll et al.1975, Ta lent et al. 1983, Orthmeyer & Ba ll 1990), and be­cause thi s mort al ity may be linked wi th food shortage(S tree t 1977 , Hi ll et al. (987).

We studie d hab itat use by ducklings in two age -class­es as we ll as hab itat use by j uve ni les. We also surveyed

181

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the amount of potential invertebrate food organismspresent in different kinds of vegetation as this may playa crucial role in habitat selection by duck young. In someNorth American studies the preference of broods for wet­lands with emergent herbaceous vegetation and floodedbushes has been shown (Ringelman & Longcore 1982,Talent et al. 1982, Monda & Ratti 1988), although in theprairies the importance of different vegetation types is of­ten uncertain due to pronounced water-level fluctuations(see refs. in Kaminski & Weller 1992). In boreal areas,the use of different vegetation types within a lake has pre­viously only been studied for duck pairs (Nummi et al.1994). In addition to considering the role of differentkinds of shore vegetation we also focus on the use offlooded wetlands by ducks .

Study areaThe study area comprised 51 lakes situated in a 39 km'boreal watershed in southern Finland (61°1O'N, 25°05'E). All lakes and small ponds containing waterthroughout the summer were included; one lake close tothe Evo Game Research Station was, however, excludedas it was influenced by human disturbance. In cons ider­ing the use of flooded areas, we included one potholewhich often dries out during late summer. Lakeshoretypes ranged from oligotrophic bog and forest with noemergent plants to more eutrophic shores with lush standsof Carex spp . and Equisetum spp.

Material and methodsHabitat measurements

Habitats were described according to Nummi & Poysa(1993, 1995). Since the water-bodies in the study areagenerally are very stable from year to year , we only useddata from one year (1989). Shoreline vegetation typeswere marked on field maps and the tota l shoreline lengthof each type was later measured from these maps for eachlake. The shoreline lengths of each vegetation type werethen pooled for all lakes , and the percentage of each veg­etation type of the total shoreline in the study area wascalculated from these pooled data . This procedure gavethe expected proportion of different habitat types withwhich the observed use by ducks was compared (see be­low). In final analyses of observed and expected habitatuse, only the three most common shore habitat types wereincluded: I) no emergent vegetation, 2) Phragmites, 3)Carex (mainly C. rostrata) . These comprised 91% of thetotal shoreline and were the only habitat types in whichsufficient duck data were obtained.

The lakes also had »floating-plant« and »open-water­habitats. These were only included in comparisons ofhabitat use by birds of different age. Data from these hab-

182

itats were not included in the examination ofobserved andexpected use of habitat types because of the difficulty oftransforming them to shoreline units comparable to thelength -based shore habitat data.

For the study of brood use of flooded shores (mainlybeaver ponds), the percentage of flooded shores of the to­tal shoreline was calculated for the years 1988- 1993. Theaverage of these yearly values was then used in analysesof observed and expected use of flooded shores .

Duck dataThe duck data were obtained during the summers 1988­93 for the three most commonly occurring species in thestudy area: mallard, teal and goldeneye. Average breed­ing densities ranged from 0.53 (goldeneye) to 0.66 (mal­lard) pairs per kilometre of shore line (Nummi & Poysa1995). Ducks were censused in the period June -August,and 35 censuses were conducted at each lake, approxi­mately one every second week. In each census all visiblebirds were counted from a fixed point using binocu lars ora telescope, whereafter the lake was circled in a boat oron foot (Koskimies & Vaisanen 1991). All observationsof broods which had not moved because of the disturb­ance caused by the observer were marked on a field mapand the vegetation type in which they were seen was not­ed . It is likely that some broods remained undetected inthe more complex habitats such as flowages (bodies ofwater formed by overflowing or damming) and Carex­stands. However, because these habitats were used bybroods more than expected (see below) the possible de­tection bias should not affect our conclusions. The age ofthe broods was determined following the classificationused by Pirko la & Hogmander (1974). As the tota l num­ber of censuses was the same for all lakes, the total num­ber of broods and juveniles was pooled over time andlakes when different habitat types were considered in theanalyses.

Throughout this study, broods were divided into twogroups: age-class I and age-class II-III. The actual agesof ducklings in these groups according to Pirkola &Hogmander (1974) are: mallard age-class I (1-18 days),age-class II-III (19-55 days) ; tea l age-class I (1-12 days) ,age-class II-III (13-35 days); goldeneye age-class I (1-22days), age-class II-III (23-63 days). The reasons for thedivision are both practical and eco logical: downy duck­lings of age-class I differ clearly in their foraging behav­iour from older ducklings (Chura 1961, Pehrsson 1979).Age-classes II and III were pooled in order to obtainenough data for the analyses.

Statistical analyses of habitat useThe G-test was used to compare observed and expecteduse of different shore habitats by broods and juveniles.

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Tab le I . Obser ved versus expec ted use of the three mo st common shore habit ats by tea l, mal­lard and golde neye young of differen t age. Th e percentage of ob served number of broods (age­classes I and II-III) or indi vid uals (juven iles) are shown; N is the number of observations. Ex­pect ed use is the percen t ava ilabi lity of the shore hab itats. The three common habitat typescomprise 91% of the total shoreline . Goldeneye ju veniles are omitted beca use of their negli­gible use of shore hab itat s, df = 2 in all cases.

floated at fixed sites on two styrofoam panels that wereattached to the bucket with metal rods.

Two activity traps and two emergence traps were usedat each trapp ing site and there were two fixed trappin gsites per lake. Animals from activity traps were identifiedand their size was assigned followi ng the taxon list andsix length categories given by Nudds & Bowlby (1984) .The most commonly occurring invertebrates in our activ­ity trap catches were Cladocera, Hydracarina, Dytiscidae ,Corixidae, Oligochaeta, Chironomidae and Ephemerop­tera (for a complete taxon list, see Poysa et al. 1994). Forca lculation of the food abundance index, the number ofindividuals within each taxon was multipl ied by the meansize of its length category. In addition to Chironomidae,only a few Ephemeroptera, Odonata and Trichopterawere present in the traps for emerging insects; thereforeno size classification was used . Food abundances are giv­en as indexes of free-swimming invertebrates per 100 trapdays , and as total numbers of emerging insects per 100trap days. The abundances are averages of the four yearsfrom all sites of each vegetation type.

The expected use assumes random habitat use, as calcu ­lated on the basis of the proportion of each shoreline typeofthe tota l shoreline in the study area . A similar observed­expected comparison was made for flooded versus non­flooded shores.

Differences in habita t use between age-classes of eachspecies were also compared with the G-tes t. In this caseboth floating -plant and open-water habitats were also in­cluded. Becau se goldeneyes in older age-classes make solittle use of shoreline habitats, the ana lyses differed forgoldeneye and dabb ling ducks. For dabbling duck s com ­pariso ns were made between habitat types: I) no vegeta­tionlPhragmites (poo led due to scarcity of data) , 2) Ca­rex, and 3) floating plants. For goldeneye the comparisonwas made between habitat types: I) Carex, 2) floatingplant s, and 3) open water.

44

29

27

4429

27

4429

27

Expected useOb served use

II- III j uv.

N = 32 N = 84

19 24

12 19

69 57

G = 24. 33 G = 33.98

P < 0.001 P < 0.00 1

II- III ju v.

N = 2 1 N = 7 1

5 10

6 48

67 72

G = 20.52 G = 66 .75

P < 0.00 1 P < 0.001

II-III

N= 19

32

16

53

G = 5.79

P > 0.05

[

N =26

I

N = 15

I

N = 35

48

88

G =44.37

P < 0.001

13

13

73

G = 13.98

P< 0.00 1

17

I I

7 1

G = 29 .78

P < 0.001

Teal

No vegetation

Phra gmi tes

Carex

Ma llard

No vege tation

Phra gmit es

Carex

Goldeneye

No vegetation

Phra gmites

Carex

Food resource samp lingNektonic invertebrates and emerging insects were contin ­uously trapped from the end of May to the end of July in1989-/992 (as in Nummi &Poysa 1993 with one addition alyear) . Traps were checked once aweek on average and trappingprocedures were identical at alllakes. However, only data fromthe three most common habitatsand from flooded shores havebeen included in this study, i.e. 16sites on nine lakes.

Free-swimming invertebrateswere caught with activity traps asdescr ibed in Murkin et al. (1983).We used glass jars equipped withwhite plastic funne ls with open­ings of 140 mm at the wide endand 20 mm at the narrow end.Trap s were suspended 20-40 cmbelow the surface of the waterwhich was 50-100 ern deep. Onnon-vegetated shores the trapswere set close to the shore where­as in vegetation stands they wereset beyond the outer edge ofemergent plants .

Emerging insects were cap­tured from the same sites as nek-ton in traps consisting of a 5-litreplastic bucket and an orange plas­tic funnel with openings of 200mm at the wide end and 40 mm atthe narrow end (Danell &Sjoberg 1977). Emergence traps

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ResultsUse of different habitattypes

Table 2. Percentage use of different types of vege tation by teals. mallards and goldeneyes ofdifferent age. N is the numb er of obser vations. The number of mallard observa tions was toolow to permit testing; the test for teal could only be done between age-class II-III and ju ve­niles.

Nummi & Poysa 1993). It is now evident that there arealso differen ces in habitat use by duck broods of differentage, and that habitat use may differ more between youngofdifferent age in one species than betwe en young of cor­responding ages in different species.

At age-class I, when the ducklings were less than threeweek s old , broods of all species preferred CaJ·ex-shores.Even goldeneye, a diving duck, made con siderable use of

Teal

Mallard

27

37

36

juv .

N = 126

GOLDENEYETEAL

45 16242

II-III

N = 37

40

27

59

14

G = 8.55. df = 2, P < 0.05

MALLARD

12

88

o

%

I

N =26

I II-III ju v.

N = 17 N = 23 N = 122

24 30 16

65 6 1 42

12 9 42

I II-III juv .

N =44 N = 84 N = 122

57 12 3

18 32 38

25 56 59

G = 60.02 , df = 4, P < 0.00 I

a

60 _

w~ 80­ouiI-oW0..XWoz«oui>a:uiCfJcoo

No vege tationlPhragm ites

Carex

Floating plants

Floating plants

No vegetation/Phraglllires

Carex

Goldeneye

Carex

Floating plants

Open water

Dabbling duck s of all ages usedCarex shores more and non-veg-etated shores less than expected,as did age-class I go ldene yes (Ta­ble I). About 70-90% of dabblingduck brood observation s weremade in Carex shores althoughthese comprised only about onequarter of all shore habitats.

Within-lake habitat use by tea ldid not seem to change much dur­ing the brood period (Table 2),but j uveni le tea l associatedstro ngly with floating vege tationas well as sedges . Sedge-sta ndswere also much used by mallardbrood s, and juveniles were ofte nfound among Carex and floatingvegetation (Table 2).

There were clear differences inhabitat use between differentage-classes of goldeneye; theCarex habitats were much preferred, but only by age­class I (Table 2). Forage-class II-III, more than two-thirdsof the observations were made in open water and amongfloating plant s. Juvenile gold eneyes made almo st no useof habitat types associated with the shore line.

Use of flooded areasThe broods of all species (age-cla sses pooled) used flood­ed areas more than expected (G-test, P < 0.00 I in allcases) (Fig . I). Teal and go ldeneye j uveni les also mademore use of the flowages than expected (G-test, P < 0.00 Iin both cases) whereas mallard juveni les did not (G-test,P > 0.05) .

Food resources on different kinds of shoresThe amount of both nektoni c invertebrates and emerginginsect s was low on the non-vegetated and Phragmitesshores surveye d, clearly higher on Carex shores, andhigher still on flooded shores (Tabl e 3).

Figure I. Observed and expected use of flooded areas (in %) in re­lation to use of other shore habitat s by duck broods in age-classes I,II and III. and by ju veniles. The hori zontal line marks the expecteduse (7%) of flowages and the numb ers of observa tions are notedabove the bars.

Discussion

Similarities and differences in habitat useof dabbling ducks and the goldeneye

It is well documented that duck brood s use well -vegetat­ed wetl ands more than do adult pairs (Patterson 1976,

Age classes : III O Juv

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Table 3. The mean and SE (based on averages of four years, N =4) of nektonic invertebratesas modified index (see melhods) of abundance and number of emerg ing insects on differentkinds of shore lines in 1989-I992. Nekto nic invertebrates as an index of anima l abundance per100 trap days and emerging insects as num bers of anima ls per 100 trap days . Number of trap­ping sites used in eac h of the four years is given in parentheses.

Carex

Phragmites

Flooded shores

nektonic invertebrates vulnerable to fish predati onabound; these include the large cladoceran s, co rixids,noton ectids and dyti scids (Erik sson et al. 1980, Bende ll& McNicol 1987 , Nummi 1989, Poysa et al. 1994) . Cla­docerans are especially exploited by teal s (Nummi 1993)although the downy young of man y other duck spec ies al­so exploit them (Co llias & Co llias 1963). The density ofthe benth os is high in older beaver flowages (McDowell& Naiman 1986, Nummi 1989) and in fish-less ponds(Mi ttelbac h 1988, McN ico l & Way land 1992).

Teals st ill used flowages a great dea l at the j uve nilestage , durin g which their diet has been found to hold ahigh percen tage of invertebrates (Nummi 1993), Howev­er, for mallard j uve niles, which consume cons iderableamo unts of seeds, flooded areas see med not to be impor­tant.

Management implicationsTh is study showed that duck broods of different age mayuse different kinds of habitats (see also Poysa & Virtanen1994). One very important habitat type to all young duck­lings are flood ed area s. Although these flowa ges maycomprise only a limited fraction of wetlands in a land­scape, their value for ducks and other wetland birds at cer­tain life stages can be great - at lea st in boreal wetlands.It is very likely that in man y areas the numb er of wet de­pression s has diminished greatly durin g the last 50 yearsas a result of inten sive drainin g (Jarvinen et al. 1977 ). Inwildlife managem ent as well as in nature co nservationmore emphas is should be placed on prese rvi ng sma ll, oc­cas ionally flooded wetlands such as the ones descri bed inthis study (for the use of sma ll ponds, see Nummi & Poysa1995). Likewise, because of the role of beaver in creat­ing suitab le habit ats fo r waterfowl (Nevers 1972, Num­mi 1992), the poss ibi lity of managing beaver popu latio nsin connectio n with ge neral wet land conservation (Ermer1984, Naiman et al. 1988) should be taken into acco untin Europe .

Nektonic invertebrates Emerging insect s

x SE x SE

330 33 21 3 (1)

446 80 26 3 (7)

1008 11 0 53 7 (6)

2127 290 172 30 (2)

No vege tation

we ll-vegetated shores at thisstage. Very young goldeneyeducklings often pick inverte­brates fro m plants and the watersurface as do dabbl ing ducks.Bengtso n ( 197 1) also found thatvery young diving duck broodsspe nt much time in habit ats ofemergent vegetation.

As dabbling duck s and gold­eneyes grew older their use ofhabit at shifted in different direc­tions. The pattern of habitat useby go ldeneyes changed clearly atage-class II-III. At this stage theyoften used open habitats which apparently refl ects the factthat goldeneyes obtain most of their food by diving at age­cla ss II-III. In dabbling ducks the habitat use pattern didnot differ much between age-cl asses I and II-III. Golden­eye juvenil es mostly used the open-water area where dab­blin g duc ks were rarel y seen. Instead, teal and especiallymallard ju venil es made more use of the floating vegeta­tion zone than the younger age-c lasses.

Importance of food and floodedareas to young ducks

Our prel imin ary data ind icated that the preferred habit attypes, COI'ex-stands and flooded shores, also harboured ahigh abunda nce of invertebrates that co nstitute poten tialfood for the yo ung ducks. The high food abundance mayexp lain why ducks prefer certain shore habit ats. It mayalso exp lain why breeding mallards are also often foundin COI'ex-vegetation (Kaminsk i & Princ e 1984, Kirby &Riechmann 1985, Nummi et al. 1994). Lowered preda­tion risk in well-vegetated shore habit ats may also play arole in explaining the habitat preferenc es observed here.But as sugges ted by Pehrsson ( 1979), a well- vegetatedshoreline may in fact be a rather dan gerous habitat be­cause of mammalian predators.

Flooded areas , mostly dammed by beaver , were espe­cially favoured by teal although young ducklings of theother spec ies also used them . Beaver impoundment s areknow n for their high inverteb rate produ ction (Mc Dowell& Nai man 1986, Nummi 1989). Downy ducklings fora geby picking invertebrates from emergent plants or floatingdebris (Beard 1953, Chura 1961, Johnston & Naiman1987, Numm i 1992). As flooded areas usually have sin­uous shore lines that tend to acc umulate decom posingvegetation, they are struct ura lly well-suited for forag ingdowny duck lings .

In new flowages or in iso lated ponds which freeze tothe bottom fish populations are low and ducks face littleco mpe tition for food from fish. In this situa tion many

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Acknowledgements - we thank the Evo Game Research Station teamfor endless walking and row ing along the lake-shores and the threeanonymous refe rees for their valuable crit icism of the manuscript.Th e grant from the Foun dation for Researc h of Natural Resourcesin Fin land to PN is much appreciated.

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