The relationship between nesting chronology and vulnerability to hunting of dabbling ducks

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1 The relationship between nesting chronology and vulnerability to hunting of dabbling ducks RBERT G. CLARK, LAWSN G. SUGDEN, R. KENT BRACE and D A N IEL J. NIEM AN Introduction T here is w idespread concern for the w ellbeing of N orth A m erican duck populations, catalysed by drought and changing land-use practices (e.g. Boyd 1985; B artonek et al. 1984). In addition there is m uch interest and speculation regarding the im pact of sport hunting on duck populations (e.g. B urnham et al. 1984; Caswell et al. 1985; T am isier 1985). T he effect o f hunting on population levels rem ains controversial, especially for fem ale ducks, because few data exist to enable the issue to be exam ined adequately. A nalyses of m ortality of fem ale M allard A nas platyrhynchos suggest th at, in som e years and in som e areas of N orth A m erica, hunting m ortality m ay be additive to natural sources o f m ortality (B urnham et al. 1984). R ecent w ork by Casw ell et al. (1985) suggests th at the size of the adult M allard cohort may be affected by hunting pressure, particularly by hunting on the breeding areas of prairie C anada. They found th at restrictive hunting regulations had a positive influence on survival of M allard, enabling the M anitoba population to grow. G iven the degraded condition o f natural habitats in the C anadian prairies, m anagem ent actions are req u ired to im prove recruitm ent and to m aintain, o r increase, d u ck p o p u la tio n s. L ittle is k n o w n, how ever, about the relationship betw een nesting chronology and susceptibility to hunting. B ecause hunting season opening dates can be adjusted to reflect breeding chronology, such changes have som e p rom ise for the m anagem ent o f duck harvests and for sustaining and possibly increasing populations. In a tw o-year study of G adw all A nas streperà, H ines and M itchell (1983) estim ated th at the percentage o f young that w ere unable to fly on the opening day of the hunting season ranged from 5 to 20%. G ilm er et al. (1977) presented evidence suggesting th at delayed breeding and m oult in fem ale M allard m ade them m ore susceptible to hunters. A necdotal accounts in the case o f M allard p ro m p te d H ochbaum (1944:138) to infer that late nesting fem ales and late hatched young w ere vulnerable to hunting in good w eather w hen hunters shot ducks along m arsh edges. G ollop (1965:35) found th at m ore late hatched M allard w ere shot within 160 km of th eir natal area and early in the hunting season (in S eptem ber and ctober) than w ere early hatched M allard. H ow ever, he found no significant difference in the proportions o f early and late hatched M allard killed by hunters up to early in the follow ing w inter. This p ap er has tw o m ain objectives. First, in o rd er to learn m ore ab o u t the potential vulnerability of young ducks to hunting, the relationship betw een the fledging dates of hatching-year ducks and opening dates of the hunting season was exam ined. Second, in o rder to learn how production and renesting effort m ight influence survival, the inter-relationships of indices of brood num bers, late nesting effort and vulnerability to hunting o f fem ale, and im m ature, ducks w ere exam ined. Methods N est initiation and clutch size data w ere collected in 1980 and 1981 and in 1983 to 1986 at the St-D enis N ational W ildlife A rea (N W A ), located approxim ately 32 km east of Saskatoon, Saskatchew an. T he N W A encom passes 385 ha, is situated n ear the southern boundary of the Prairie P arkland, has m oderately rolling terrain, and contains over 100 w etlands w hich vary in perm a nency (see S ugden and B eyersb erg en 1985). B ecause the N W A is located at the ecotone of the parkland and grassland biom es, the data should be reasonably re p re se n ta tiv e o f conditio n s in southcentral Saskatchew an. N est searches w ere conducted three or four tim es annually betw een early M ay and early July. W eather and breeding population levels dictated w hen searches began and ended. N ests w ere located by w alking through nesting habitat and "beating the cover, or by using the "cable-drag m ethod (H iggins et al. 1969). Initiation date was calculated for all nests 137 W ild fowl» (I SS):

2 138 R obert G. C lark, Law son G. Sugden, R. Kent Brace and D aniel J. Nieman found o f M allard, G adw all, N orthern Pintail A. acuta, B lue-w inged T eal A. discors, N orthern Shoveler A. clypeata and A m erican W igeon A. am ericana. To derive a date of fledging for ducklings from each nest, we sum m ed the num ber of days to lay a clutch using the data in T able 1, i.e. m odal clutch sizes, lengths o f incubation after B ellrose (1976) and m edian age at fledging derived from G ollop and M arshall (1954). T hough arbitrary adjustm ents to reflect seasonal changes in clutch size o r differential grow th rates of early and late hatched ducklings w ere not m ade, seven days from fledging dates was considered to represent the period during w hich new ly-fledged ducks becom e m ore com petent flyers. N orthern Pintail and G adw all nests w ere not found in 1984 and N o adjustm ents to the d ata for o th er years w ere m ade because the values derived from the nests o f these two species are to som e extent com pensatory. Pintail being early and G adw all late nesters (see D u eb b ert and Frank 1984). E ach year, th e percentage o f broods younger than one w eek post-fledging was determ ined during the follow ing periods: before 1 S eptem ber; 1-6 Septem ber; 7-14 S ep tem b er (w hen h u n tin g has usually o p ened); S eptem ber and after 30 Septem ber. T he six species considered here (T able 1) accounted for m ost o f th e total estim ated retrieved kill of ducks in southern Saskatchew an. B ecause M allard typically com prise m ore than th ree-q u arters of the ducks harvested, we also exam ined their data separately. All w etlands on th e N W A w ere censused w eekly betw een spring m elt and freeze-up each year, Visits w ere m ade to betw een 111 and 114 w etlands each year and the percentage containing w ater on 1 M ay and 1 July was calculated. B ased on spring and sum m er w ater conditions subjective assessm ents w ere m ade of the potential for renesting each year. Y ears w ith good to excellent spring and sum m er pond conditions w ere considered to be good years for renesting. In the prairies, the brood and late nesting indices are derived from aerial census data. Flights are m ade in the m orning in good w eather from 1 July to 21 July. T he num bers of duck broods (by age class), single lone drakes, and pairs are counted, species being determ ined w here possible. Large counts of lone drakes and pairs indicate m uch late nesting, while large brood counts indicate good fledging success potential. T hese duck surveys are m ade annually (e.g. B enning 1986). L ate nesting (renesting) is caused by initial nest failures rath er than by hens attem pting to raise a second brood. Incidence o f late nesting largely reflects w etland abundance in late June and early July w hich, in tu rn, is d ep endent upon the num ber of ponds in early spring and the rate o f drying betw een early spring and early sum m er. N est predation is usually severe, so th at w hen w etlands are abundant in July, and drying is m oderate, renesting effort is w idespread (e.g. C ow ardin et al. 1985). R esults o f the N ational H arvest Survey (M etras and W endt 1986) w ere used to calculate th e ratio of adult fem ales:adult m ales, and o f im m atures:adults, killed by hunters. W ith the exception o f 1978 and 1984 w hen th ere w ere sam pling problem s, reliable data w ere available from 1972 to 1985 (L. M etras, CW S, pers. com m.). The ratios w ere calculated for all dabbling ducks, and also separately for M allard. B ecause the brood index was based on all species p ooled, a separate analysis of M allard could n o t be perform ed. Table 1. Clutch size, length of incubation, and age at fledging of the most common dabbling ducks nesting at St-Denis NWA, Saskatchewan. Also shown for each species is its percentage representation in the duck harvest in southern Saskatchewan and Clutch size (mode) Length of incubation (days) Age at fledging (days) Representation in harvest (% ) Mean Range Mallard (73-86) N orthern Pintail ( ) Blue-winged Teal ( ) N orthern Shoveler ( ) Gadwall ( ) American Wigeon ( )

3 Nesting chronology and hunting vulnerability 139 R esults T he chronology of nest initiation varied from year to year at the St-D enis N W A (Figure 1 ). This produced substantial variation in the num ber of fledged broods across the four tim e periods (last tw o periods pooled), for all species (Figure 2) and for M allard alone (Figure 3). T he G -test of Z ar (1984), qualitatively sim ilar to the Chisquared test, was used to see if the pro p o r tions of broods fledged in each tim e period differed across the years. It gave, respectively, G = 6 8.5, P c. l, d f= 15; and G = 39.3, P c. l, df= 15. ver the six years, 15% of dabbling duck (22% of M allard) broods w ere less than one w eek postfledging w hen the hunting season opened. In 1981 and 1983, substantial proportions of young ducks m ust still have been inexperienced flyers w hen the hunting season opened (T able 2). In all but one year M allard w ere m ore vulnerable than o th er species. T he trend was m ost pronounced in 1 MAY MAY MAY MAY MAY - 4 JUNE JUNE JUNE 18 JUNE DATE Figure I. Nest initiation chronology of dabbling ducks at the St-Denis NWA, Saskatchewan in 1980 ( ), 1981 ( ), 1983 ( ), 1984 ( ), 1985 () and 1986 (A ). Shown is the cumulative percent of nests initiated during the spring. The dotted lines join the minimum and maximum percentage of nests initiated within each time period.

4 140 Robert G. Clark, Lawson G. Sugden, R. Kent Brace and Daniel J. Nieman Dabbling duck broods li z 0 Q 80- LLJ C CL 60- * UJ UJ 40 C 20' C m YEAR Figure 2. Percentages of dabbling duck broods younger than one week post-fledging during the late summer and fall in south-central Saskatchewan. Numbers of nests found each year are indicated within the histograms. The percentage of broods is shown for the following periods: before 1 Sept ( ); 1-6 Sept ( ); 7-14 Sept (Sä); Sept (H ); after 30 Sept (K ). Hunting season usually opened during the period 7-14 Sept. (Table 2). Table 2. Pond conditions at St-Denis NWA and in Survey strata 30 and 32, southern Saskatchewan, subjective assessment of renesting potential, hunting season opening dates, and percentages of broods less than one week post-fledging when hunting season opened. St-Denis NWA % wetlands with water 1 May 1 July Strata ponds x 103 Renesting May potential Season opening % of broods <1 week post fledging m oderate 8 Sept 9.4 (19.2)* poor 7 Sept 30.0 (32.0) good 8 Sept 41.8 (55.6) poor 5 Sept 14.6 (13.8) good 11 Sept 9.7 (21.9) moderate 15 Sept 4.3 (9.8) Percentage of Mallard broods shown in parentheses. Derived from Figures 2 and 3. In 1980 and 1983, the percentage of broods in the 7-14 Septem ber range was multiplied by a factor of 0.8 before adding to the percentages in later groups; the factor in 1985 was 0.4. These factors were used to compensate in part for differences among years in the overlap of opening dates and the selected time period.

5 CD z Ü Q UJ 100' 80 Nesting chronology and hunting vulnerability 141 M allard broods I C 6 0 o * UJ LU 5 40 C o DC CÛ YEAR Figure 3. Percentages of Mallard broods younger than one week post-fledging. Details follow Figure , 1983 and 1985 w hen renesting p o ten tial at the N W A was either m oderate or good (T able 2). T he data on spring pond conditions obtained at the N W A w ere com parable w ith those in adjacent survey strata (T able 2) derived from B enning ( 1986). The years 1981 and 1984 w ere dry, w hereas 1983,1985 and, to a lesser ex ten t, 1986 were relatively w et. B ased on th e N ational Flarvest Survey statistics and indices derived from annual production surveys, som e potentially im p o rtan t associations w ere found w ith the ratios o f ducks killed by hunters. T he ratio of adult fem ale:adult m ale dabbling ducks was positively and significantly associated with the brood index (T able 3). T hus, w hen th e breeding effort was strong (and hens Table 3. The association between brood and late nesting indices, and the ratio of adult females:adult males, and immatures:adults, in hunter survey samples. Shown are Spearman rank correlation coefficients. All species pooled Mallard alone Brood index correlated with: Adult females:adult males 0.85** t Im matures:adults 0.38 Late nesting index correlated with: Adult females:adult males 0.56* 0.13 Im matures:adults 0.55* 0.73** *P<0.05; "P c. l; N =12 years (Z ar 1984) tn ot available because Mallard broods were not recorded separately.

6 142 R obert G. Clark, Law son G. Sugden, R. Kent Brace and D aniel J. Nieman w ere generally successful), a higher p ro p o r tion o f adult fem ales was included in the harvest. T he association w ith the late nesting index also was positive. T he ratio of im m aturesradults in the harvest was associated positively, b u t not significantly with the brood index; how ever with the late nesting index th e association was significant. T hus, in years w hen breeding was prolonged, the num ber of im m atures killed relative to adults increased. B ecause the kill of im m ature ducks may have been associated both w ith late nest frequency and w ith overall production (brood index), the association betw een the ratio o f im m atures: adults harvested and the late nesting index was gauged while controlling for the effects of the brood index. T he strength of the association was then dim inished (species pooled, partial rs= 0.44, P < ). A ssum ing th at the dabbling duck brood index was also indicative of M allard brood production, the relationship was significant for th at species alone when the brood index was rem oved statistically using a partial rank correlation analysis (partial rs= 0.68, P < 0.05). Discussion H eavy egg p red atio n on early nesting species such as th e M allard increases the im portance of renesting in reproductive success and recruitm ent (Sargeant et al. 1984; C ow ardin et al. 1985). In years when renesting was w idespread, large num bers of young M allard and, to a lesser extent, o th er dabbling ducks w ould scarcely be able to fly w hen hunting seasons opened, a situation sim ilar to th at found in m any regions of E u ro p e (T am isier 1985). These results also suggest th at late nesting may result in relatively high harvests of im m a ture dabbling ducks, irrespective of the level o f production indices. T he higher vulnerability of hatching-year ducks is well know n (e.g. G ollop 1965; H ochbaum and W alters 1984), and was ap p aren t in data collected 100 km south of the St-D enis N W A w here young M allard w ere tim es m ore vulnerable to hunting than adults (G reenw ood et al. 1986). C ooch and B oyd (1984) found a highly significant positive relationship betw een M allard production and (hunter) kill in southern Saskatchew an. N ichols and H ines (1983) reported th at young fem ale M allard survived b etter during years of low harvest rate than during years of high harvest rate. T heir conclusion was based on data collected in m ajor M allard production areas of central C anada and the U nited States. H ow ever, results from southern M anitoba indicated no substantial im provem ent in survival of young fem ales in years w hen liberal regulations w ere changed to restrictive harvest regulations (Caswell et al. 1985). T he present analysis suggests that successful b reed in g m ay predispose adult fem ales to increased hunting m ortality. ne plausible explanation is th at, owing to high predation rates on early nests, m ost fem ales nest successfully relatively late in the year after which they accom pany young and m oult late in the sum m er o r early autum n. T hese hens w ould be in relatively p o o r physical co n dition w hen hunting opened. In M innesota, high predation on M allard nests led to renesting and late m oult (G ilm er et al. 1977); com pared with early and unsuccessful b reeders, these renesting fem ales w ere m ore vulnerable to hunters. Several o th er studies have found th at ducks w ith good body conditions are less vulnerable to hunters than those with relatively poor condition (G reenw ood et al. 1986; H aram iser«/. 1986; H epp et al. 1986). In the present study, there was som e indication th at late nesting resulted in higher harvest rates of adult fem ale dabbling ducks relative to adult m ales (T able 3). T he associations observed betw een nesting chronology and hunting m ortality of dabbling ducks are interesting, but m ust be view ed w ith caution. M ore data are needed to d eterm ine adequately the im pact of hunting m ortality on survival of young ducks and adult fem ales, and consequent changes in duck population size. The authors are unaw are of any analysis which has exam ined th e influence of hunting m ortality on survival of young and fem ales (and change in population size) while controlling for the influence of breeding chronology. Small changes in the tim ing of the opening day could greatly alter the num ber of young ducks which are poor flyers on the opening day o f hunting season. Indirectly, these findings suggest th at adjustm ent of opening day can affect the body condition of ducks shot by hunters, particularly that of breeding hens. D elaying of opening dates has potential for protecting breeding hens, and w ould probably be m ost effective w here over-w ater hunting is com m on. M an

7 Nesting chronology and hunting vulnerability 143 ipulating the date o f opening is preferable to changing bag limits. It is much m ore difficult to enforce bag limits than to detect "out of season " shooting violations. A t present, brood and late nesting survey results are reported in July. In prairie C anada decisions regarding the likelihood o f late nesting m ust be m ade before mid- June because o f the long tim e th at it takes to get the governm ent to approve hunting regulations. This used to be com pleted in less that tw o w eeks, but now takes a m inim um of four to six w eeks. A n approach which provided inform ation by m id-june on w etland conditions and hence potential for late nesting and breeding success w ould im prove the setting of C anadian hunting regulations. T o conclude, in terestin g associations w ere found betw een nesting chronology, breeding effort and vulnerability to hunting which have im portant im plications for duck population m anagem ent in prairie C anada, and possibly elsew here. T he strength o f the relationships found are also notew orthy because, in Saskatchew an, m ost ducks are killed in the last w eek of S eptem ber through m id - cto b er (D.J. N iem an, u n p u b l.). G iven nesting chronology sim ilar to th at described here, it is predicted that these relationships w ould be stronger in areas w here g reater proportions o f ducks are killed early o r w here hunting over w ater (rath er than in stubble fields) is w idespread. L ater season opening dates offer one alternative for m anaging harvest rates which m ay have a direct bearing on survival of young ducks and adult fem ales. F u rth er w ork is needed concerning: ( 1) the relationships betw een breeding effort, physical condition and hunting m ortality in adult fem ale dabbling ducks, especially the M allard; and, (2) the im pact of hunting m ortality on recruitm ent and duck population size. Acknowledgements M.R. M cl andress, J.M. S u th erland, E.J. W oodsworth and many other CWS staff and summer students helped to collect information on nesting ducks and we thank them all. We appreciate the assistance of J.B. Ignatiuk and A.M. Tuchscherer in sorting and compiling the nest data, of G.W. Beyersbergen in drawing the illustrations, and of L. M etras and J.R. Smith in providing data. H. Boyd, R.. Bailey, R. Edwards and G. Hochbaum generously offered constructive criticisms of an early draft. Summary Between 1980 and 1986 in south-central Saskatchewan, Canada, the percentage of broods younger than one week post-fledging when the hunting season opened was 15% (22% for Mallard), and ranged from 41% (55% Mallard) in 1983 to 4% (9% M allard) in The associations between the relative proportion of adult females killed and both the brood index and late nesting were significant. The ratio of imm ature:adult ducks killed by hunters was positively associated with the late nesting index, but not with the brood index. The results suggested that, depending on the cohort examined, hunting mortality was related to breeding chronology and effort. Delaying the opening day of hunting holds promise for reducing the kill of adult hens in years when breeding effort is prolonged. References B artonek, J.C., G am ble, K.E., Blohm, R.J., Miller, H.J., Brace, R.K., Pospahala, R.S.. Caswell. F.D. and Smith, M.M Status and needs of the Mallard, Trans. North A m. Wildl. Natur. Resour. Conf. 49: Bellrose, F.C Ducks, geese und swans o f North America. Stackpole Books, Harrisburg, PA. Benning, D.S W aterfowl breeding population survey, southern Saskatchewan, U.S. Fish Wildl. Seri., Can. Wildl. Serv. Intern. Rep. Boyd, H The large-scale impact of agriculture on ducks in the Prairie Provinces, Can. Wildl. Serv. Progr. Notes 147. Burnham, K.P., W hite, G.C. and Anderson, D.R Estimating the effect of hunting on annual survival rates of adult Mallards. J. Wild. Manage. 48: Caswell, F.D.. Hochbaum, G.S. and Brace, R.K The effect of restrictive regional hunting regulations on survival rates and local harvests of southern Manitoba Mallards. Trans. N. Amer. WÌldl. Natur. Resour. Conf. 50: Cooch, F.G. and Boyd, H Changes in the net export of Mallard from western Canada and the contiguous US, Can. Wildl. Serv. Prog. Notes 142. Cowardin, L.M., Gilmer, D.S. and Shaiffer, C.W Mallard recruitm ent in the agricultural environm ent of North Dakota. Wildl. Monogr. 92.

8 144 Robert G. Clark, Lawson G. Sugden, R. Kent Brace and Daniel J. Nieman D uebbert, H.F. and Frank, A.M Value of prairie wetlands to duck broods. Wildl. Soc. Bull. 12: Gilmer, D.S., Kirby, R.E., Ball, I. J. and Riechmann, J.H Post-breeding activities of Mallards and W ood Ducks in north-central Minnesota. J. W ildl Manage. 41: Gollop, J.B Dispersal and annual survival o f the Mallard. Ph.D. Thesis, Univ. of Saskatchewan, Saskatoon. Gollop, J.B. and M arshall, W.H A guide fo r ageing broods in the field. Mississippi Flyway Council, Tech. Sec. mimeo. Greenw ood, H., Clark. R.G. and W eatherhead, P.J Condition bias of hunter-shot Mallards (Anas platyrhynchos). Can. J. Zool. 64: Haram is, G.M., Nichols, J.D., Pollock, K.H. and Hines, J.E The relationship between body mass and survival of wintering Canvasbacks. A uk 103: Hepp, G.R., Blohm, R.J., Reynolds. R.E., Hines, J.E. and Nichols, J.D Physiological condition of autum n-banded Mallards and its relationship to hunting vulnerability. J. Wildl. Manage. 50: Higgins, K.F., Kirsch, L.M. and Ball, I.J., Jr A cable-chain device for locating duck nests. J. Wildl. Manage. 33: Hines, J.E. and Mitchell, G.J Breeding ecology of the Gadwall at W aterhen Marsh, Saskatchewan. Can. J. Zool. 61: Hochbaum, G.S. and W alters, C.J Com ponents of hunting m ortality in ducks. Can. Wildl. Serv. ccas. Paper 52. Hochbaum, H.A The Canvasback on a prairie marsh. The Am erican Wildl. Inst., W ashington, D.C. M etras, L. and W endt, S Migratory birds killed in Canada during the 1984 season. Can. Wildl. Serv. Prog. Notes 161. Nichols, J.D. and Hines, J.E The relationship between harvest and survival rates of Mallards: a straightforward approach with partitioned data sets. J. Wildl. Manage. 47: Sargeant, A.B., Allen, S.H. and E berhardt, R.T Red fox predation on breeding ducks in midcontinent N orth America. Wildl. Monogr. 89. Sugden, L.G. and Beyersbergen, G.W Prediction of duck nest survival in conventional and zero-tilled stubble fields. Can. Wildl. Serv. Progr. Notes 156. Tamisier, A Hunting as a key environmental param eter for the W estern Palearctic duck populations. Wildfowl 36: Zar, J.H Biostatistical analysis. Prentice-Hall Book Co., Englewood Cliffs, NJ. Robert G. C lark, Lawson G. Sugden, R. Kent Brace and Daniel J. Nieman. Canadian Wildlife Service, Prairie M igratory Bird Research Centre, 115 Perim eter Road, Saskatoon, Saskatchewan S7N 0X4, Canada.