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The Wilson Journal of Ornithology 120(4):725–731, 2008 LESSER SNOW GEESE AND ROSS’S GEESE FORM MIXED FLOCKS DURING WINTER BUT DIFFER IN FAMILY ´ N EINAR JO´NSSON1,2,4 AND ALAN D. AFTON3 ABSTRACT.—Smaller species are less likely to maintain families (or other forms of social groups) than larger species and are more likely to be displaced in competition with larger species. We observed mixed-species flocksof geese in southwest Louisiana and compared frequencies of social groups and success in social encounters ofLesser Snow Geese (Chen caerulescens caerulescens; hereafter Snow Geese) with that of the smaller, closely-related Ross’s Geese (C. rossii). Less than 7% of adult and Ͻ4% of juvenile Ross’s Geese were in families,whereas 10–22% of adult and 12–15% of juvenile Snow Geese were in families. Snow Geese won 70% ofinterspecific social encounters and had higher odds of success against Ross’s Geese than against individuals oftheir own species. The larger Snow Geese maintain families longer than Ross’s Geese, which probably contrib-utes to their dominance over Ross’s Geese during winter. Predator vigilance probably is an important benefit ofmixed flocking for both species. We suggest the long-standing association with Snow Geese (along with asso-ciated subordinate social status) has selected against family maintenance in Ross’s Geese. Received 23 August2007. Accepted 27 February 2008. Body size seemingly influences social be- (Krause and Ruxton 2002). Smaller species havior and foraging behavior in many animals generally are more vulnerable to predator at- (Calder 1996). Body size has important phys- tacks than larger species, but can benefit by iological implications for birds: (1) rate of forming mixed flocks with larger species, heat loss increases with decreasing body size which at times have better predator detection because of increasing surface to volume ratio capabilities (McWilliams et al. 1994, Kristian- (Goudie and Ankney 1986, Calder 1996); (2) mass-specific metabolic rate is inversely re- lated to body mass (Kendeigh 1970, Calder (Chen caerulescens caerulescens; hereafter 1996); (3) gut size scales linearly with body size and partly affects the rate of energy ex- breeding season to the beginning of the next traction from food (Demment and Van Soest (family social system) (Boyd 1953; Raveling 1985); and (4) larger species generally have 1970; Prevett and MacInnes 1980; Black and greater fasting endurances than smaller spe- cies (Goudie and Ankney 1986, Calder 1996, Kalmbach 2006). Larger goose families gen- Jo´nsson et al. 2007). Smaller species are rel- erally are dominant over smaller families, atively less likely to maintain social groups, pairs, and lone geese (Loonen et al. 1999, generally select more sheltered habitats, and Stahl et al. 2001, Kalmbach 2006). Parents ap- consume more specialized diets (Jarman 1974, parently profit from juvenile assistance when Shelley et al. 2004). Smaller species also are defending patches of food from other flock more likely to be displaced in competition members (contributor effect hypothesis, Black with larger species, regardless of numbers present (Shelley et al. 2004) and often use Conversely, smaller goose species may not maintain families in winter (McWilliams andRaveling 1998). In California, Ross’s Geese 1 School of Renewable Natural Resources, Louisiana (C. rossii) form denser flocks than larger State University, Baton Rouge, LA 70803, USA.
goose species when foraging on grasslands 2 Current address: University of Iceland, Snæfellsnes Research Centre, Hafnargata 3, 340 Stykkisho´lmur, Geese (Branta hutchinsii) (Johnson and Rav- eling 1988, McWilliams and Raveling 1998).
3 USGS, Louisiana Cooperative Fish and Wildlife Only a small percentage of Ross’s Geese in Research Unit, Louisiana State University, BatonRouge, LA 70803, USA.
California are paired or in families (dense- 4 Corresponding author; e-mail: [email protected] flock social system) (Johnson and Raveling THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 120, No. 4, December 2008 1988, McWilliams and Raveling 1998). Fam- graphical location, season, and climate (Jo´ns- ily maintenance varies in relation to food choices (seeds vs. leafs and tubers) and fine-scale spatial distribution of selected food plants among habitats (McWilliams and Rav- Study Area.—We observed Snow Geese and Ross’s Geese in the rice prairie region of southwest Louisiana in winters 2002–2003 from avian predators than are larger goose and 2003–2004 (Jo´nsson 2005; Jo´nsson and species and predation pressure may have been Afton 2006, 2008). Rice prairies are former an important evolutionary factor (albeit not tall-grass prairies which are extensively cul- the only factor) influencing their social system tivated and managed, mostly for rice, but also during winter (McWilliams et al. 1994). In ad- as pasture for cattle (Alisauskas et al. 1988, dition, denser flocks may convey benefits of Bateman et al. 1988). This area has been de- decreased nearest neighbor distance, i.e., birds scribed by Alisauskas et al. (1988) and Bate- in denser flocks are able to spend more time feeding and less time vigilant (Fernandez-Jur- comprised of Snow Geese and Ross’s Geese Southwest Louisiana is a historical winter- using foraging habitats, i.e., non-flooded rice- ing area for Snow Geese (Jo´nsson and Afton fields, which were uncut, stubble, tilled, or fal- 2006 and citations therein), but Ross’s Geese low (Alisauskas et al. 1988, Jo´nsson 2005).
only began wintering in Louisiana during the Ross’s Geese comprised, on average, 7% of last decade (Jo´nsson 2005). Ross’s Geese in observed mixed white goose flocks during ourstudy. Estimated combined Snow Goose and Louisiana occur only in rice-prairies (cultivat- Ross’s Goose numbers on our study area were ed former tall-grass prairie) where they pri- 257,119 in 2002–2003 and 360,487 in 2003– marily forage in rice fields in mixed flocks with Snow Geese. Ross’s Geese have small Sampling of Focal Geese.—Three trained bills that are adapted for grazing on grass (Ry- observers and JEJ collected behavioral data in der and Alisauskas 1995). Thus, they are rare- winters 2002–2003 and 2003–2004; JEJ was ly found in marshes along the Gulf Coast, the only observer present in both winters and which are historical habitats of Snow Geese trained other observers, until results of obser- (which have larger bills adapted for excava- vation of the same focal birds were nearly tion of marsh plants) (Alisauskas 1998).
identical among observers (Jo´nsson and Afton Dominant goose species can affect feeding 2006, 2008). We are confident that inter-ob- behavior, distribution, and food selection of server variation between or within years did less aggressive species when feeding in mixed flocks (Kristiansen and Jarrett 2002). Interspe- cific dominance relationships often are affect- ed by the number of individuals present from winter. Observations were made during day- each species (i.e., the more numerous species light between 0800 and 1700 hrs CST. Mixed flocks were large (a few hundred to a few thousand), mobile, and flushed often; thus, although exceptions are known (Kristiansen risk of repeated sampling of individuals was We present the first quantitative comparison of (1) frequencies of pairs and families, and collected 5 to 10-min focal sampling obser- (2) frequencies and outcomes of intra- and in- vations of randomly selected individuals (Alt- terspecific social encounters of Ross’s Geese mann 1974). We used sequences of 20 random and Snow Geese in mixed wintering flocks.
numbers to select focal geese within a field of vision, counting from left or right until a closely-related species of varying size forag- goose was located that corresponded to each ing together on the same plant species and controls for effects of macro habitat, geo- Time of day was not a variable of biological Jo´nsson and Afton • MIXED FLOCKS AND FAMILY MAINTENANCE IN WINTERING GEESE 727 interest in this study. We attempted a priori based on normal and Poisson distributions; the to control for time of day variation in behavior Poisson log-linear model is equivalent to a lo- by alternating between species during field gistic regression based on the multinomial dis- observations to ensure that comparisons be- tribution (Agresti 1996). We evaluated good- tween species were unbiased (Jo´nsson and Af- ness of fit for these models by comparing ra- ton 2006, 2008). Both species were sampled tios between degrees of freedom (df) and de- equally during mid-day (1100 –1300 hrs) viance of the models; a ratio of deviance/df when geese were relatively prone to cease ac- close to 1.0 indicates a good model fit (Agresti 1996). A linear model based on the normal We assigned age classes to Snow Geese and distribution fit the data reasonably well (de- Ross’s Geese based on plumage color and pat- viance ϭ 24.0, df ϭ 15), whereas the Poisson terns (Ryder and Alisauskas 1995, Mowbray model gave a poorer fit (deviance ϭ 149.6, df et al. 2000). We assigned pair and family sta- tus to individuals under observation based on We calculated probabilities of winning en- mutual participation in social encounters, mu- tual chasing or avoiding other geese, and co- ordinated directions of locomotion (Raveling specific social encounters versus intraspecific social encounters for both species. Our goal Ankney 1990). We categorized focal individ- was to quantify success in interspecific en- uals into five social groups (after Boyd 1953, counters, using success in intraspecific en- Raveling 1970, Gregoire and Ankney 1990): counters as a baseline value. We calculated (1) lone adult, a lone after-hatch-year goose; odds ratios of winning against the other spe- (2) parent, adult goose bonded (i.e., paired) cies over the odds of winning against a con- with another adult goose, accompanied by at least one hatch-year bird; (3) paired non-par- ent, adult goose bonded with another adult goose without hatch-year birds; (4) juvenile in ϭ Probability of winning (P )/(1 Ϫ P )} family, hatch-year goose accompanied byadult parents; and (5) lone juvenile, a lone We recorded frequencies of social encoun- ters between focal geese and other geese, scor- We assumed that differing odds of success ing wins if opponents responded to interac- (unequal odds ratio) between Snow Geese and tions by avoiding or fleeing focal geese; focal Ross’s Geese indicated that one species was birds were assigned a loss if an opponent dominant over the other species, whereas odds ratios of ϳ1 indicated equal success in inter- Ankney 1990). We only recorded social en- specific social encounters and equal social sta- counters directly involving focal geese, their Statistical Analyses.—We used a general- Frequencies of social groups differed be- (Agresti 1996, SAS Institute 1999) to estimate tween species (␹2 ϭ 6.12, P ϭ 0.013) and age whether frequencies of social groups (parents, groups (␹2 ϭ 35.55, P Ͻ 0.001), but not be- non-parental pairs, and lone geese) differed tween winters (␹2 ϭ 0.53, P ϭ 0.466). The between species, age groups, and winters, ratio of juveniles to adults was relatively high- which were categorical explanatory variables.
er for Snow Geese in both winters (Table 1).
Final models were selected using backwards Less than 7% of adult and Ͻ4% of juvenile stepwise model selection (Agresti 1996), ex- Ross’s Geese were in families, whereas 10– cept the age ϫ social group interaction was 22% of adult and 12–15% of juvenile Snow fixed (regardless of significance) in this model because pairs without juveniles were not ob- to 10 times more likely to have intraspecific encounters than interspecific encounters (Ta- THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 120, No. 4, December 2008 Age and social groups (% of observations) of focal Lesser Snow Geese and Ross’s Geese in the rice prairies of southwest Louisiana during winters, 2002–2003 and 2003–2004.
a Percent juveniles within N.
b Number of focal individuals.
ble 2). In contrast, focal Ross’s Geese engaged Geese, and one win was against an adult pair.
in intra- and interspecific social encounters Focal Ross’s Geese did not win social en- with equal frequency in 2003–2004, but had counters against Snow Geese in families.
3 times more interspecific social encountersthan intraspecific social encounters in 2002– 2003 (Table 2). Focal Snow Geese were more Family Maintenance.—Our data from Lou- likely to win social encounters with Ross’s isiana, combined with that from other loca- Geese than with other Snow Geese (Table 2).
tions, indicate Ross’s Geese maintain families Focal Snow Geese were relatively more suc- for shorter periods than Snow Geese through- cessful in interspecific social encounters; out their current wintering ranges. Timing of Snow Geese won 30 of 52 social encounters family break-up is known to vary among spe- in 2002–2003, and 32 of 33 social encounters cies, individuals, and years (Prevett and Mac- in 2003–2004 (Table 2). Snow Geese won 63 Innes 1980, Black et al. 2007). Eighty percent of 87 (72.4%) interspecific social encounters of all juvenile Snow Geese wintering in the observed when all focal observations of both Mississippi Flyway are in families from 20 species were combined. Focal birds of both December to 15 March, whereas Ͻ50% of all species were more successful in intraspecific goslings are in families on staging areas in social encounters in 2003–2004 than in 2002– late March and early April (Prevett and Mac- Innes 1980). Generally, over 50% of all gos- Overall, focal Snow Geese lost only 10 so- ling Barnacle Geese (Branta leucopsis) leave cial encounters against Ross’s Geese; all their parents by December and Ͻ20% remain Ross’s Goose wins were against lower ranked with their parents until April; however, this Snow Geese (i.e., non-parental pairs and lone distribution varies among years (Black et al.
birds); six were against lone juvenile Snow 2007). We observed more families in winter Geese, three were against lone adult Snow 2003–2004; family breakup on our study area Frequencies of social encounters (n/hr) of focal Lesser Snow Geese and Ross’s Geese, and odds of their success in social encounters in the rice prairies of southwest Louisiana during winters, 2002–2003 and2003–2004.
Percentage of intraspecific social encounters won Percentage of interspecific social encounters won a Odds of interspecific success ϭ odds of winning against other species/odds of winning against own species.
Jo´nsson and Afton • MIXED FLOCKS AND FAMILY MAINTENANCE IN WINTERING GEESE 729 may have occurred earlier for both species in maining well within flock boundaries (JEJ, winter 2002–2003 than in winter 2003–2004.
pers. obs.; R. C. Drewien, pers. comm.). Thus, dominance of Snow Geese seemingly does not Flocks.—Snow Geese were dominant over drive Ross’s Geese towards flock edges; in- Ross’s Geese, as indicated by their relatively dividuals on flock edges often are subordi- higher odds of winning against Ross’s Geese.
We did not observe a Ross’s Goose win a so- cial encounter against a Snow Goose in a fam- ily group. The relatively higher success of goose species; recent genetic studies show Snow Geese in interspecific encounters in 2003–2004 corresponded to a higher frequen- Geese and Ross’s Geese over historical time cy of families in that year. Effects of species (Weckstein et al. 2002). New pairs are formed and family maintenance on outcomes of social on wintering grounds or during spring migra- tion (Ganter et al. 2005); thus, the two species Geese may be more successful in interspecific exchange genetic material via mutual winter- social encounters because they maintain fam- ing areas (Mowbray et al. 2000). We suggest ilies. Similarly, family maintenance, rather that along with predation pressure (cf. Mc- than species or body size may explain differ- Williams et al. 1994), the historical associa- ent time-budgets of the two species (Jo´nsson tion of Ross’s Geese with Snow Geese, along and Afton 2008). Single-species flocks are with the former’s associated subordinate so- known for both species in other locations cial status, has selected against family main- (Johnson and Raveling 1988, Ryder and Ali- tenance in Ross’s Geese. However, we ob- sauskas 1995, Mowbray et al. 2000), but we served a small proportion of Ross’s Geese in only observed one single-species flock of families each winter. Thus, family mainte- Ross’s Geese, in winter 2003–2004 (Jo´nsson nance probably represents individual choice within a species, rather than a species-fixed Predator vigilance (via ‘‘many eyes’’ and evolutionary constraint (Black et al. 2007).
dilution effect) probably is an important ben-efit of mixed flocking in both species (Kris- tiansen et al. 2000, Krause and Ruxton 2002, Our study was funded by the Canadian Wildlife Ser- vice, Louisiana Department of Wildlife and Fisheries may find it increasingly difficult to select prey, (LDWF), Delta Waterfowl Foundation, Rockefeller when prey choice requires choice of differing Scholarship program, a Research Partnership Proposal(RPP) Grant from Cameron Prairie National Wildlife types and each prey type has differing cost- Refuge (NWR) and the U.S. Fish and Wildlife Service, benefit relationships for the predator (confu- and by the U.S. Geological Survey-Louisiana Coop- sion effect; Sinclair 1985, FitzGibbon 1990, erative Fish and Wildlife Research Unit, Graduate School, School of Renewable Natural Resources at have relatively better predator detection ca- Louisiana State University (LSU), and LSU AgCenter.
pacities because they are taller (cf. Randler Rockefeller State Wildlife Refuge, Cameron PrairieNWR, and LDWF provided housing and valuable lo- 2004) and may have greater visual acuity, giv- gistical support. We thank C. J. Michie, Brandt Meix- en acuity is positively correlated with eye ell, M. G. Pollock, T. W. Blair, and J. M. Yurek for size, which scales positively with body size help with data collection, and D. C. Blouin and M. D.
(Fernandez-Juricic et al. 2004). We often ob- Kaller for advice on statistical analyses. We thank R.
served Red-tailed Hawks (Buteo jamaicensis) N. Helm, Guthrie Perrie, T. J. Hess, J. T. Linscombe, fly by the geese, causing them to respond by and Daniel Gary for valuable assistance with our proj- becoming alert. Bald Eagles (Haliaeetus leu- ect. We acknowledge D. G. Homberger, W. G. Henk,M. J. Chamberlain, S. R. McWilliams, and an anony- cocephalus) also attacked goose flocks (Jo´ns- mous referee for suggestions that improved this paper.
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