25

Why should Greylag Goose Anser anser parents rear offspring of others?

L. Nilsson & H. Kampe-Persson

Department of Animal Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden.

leif. [email protected]. se, [email protected]

The benefits and costs of rearing large broods were studied in a popula­ tion of neck collared Greylag Geese in Scania, southernmost Sweden. Families were divided into four groups according to brood size: 1-4, 5-8, 9-12 and >12 young. Gosling survival showed a significant positive rela­ tionship with brood size on arrival at the rearing area. First-year and third-year local survival, as well as breeding recruitment, showed a sig­ nificant positive relationships with brood size at fledging. For adults rearing large broods, no costs were detected during the year following that when they fledged >8 young; both survival rate and reproductive success exceeded the long-term averages.

Key Words: neck collared population, adoption, survival, recruitment, south Sweden

Greylag Geese Anser anser are 1983). In addition to these very large sometimes seen with very large broods broods, there are also many cases of small young, some broods being so where just one or two eggs are added to large (up to 55 young) that it is impossi­ a clutch, or one or a few young are ble for a single female to have adopted into a brood. incubated so many eggs (Karlsson et al. The habit of intra- and inter-specific 1982; Jensen 2000; Persson 2002). It is egg parasitism and the adoption of well-known that Greylag Geese, like a small young is widespread among dif­ number of other waterfowl species, ferent species of waterfowl (Eadie et ai sometimes lay their eggs in the nest of 1988; Lank et ai 1989; Weigmann & another female, but excessively large Lamprecht 1991; Williams 1994; clutches laid by two or more females Beauchamp 1997; Andersson & Åhlund rarely hatch (Hauff 1982; Witkowski 2001). The adoption of foreign young is

©Wildfowl & Wetlands Trust Wildfowl [2003) 54: 25-37 26 Why should Greylag parents rear offspring of others? a prevalent habit of the Common Eider (Gregoire & Ankney 1990). Somateria mollissima and has been In this study, these questions are studied by a number of workers, who addressed for the Greylag Goose by have discussed the advantages for studying pre- and post-fledging sur­ females that Leave their young in the vival rates and recruitment rate in a care of others (eg Bustnes & Erikstad neck collared population that had been 1991; Pöysä 1995; Kilpi et al. 2001; studied intensively since 1985. Bustnes et al. 2002). Eadie & Lumsden (1985; cf. also Arnat 1987) concluded Study population that nest parasitism would sometimes The study population has never not be disadvantageous for the host but been manipulated during nesting, might be of advantage. because of the risk of influencing the Modern DNA techniques have breeding performance (cf. Witkowski revealed that there is a high frequency 1983). of mixed parentage due to nest para­ Pre-hatch brood amalgamation has sitism and adoption of young both in not been studied in this population. For ducks (Andersson & Åhlund 2001) and the present, there is only circumstan­ geese [Choudhury et al. 1993; Larsson tial evidence of pre-hatch brood et al. 1995). Among geese, parental amalgamation, such as the occurrence care in the form of vigilance and pro­ of nests containing >12 eggs and the tection can serve all members of a nest of a Barnacle Goose Branta leu­ brood, and the acceptance of extra copsis which contained four of its own young would therefore have no negative eggs and six Greylag Goose eggs effects on the host family (Lazarus & (Nilsson & Persson 1994; Persson Inglis 1986). Cooch (1991) found a 1997). A clutch of 12 eggs is widely faster growth rate in large families and accepted as the largest a Greylag suggested that the adoption of young Goose could lay naturally (Kampe- could lead to positive effects for the Persson 2002). In a German study, 7% host family. Larger families have a bet­ of 467 nests contained >12 eggs, and ter chance of detecting predators at an 18% of all eggs were found in nests early stage, and the strategy of adopt­ containing 13-40 eggs (Hauff 1982). In a ing foreign goslings may dilute the Polish study, only two clutches out of predation on their own offspring (Eadie 629 contained >12 eggs; all clutches & Lumsden 1985; Eadie et al. 1988). containing 1-6 eggs were laid by a sin­ Larger families are socially dominant gle female, while more than half of over smaller families (Black & Owen those containing 9-12 eggs were laid by 1989; Boyd 1953; Hanson 1953; two females [Witkowski 1983). Overall, Raveling 1970), which was found to be 4% of all nests contained eggs laid by of advantage in wintering Lesser Snow two females; in 30% of these, two Geese Anser caerulescens caerulescens females had laid two complete clutches Why should Greylag parents rear offspring of others? 27 in a single nest, while in the remaining area, and early-formed broods always 70%, one female had laid a complete consisted of similar-aged goslings clutch and another female had added (Persson 2002). In the rearing areas, on 1-3 eggs. The Swedish study popula­ the other hand, permanent brood mix­ tion, established in the late 1960s, grew ing was an extremely rare event, while on average 15.3% per annum (Nilsson pairs frequently lost or gained one or a et al. 2002), and now numbers >1,000 few goslings. Warhurst & Bookhout breeding pairs. As a consequence, (1983) suggested that post-hatch brood pairs nest close together, either in amalgamation in Canada Geese Branta reedbeds or on small islands (Nilsson canadensis occurs due to accidental & Persson 1994), which increases brood mixing shortly after hatching in opportunities to lay eggs in other areas of high brood density. The same females' nests (Brown 1984). seems to apply to the study population. Consequently, there is no reason to During the study period, the most assume pre-hatch brood amalgama­ common brood size of families arriving tion to be less frequent in the study at the rearing area was five, but four population than in other goose species was almost as common (Figure 1). The (Lank et al. 1989; Weigmann & commonest brood size at fledging was Lamprecht 1991). three, closely followed by two and four Almost all families with >12 young young. The largest brood seen during formed before they reached the rearing the study had 27 young, followed by

Figure 1. Distribution of brood sizes in the study area, Scania, south Sweden, on arrival to the brood- rearing area (light columns) and at fledging (dark columns), pooled over 1984-2002. All families having at least one neck collared parent were included. 28 Why should Greylag parents rear offspring of others? families of 22, 21 and 17 young. The area on the coast, where a large pro­ largest family at fledging had 22 young. portion of the Greylag Geese from the A total of nine broods numbering >12 study area gather (cf. Nilsson & young when first seen at the rearing Persson 1992). area was found, six at Lake Yddingen For analysis, families were divided and three at Lake Börringesjön. The into four groups according to brood mean number of young in these broods size, on arrival at the rearing area and decreased during brood-rearing, from at fledging, respectively: 1-4, 5-8, 9-12 17.4±1. 63 to 14.6+1. 4-9. and >12 young. Rearing survival was expressed as Method the number of geese that fledged as a percentage of the number that had Greylag Goose families were cap­ reached the brood-rearing area. tured and neck collared annually in a Calculations were based on the num­ study area in southwestern Scania, ber of young in families having at least southernmost Sweden, from 1985 one neck collared parent. All such fam­ onwards (Nilsson & Persson 1992; ilies recorded in a rearing area within Persson 1994). A total of 575 adults and ten days of hatching were included. 1, 700 goslings had been neck collared Account was taken of pairs that lost all by 2000 (Persson 2000a). Goslings were their young and left the study area to neck collared on average 22±0 days moult elsewhere, as well as of pairs before they fledged (Nilsson et al. 1997). that left the main rearing area before Individuals marked with poor quality fledging. Permanent brood mixing was neck collars (mainly 1984-1985) were accounted for by assuming that gains excluded from the analyses (cf. Persson and losses of goslings were indepen­ dent of family size. In this way, survival 2000b). During 1985-2002, regular searches estimates were not influenced as the for neck collared geese were undertaken individual was used as a sampling unit from their arrival in spring until their when following changes in brood size. autumn departure. In spring, the study During the latter half of the rearing area was visited several times a week period, goslings commonly left their in order to establish the return of parents without joining another family for shorter or longer periods (up to marked birds and their breeding per­ formance. From late May until the last three weeks), but all recorded separa­ young were fledged, the breeding area tions ended with a complete was visited almost daily to establish the re-grouping of the family. As signifi­ number of fledglings in each family. In cantly fewer small young survived to summer and autumn, weekly searches fledging at Lake Fjöllfotasjön than at were extended to include neighbouring the three lakes, Klosterviken, staging areas, especially the Foteviken Börringesjön and Yddingen (45% vs 68- Why should Greylag parents rear offspring of others? 29

71%; Nilsson et at. 2002], only families Results from the latter three lakes were includ­ ed in the analysis. Rearing survival First-year and third-year local sur­ vival were expressed as the number of The survival of young during brood- geese re-sighted after 1 July in years rearing was significantly higher with t+1 and t+3 respectively, as a percent­ increasing family size [Figure 2; age of the number that fledged in year χ23= 1 7.94-, P<0.001). Restricting the t. Such a simple method was possible analysis to lakes and years when because of the extremely high re-sight­ broods of >12 young were recorded, the ing frequency of all groups of geese outcome did not change: differences [Nilsson & Persson 1993), supporting were still highly significant (χ23=17.98, the assumption that the probability of P<0.001). detecting a surviving neck collared individual present in southwest Scania Local survival rates after the cut-off date (1 July) equals Both first-year and third-year local one. The resulting estimates were survival were significantly higher with probably slightly lower than the true increasing family size [Figure 3; survival rates as they took into account χ23=16.11, P<0.01 and χ23=9.31, P<0.05, neither early natal dispersal to sites respectively). outside southwest Scania [Nilsson & In an earlier study, Nilsson et al. Persson 2001a) nor neck collar losses (1997) showed that survival rates dif­ (Persson 2000b). In this study, underes­ fered among lakes. Making the analysis timates of the true survival rates were separately for the four main study assumed to be evenly distributed over lakes, the same general pattern was brood sizes, small and of no impor­ found in all lakes but differences were tance for the outcome of the analyses. significant only at Lake Yddingen, for Greylag Geese were considered to be first-year as well as third-year local recruited to a breeding area if they were survival [n=461; χ73=9.32, P<0.05 and seen with a nest or unfledged young in the χ23=8. 71, P<0.05, respectively). area (Nilsson & Persson 2001a). In addi­ tion, geese seen during the first two Recruitment rate weeks after fledging were considered to be recruited to the same area. Most indi­ The recruitment rate, when calcu­ viduals had recruited to the breeding lated for all fledged young, was population ny the time they were five significantly higher with increasing years old, but new parents were often family size (Figure 4), for all (χ23=13.38, unsuccessful. A paired bird in the fifth cal­ P<0. 01) as well as for successful endar year was counted as recruited to a (χ23=13.26, P<0.01) recruits. Restricting site if it remained faithful to it. the analysis to birds surviving at least 30 Why should Greylag parents rear offspring of others?

Figure 2. Survival (%) of young during brood-rearing in relation to brood size on arrival at the brood-rear­ ing area, at lakes Klosterviken, Börringesjön and Yddingen, Scania, south Sweden, pooled over 1986-2002. All families having at least one neck collared parent were included. The number of young in each group is given above the columns.

Figure 3. First year (light columns] and third year (dark columns) local survival 1%) in relation to brood size at fledging of neck collared young fledged in the study area, Scania, south Sweden, in 1984--1998. Num ber of young in each group is given above the columns. Why should Greylag parents rear offspring of others? 31

three years after fledging (ie normal (n=77; χ;',=0.00, n.s. and %;j=Q.00, n.s.). earliest age at recruitment; Nilsson & Persson 1994), recruitment rate still Parents in year t+1 correlated positively with family size (Figure 5), differences in recruitment Only one individual was seen with rate being significant for all recruits >12 young in more than one year: a (%23=7.92, P<0.05), whereas they were female in Lake Börringesjön, with not significant for successful recruits unmarked partner(s), fledged eight of (χ23=7.27, n.s.). 14 young in year t and 16 of 16 in year Separate analyses of the different t+1, while data on fledging success lakes showed a significantly higher rate were missing for year t+2. of recruitment (all and successful Of adults that fledged 9-12 and >12 recruits, respectively) with increasing young respectively, in yeart, 88% (n=34) family size for Lakes Yddingen (n=448; and 82% (n=17) were alive during the χ23=1 3.85, P<0.01 and χ23=14.35, P<0.01) following breeding season. Of 22 pairs and Klosterviken (n=291 ¡ χ22=9.66, that fledged 9-12 young in year t, 12 P<0.01 and χ22=9.76, P<0.01), whereas fledged 2-8 young and 10 failed in year no such correlation was found for t+1, while of eight pairs that fledged >12 Lakes Börringesjön (n=78; χ2,=0.08, n.s. young in year t, four fledged 2-5 young and χ2,=0.41, n.s.) and Fjällfotasjön and four failed in year t+1.

Figure 4. Breeding recruitment [%) in relation to brood size at fledging of neck collared young fledged in the study area, Scania, south Sweden, in 1984-1998. All (light columns) and successful (dark columns] recruits are shown. The number of young in each group given above the columns. 32 Why should Greylag parents rear offspring of others?

Figure 5. Breeding recruitment (%) in relation to brood size at fledging of neck collared young fledged in the study area, Scania, south Sweden, in 1984-1998 and surviving to the age of three. All (light columns) and successful (dark columns) recruits are shown. The number of young in each group given above the columns.

Discussion young and inexperienced parents among the smallest brood sizes, such Large broods may benefit in having parents generally having a lower rear­ faster gosling growth rates (Cooch et at. ing success than experienced breeders 1991; Lepage etat. 1998; Loonen et at. (Raveling 1981; Zicus 1981; Rockwell et 1999) and higher dominance rank (Boyd at. 1993; Nilsson 1998; but see 1953; Hanson 1953; Raveling 1970; Forslund & Larsson 1992). However, Black & Owen 1989), allowing them to the exclusion of broods numbering 1-4 compete more successfully for and young when first seen in the rearing obtain better access to food resources area does not change the outcome of (Gregoire & Ankney 1990), which con­ the analysis. sequently results in higher survival In the present study, it was not pos­ (Lepage et at. 1998; Loonen et at. 1999; sible to differentiate between Nilsson et at. 1997; this study) and within-pair and extra-pair young recruitment probabilities (Nilsson etat. among the neck collared individuals. 1997; this study). Thus, it was not possible to analyse The pre-fledging survival estimate whether there were any differences in given in this study might have been survival and breeding recruitment inflated by an over-representation of between these two categories. Why should Greylag parents rear offspring of others? 33

However, in studies of other goose other hand, had both a survival rate species no significant differences were (88%) and a reproductive success (55%) found between within-pair and extra­ that exceeded the long-term averages, pair young, in either pre-fledging which might be explained by their being survival (Lank et al. 1990; Williams ’better-quality' parents. 1994), post-fledging survival (Lank et al. Published studies do not reveal any 1990; Larsson et al. 1995) or breeding substantial cost to parents of rearing recruitment (Lank et al. 1990; Larsson large broods, with or without extra-pair et al. 1995). young. Even though feeding time was The fate of individual goslings negatively correlated and the amount of between hatching and breeding vigilance positively correlated with recruitment could not be studied in this brood size during brood-rearing, there population, as all marking took place was no relationship between number of when goslings were 5-6 weeks old goslings reared and probability of (Persson 1994). An indication of a return or timing of breeding in the fol­ gosling's probability of future breeding lowing year (Schindler & Lamprecht recruitment can be obtained, however, 1987; Forslund 1993; Williams et al. as soon as it arrives at the rearing area 1994; Loonen et al. 1999). by combining Figures 2 and 4. Available data clearly indicate that The fact that both pre- and post- goose parents can enhance both the fledging survival, as well as survival and breeding recruitment of recruitment rate, showed a positive their young by rearing extra-pair young, relationship with brood size ought to the more the better, which ought to favour adoption in this population, pro­ make adoption a favoured breeding vided there are no large costs to strategy. As this strategy is beneficial parents rearing additional young. The for the adults, it is surprising that it is data in question for parents that not more commonly reported but, on fledged >12 young in year t were based, the other hand, the opportunity to adopt however, on small sample sizes and do is greatest in a dense breeding popula­ not reveal any such costs. Survival tion and it is difficult to observe the between years t and t+1 (82%) was different pairs when the population is close to the long-term average of 83% unmarked. (Nilsson & Persson 1993), and repro­ The actual mechanism involved in ductive success in year t+1 (50% of gosling adoption is not clear. It is noted pairs producing fledglings) equalled almost daily in the study population the long-term average, as 48% of all that some stray goslings are chased breeding attempts by experienced away while others are adopted, some breeders resulted in at least one fledg­ adoptions involving goslings that are up ing (Nilsson & Persson 2001 b). Adults to eight weeks old. These goslings are that fledged 9-12 young in year t, on the adopted at an age when both parents 34- Why should Greylag parents rear offspring of others? and offspring are capable of recognis­ References ing each other (cf. also Choudhury et al.

1993; Williams 1994; Zicus 1981). A Amat, J. 1987. Is nest parasitism among plausible explanation may be that "suc­ ducks advantageous to the host? cessful adults always adopt a gosling American Naturalist 130: 454-457. as long as it is not younger than and/or Andersson, M. & Åhlund, M. 2001. Protein in an inferior physical condition to their fingerprinting: a new technique reveals own young". This seems to apply to the extensive conspecific brood parasitism. Scanian Greylag Goose, and very likely Ecology 82: 1433-1442. also to other goose populations. Beauchamp, G. 1997. Determinants of Support for this hypothesis comes from intraspecific brood amalgamation in the fact that adopted goslings are near­ waterfowl. The Auk 114: 11-21. ly always impossible to single out from a mixed brood (authors' observations). Black, J.M. & Owen, M. 1989. Agonistic To date, there have been no reports of behaviour in Barnacle Goose flocks: assessment, investment and reproductive geese stealing' young to increase the success. Animal Behaviour 37: 199-209. size of their own family, but it is possi­ ble given that a Greylag Goose pair in Boyd, H. 1953. On encounters between wild Utterslev Mose (Copenhagen, White-fronted Geese in winter flocks. Denmark) reared 15, 41 and 35 young, Behaviour 5: 85-129. respectively, in 1999, 2000 and 2001 Brown, C.R. 1984. Laying eggs in a neigh­ (Henning Jensen, in litt. ). bour’s nest: benefits and costs of colonial nesting in swallows. Science 224: 518-519. Acknowledgements Bustnes, J.O. & Erikstad, K. E. 1991. Parental care in the Common Eider Financial support for field studies Somateria mollissima, factors affecting carried out in 1997-2000 was obtained abandonment and adoption of young. from Øresundskonsortiet. Financial Canadian Journal of Zoology 69: 1538-1545. support for the neck collaring pro­ Bustnes, J. O., Erikstad, K.E. & Bjørn, T.H. gramme was also obtained from the 2002. Body condition and brood abandon­ Nordic Council for Wildlife Research, ment in Common Eiders breeding in the Swedish Environmental Protection High Arctic. Waterbirds 25: 63-66. Agency, Swedish Sportsmen’s Choudhury, S., Jones, C.S., Black, J. M. & Association, Swedish Ornithological Prop, J. 1993. Adoption of young and Society (Gustaf Danielsson Fund) and intraspecific nest parasitism in Barnacle Carl Trygger's Foundation for Scientific Geese. Condor 95: 860-868. Research. Cooch, E.G., Lank, D.B., Dzubin, A., Rockwell, R.F. & Cooke, F. 1991. Body size variation in Lesser Snow Geese: environ­ mental plasticity in gosling growth rates. Ecology 72: 503-512. Why should Greylag parents rear offspring of others? 35

Eadie, J. McA. & Lumsden, H. G. 1985. Is Breeding of the Greylag Goose Anser nest parasitism always deleterious to anser in Skåne, South Sweden, 1978- Goldeneyes? American Naturalist 126: 1982. ] Anser 21: 223-232. 859-866. Kilpi, M., Öst, M., Lindström, K. & Rita, FI. Eadie, J. McA., Kehoe, F.P. & Nudds, T. D. 2001. Female characteristics and 1988. Pre-hatch and post-hatch brood parental care mode in the crèching sys­ amalgamation in North American tem of Eiders, Somateria mollissima. Anatidae: a review of hypotheses. Animal Behaviour 62: 527-534. Canadian Journal of Zoology 66: 1709- 1721. Lank, D. B., Cooch, E. G., Rockwell, R.F. & Cooke, F. 1989. Environmental and demo­ Forslund, P. 1993. Vigilance in relation to graphic correlates of intraspecific nest brood size and predator abundance in the parasitism in Lesser Snow Geese Chen Barnacle Goose, Branta leucopsis. Animai caerulescens caerulescens. Journal of Behaviour 4-5: 965-973. Animal Ecology 58: 29-45.

Forslund, P. & Larsson, K. 1992. Age-relat- Lank, D.B., Rockwell, R. F. & Cooke, F. 1990. ed reproductive success in the Barnacle Frequency-dependent fitness conse- Goose. Journal of Animal Ecology 6 1 :195- guences of intraspecific nest parasitism 204. in Snow Geese. Evolution 44: 1436-1453.

Grégoire, P. E. & Ankney, C.D. 1990. Larsson, Κ., Tegelström, Fl. & Forslund, P. Agonistic behaviour and dominance rela­ 1995. Intraspecific nest parasitism and tionships among Lesser Snow Geese adoption of young in the Barnacle Goose: during winter and spring migration. The effects on survival and reproductive per­ Auk 107: 550-560. formance. Animal Behaviour 50: 1349-1360. Hanson, H.C. 1953. Inter-family dominance in Canada Geese. The Auk 70: 11-16. Lazarus, J. & Inglis, I.R. 1986. Shared and unshared parental investment, parent-off- Hauff, P. 1982. Bestandsentwicklung und spring conflict and brood size. Animal Brutbiologie der Graugans, Anser anser, Behaviour 34: 1791-1804. im NSG Kuhlrader Moor and Röggeliner See. Beiträge vor Vogelkunde 28: 48-58. Lepage, D., Gauthier, G. & Desrochers, A. 1998. Larger clutch size increases fledging Jensen, FI. 2000. Ekstremt stort success and offspring quality in a precocial grågåsekuld Anser anser-. Adoption eller species. Journal of Animal Ecology 67: 210- ægdumpmng af flere hunner? [In Danish 216. with English summary: An extremely large brood of Greylag Goose Anser anser: Loonen, M.J. J.E., Bruinzeel, L.W., Black, J. M. Adoption or egg dumping by several & Orent, R. H. 1999. The benefit of large females? ] Ornis Svecica 10: 1 73-177. broods in Barnacle Geese: a study using natural and experimental manipulations. Kampe-Persson, H. 2002. Greylag Goose Journal of Animal Ecology 68: 753-768. Anser anser. BWP Update 4(3): 181-216. Nilsson, L. 1998. The Greylag Goose Anser Karlsson, J., Nilsson, L. & Persson, FI. 1982. anser as a model species for the study of Grågåsen som häckfågel i Skåne 1978- waterfowl breeding ecology. Acta Zoologica 1982. [In Swedish with English summary: Lituanica, Ornithologia 8: 20-28. 36 Why should Greylag parents rear offspring of others?

Nilsson, L. & Persson, H. 1992. Feeding Persson, H. 2000a. Halsringmärkning av areas and local movement patterns of grågäss i Skåne 1984-2000. [In Swedish post-breeding Greylag Geese Anser anser with English summary: Neck collaring of in South Sweden. Ornis Svecica 2: 77-90. Greylag Geese Anser anser in Scania, 1984-2000.] Anser 39: 167-172. Nilsson, L. & Persson, H. 1993. Variation in survival in an increasing population of the Persson, H. 2000b. Neck collar retention in a Greylag Goose Anser anser in Scania, Greylag Goose Anser anser population. southern Sweden. Ornis Svecica 3: 137- Ornis Svecica 10: 155-160. 146. Persson, H. 2002. Kullsammanslagning hos Nilsson, L. & Persson, H. 1994. Factors grågås Anser anser. [In Swedish with affecting the breeding performance of a English summary: Brood amalgamation in marked Greylag Goose Anser anser popu­ the Greylag Goose Anser anser. ] Ornis lation in south Sweden. Wildfowl 45, Svecica 12: 96-99. 33-48. Pöysä, H. 1995. Factors affecting abandon­ Nilsson, L. & Persson, H. 2001a. Natal and ment and adoption of young in Common breeding dispersal in the Baltic Greylag Eiders and other waterfowl: a comment. Goose Anser anser. Wildfowl 52: 21-30. Canadian Journal of Zoology 73: 1575-1577.

Nilsson, L. & Persson, H. 2001b. Change of Raveling, D.G. 1970. Dominance relation­ mate in a Greylag Goose Anser anser pop­ ships and agonistic behaviour of Canada ulation: effects of timing on reproductive Geese in winter. Behaviour 37: 291-319. success. Wildfowl 52: 31-40. Raveling, D.G. 1981. Survival, experience, Nilsson, L, Green, M. & Persson, H. 2002. and age in relation to breeding success of Field choice in spring and breeding per­ Canada Geese. Journal of Wildlife formance of Greylag Geese Anser anser in Management Ab: 817-829. south Sweden. Wildfowl 53: 7-25. Rockwell, R. F., Cooch, E. G., Thompson, C. B. Nilsson, L., Persson, H. & Voslamber, B. & Cooke, F. 1993. Age and reproductive 1997. Factors affecting survival of young success in female Lesser Snow Geese: Greylag Geese Anser anser and their experience, senescence and the cost of recruitment into the breeding population. philopatry. Journal of Animal Ecology 62: Wildfowl 48: 72-87. 323-333.

Persson, H. 1994. Halsrmgmärkning av Schindler, M. & Lamprecht, d. 1987. Increase grågäss Anser anser i Skåne 1984-1993. of parental effort with brood size in a nid- [In Swedish with English summary: Neck- ifugous bird. The Auk 104: 688-693. banding of Greylag Geese Anser anser in Scania, 1984-1993.] Anser 33: 101-106. Warhurst, R. A. & Bookhout, T. A. 1983. Effect of gang-brooding on survival of Canada Persson, H. 1997. Den vitkindade gåsens Goose goslings. Journal of Wildlife Branta leucopsis etablering i Skåne. [In Management 47: 1119-1124. Swedish with English summary: Breeding of the Barnacle Goose Branta leucopsis in Weigmann, C. & Lamprecht, d. 1991. Skåne, 1986-1996.] Anser 36: 11-15. Intraspecific nest parasitism in Bar-head­ ed Geese Anser indicus. Animal Behaviour 41: 677-688. Why should Greylag parents rear offspring of others? 37

Williams, T.D. 1994. Adoption in a precocial species, the Lesser Snow Goose: inter- generational conflict, altruism or a mutually beneficial strategy? Animat Behaviour 47: 101-107.

Williams, T. D., Loonen, M.J. J. E. & Cooke, F. 1994. Fitness consequences of parental behaviour in relation to offspring number in a precocial species: the Lesser Snow Goose. The Auk 111: 563-572.

Witkowski, J. 1983. Population studies of the Greylag Goose Anser anser breeding in the Barycz valley, Poland. Acta Ornithologica 19: 179-216.

Zicus, M. C. 1981. Canada Goose brood behaviour and survival estimates at Crex Meadows, Wisconsin. Wilson Bulletin 93: 207-217.