Hybridization between Lesser White-fronted Goose Anser erythropus and Barnacle Goose Branta leucopsis

125 Citation: Liljebäck N, Koffijberg K, Kowallik C, Månsson J & Andersson Å. 2021. Use of foster parents in species conservation may cause conflicting objectives: hybridization between Lesser White-fronted Goose Anser erythropus and Barnacle Goose Branta leucopsis. Ornis Svecica 31: 125–138. https://doi.org/10.34080/os.v31.22430. Copyright: © 2021 the author(s). This is an open access article distributed under the CC BY 4.0 license, which allows unrestricted use and redistribution, provided that the original author(s) and source are credited. R E S E A RCH PA PE R

). Contrary to hybridization among ducks, the extent of hybridization in geese is poorly understood (for review, see Ottenburghs et al. 2016b). Only recently, more insight has emerged, both from geese in the wild and in cap-

Introduction and background
Hybridization has been highlighted as a potential threat to endangered taxa (Allendorf et al. 2001, Jackiw et al. 2015. Under certain conditions, hybrids may constitute an issue for conservation of small populations and translocations of captive-bred individuals may be of special concern in this context (IUCN/SSC 2013). Even if hybridization is a natural and pronounced process in the genetic ancestry of goose species (Ottenburghs et al. 2016a), introgression of alien genes may alter species barriers or diminish population-specific adaptations (e.g. Muñoz-Fuentes et al. 2007, Todesco et al. 2016. One case where hybrids have gained special attention in relation to translocations concerns hybrids between Lesser White-fronted Goose Anser erythropus and Barnacle Goose Branta leucopsis (hereafter LWfG × BG). The existence of hybridizing pairs and their offspring have provoked disputes regarding the potential threat they may pose to the small, and highly endangered, populations of Lesser White-fronted Goose breeding in Fennoscandia ( Jones et al. 2008).
Geese and ducks are among the bird taxa most likely to hybridize in nature (Randler 2008, Nijman et al. 2010, Ottenburghs et al. 2016a) and recurrent hybridizations may account for their low level of genetic differentiation adopted the migratory habits of Barnacle Geese, overlapping very little with sites used by Lesser White-fronted Geese. We find no evidence that the hybrids ever posed a serious threat to Lesser White-fronted Geese breeding in Fennoscandia.
Keywords: translocation | intergeneric hybrid | distribution | wintering grounds | migration route | Anatidae hunting pressure on stopover and wintering sites along known eastern migration routes ( Jones et al. 2008). To meet this objective, the Swedish conservation program adopted a translocation approach, using semi-domestic Barnacle Goose pairs as foster parents, with known wintering areas in Western Europe (mainly the Netherlands;von Essen 1991). In 1981, the first captive-bred goslings were released together with their foster parents in Swedish Lapland, in an area that was still inhabited by breeding Lesser White-fronted Geese at that time (von Essen 1991, Andersson & Holmqvist 2010. The project was successful in the sense that released young Lesser White-fronted Geese adopted the migration route presented by their foster parents and subsequently established a wintering tradition in the Netherlands. Following this human-mediated migration route, the population avoided the previously observed high mortality during migration. Consequently, the population size slowly started to increase (von Essen 1991, Koffijberg et al. 2005Koffijberg et al. , 2006. However, the use of foster parents also had the unfortunate effect that a small number of captive-bred male Lesser White-fronted Geese became erroneously imprinted on the foster parent species and later paired with female Barnacle Geese and successfully produced hybrid offspring. Hybrid families were frequently reported from staging sites in Sweden and from wintering areas in the Netherlands. As a response, the Swedish conservation program decided at an early stage to remove Lesser White-fronted Goose males in mixed pairs, and their offspring, whenever possible. Kampe-Persson & Lerner (2007) provide a general overview of goose hybrids recorded in Sweden, including LWfG × BG hybrids. So far, no attempt has been made to investigate numbers and dispersal of LWfG × BG hybrids in detail, nor to evaluate whether the culling of mixed pair males and their hybrid offspring were successful to reduce putative conservation problems.
In this study, we investigate the occurrence of free-flying hybrids of LWfG × BG in Western Europe. Based on datasets retrieved from citizen science portals and goose census schemes, we analyse numbers, dispersal/distribution, and trends in six different countries. Furthermore, archives of the Swedish conservation program were searched for additional information about the sites used by mixed pairs and annual production of hybrid offspring. We compare data on reproduction with the information gathered during staging and wintering along the migration route. Based on our findings we discuss whether the hybrids with Barnacle Geese pose a risk in the conservation of Lesser White-fronted Goose populations breeding in Fennoscandia.

DATA ON MIXED PAIRS ON BREEDING SITES
Between 1981 and 1999, a total of 301 young Lesser White-fronted Geese were released in Swedish Lapland. Of them, 257 were released accompanied by their Barnacle Goose foster parents. The remaining 44 young were released in the same area but without company of, or having been associated with, foster parents. All goslings carried individual combinations of colour rings. The 44 birds released without foster parents generated few resightings and are not treated further in this study.
Mainly volunteer ring readers along the flyway reported resightings of ringed Lesser White-fronted Goose individuals (including information on family status). Reports were filed and stored in a database by the conservation program. From this database we retrieved data on the number of ringed Lesser White-fronted Goose males in mixed pairs, including information about documented number of hybrid offspring produced on an annual basis. In addition, we found information about (1) site use by Lesser White-fronted Goose males, (2) when Lesser White-fronted Goose males in "mixed pairs" were culled, and (3) culling of hybrids, but also (4) supporting information, e.g., about local movements (see also Andersson 2019).

CITIZEN SCIENCE DATA ON OBSERVED HYBRIDS FROM STAGING AND WINTERING SITES
From about 2000 onwards many European countries have launched web-based species gateways (data portals) where the public can report observed species with various additional information. Most reports are submitted by enthusiastic volunteers but some also by professional conservation officials and researchers. Even though citizen science data are non-systematic, the large data volume provide useful information especially for rarer taxa, including hybrids (e.g. Snäll et al. 2011). We extracted data from such species gateways in Norway (https://www.artsobservasjoner.no), Sweden (https://www.artportalen.se), Finland (https://www. tiira.fi), Denmark (https://dofbasen.dk), Germany (https://www.ornitho.de) and the Netherlands (https:// waarneming.nl) until spring 2017. Furthermore, a large number of ornithological reports (e.g., national, regional, or local annual bird reports) were screened for information. For the Netherlands, we also retrieved records from the national waterbird monthly census scheme, co-ordinated by Sovon Vogelonderzoek Nederland (e.g. Hornman et al. 2020). From these sources 1,303 reports were extracted (including obvious mis-identifications; see Table 1 for a complete overview of data sources and references used).
Since proper identification of hybrid geese may pose a significant challenge (McCarthy 2006), a verification was carried out for observations associated with photographic documentation. Moreover, specific remarks on observation details were checked, e.g., when administrators of the species gateways had approved, or added comments about, observations. From the 1,303 reports scrutinized for our analyses (Table 1), 37 reports (3 %) undoubtedly referred to hybrids of other goose species. For Germany in the period 2011-2017 many reports included photos and 26 % of all records submitted involved other goose species. Common mistakes were misidentification of Greylag Goose × Canada Goose Branta canadensis and Greylag Goose × Barnacle Goose, but also Barnacle Goose × Snow Goose or Ross's Goose A. caerulescens/rossii. Certainly, the most challenging hybrid type for identification in the context of this study relates to hybrids of Barnacle Goose × Greater White-fronted Goose. In Germany and the Netherlands, hybrid young of mixed pairs of these two species were indeed sometimes found in reports (documentation showing the adults as well), but hybrid offspring was none the less reported as Lesser White-fronted Goose hybrids, likely because observers were aware of this hybrid type and its conservation issues. In this study, we assume that all reports lacking documentation or additional remarks made by administrators, were correctly identified hybrids and hence included them in the TABLE 1. Overview of data sources used in the study to describe distribution of hybrids between Lesser White-fronted Goose Anser erythropus and Barnacle Goose Branta leucopsis. We present the total number of reports (including multiple records of the same flock or individuals) for each region before examination are stated, followed by the number of rejected reports within brackets. Rejection of reports was based on verification of photos or through enquiries to observers. Note that data was downloaded in 2017-2018, consequently reports in later years are not covered.
-Översikt för datakällor för att beskriva utbredningen av hybrider mellan fjällgås Anser erythropus och vitkindad gås Branta leucopsis. Det totala antalet rapporter (som kan inkludera flera rapporter gällande samma flock eller individ) för varje region anges, följt av antalet rapporter som uteslutits inom parentes. Uteslutning av rapporter baserades på granskning av fotografier eller på direktförfrågningar till observatörer. Notera att dessa data laddades ned 2017-2018 och att rapporter sålunda kan ha tillkommit efter vårt datauttag. analysis. The identified errors show that we should be careful about the absolute numbers presented, especially as the majority of reports lacked any documentation to support identification. Observer bias is an underlying potential weakness in any analyses based on data from public gateways. The reporting rate of hybrids to the online platforms may have changed over time, e.g., as observers became more aware of the conservation issues with specific hybrids of Barnacle Goose and Lesser White-fronted Goose as described above. However, as the hybrid families were intensively monitored by the Swedish Lesser White-fronted Goose project and individual observers-and enquiries were made among observers reporting hybrids in Sweden and the Netherlands-we assume that the data presented here do give a good overview of the trends in numbers and dispersal of hybrids.
For annual estimates of hybrid numbers (Figure 1), we restricted data to reports from Sweden and the Netherlands, which constituted the most long-term and comprehensive datasets. Reports from Norway, Finland, Germany and Denmark were scattered, and in this study only used to describe the geographical distribution of hybrids. In Sweden, our estimated numbers of hybrids and their distribution were verified directly with local ornithologists at most sites, to avoid misinterpretations. Sightings in the Netherlands were screened for possible errors and, when necessary, enquiries with the observer were made to clarify details.

BARNACLE GEESE BREEDING IN SWEDEN
In total, six Lesser White-fronted Goose males (1.7 % of all released young with foster parents) were observed to produce hybrid offspring with Barnacle Goose females (Table 2, Figure 2). All the males originated from releases in two consecutive years, 1996 (2) and 1997 (4). One male bred in 1999 at Öster Malma close to Stockholm ( Figure 2). This male (ID 431; Table 2) and its two hybrid offspring were taken out immediately at the breeding site, hence these two hybrids were not included in estimated production of free-flying hybrids ( Figure 1). From 2002 and onwards one or more of the first-year birds (dark grey), and (b) the Netherlands during wintering. The Dutch numbers are the estimates used in our analyses (solid grey), and addi tion ally the highest possible number (hashed grey), based on maximum numbers at each site, a total which includes potential duplicate counts and likely is a biased overestimation. For comparison, we show (c) the cumulative number of hybrids produced in Sweden (cf. other five males were observed to breed ( Table 2) in colonies of Barnacle Geese in the archipelago near Söderhamn and Gävle, along the southwestern coast of the Bothnian Sea, or at one site close to Krokom in Jämtland County ( Figure 2, Table 2). In total, 42 hybrid offspring could be assigned to the known mixed breeding pairs in the period 2002-2013. The number of young and the number of years with successful breeding varied among mixed pairs, with one pair raising in total 15 hybrids during six breeding seasons (male 841, Table 2). In addition, seven (5 + 2) hybrid offspring were reported but without the ID (colour ring code) of the male Lesser White-fronted Goose having been determined. Thus, our data indicate a total production of at least 49 fledged and free-flying LWfG × BG hybrids in Sweden between 2002 and 2013.
Besides the birds released within the Swedish conservation program, an early mixed breeding pair in Sweden was reported already in 1985. According to Kyrk (1987) and Kampe-Persson & Lerner (2007) this pair consisted of an escaped Lesser White-fronted Goose male (from a park) breeding with a female Barnacle Goose in a feral population of the latter species close to Stockholm, thus having no connection to  -Karta som visar lokaler nämnda i denna studie, med avseende på blandpar av fjällgås Anser erythropus × vitkindad gås Branta leucopsis och deras hybridavkomma.  Figure 1).

AND WINTERING SITES
Reports of hybrids LWfG × BG were made annually from the winter 2004/2005 onwards ( Figure 1). In Sweden, after the first known mixed breeding in the wild (in 2002, see  mean: 8.7). Four hybrids were culled in Sweden prior to 2015, and another eight during 2015-2017. The cumulative number of hybrids produced was not reflected by increasing numbers being observed, neither at staging sites in Sweden nor at wintering sites in the Netherlands. Instead, the numbers in both countries remained at fairly stable levels, but started to decrease immediately after the last known successful breeding in 2013 (Figure 1). In Sweden, observations of hybrids were mainly done during the post-breeding period in August, lasting until onset of autumn migration in September-October, and during April-June. In the Netherlands, most arrivals were recorded in October, likely to directly follow departures from Sweden, where number of reports decreased simultaneously (Figure 3). Monthly number of reports in the Netherlands showed a typical pattern for wintering birds, with birds leaving the country again mainly in April and May, associated with departure of Barnacle Goose flocks. Concurrently, numbers in Sweden increased again, especially in April (Figure 3).

DISPERSAL AND DISTRIBUTION
The majority of reports on LWfG × BG hybrids originated from Sweden (68 %, N = 1,266) and the Netherlands (26 %; Table 1, Figure 4b). Scattered observations were also reported from Germany (4 %) and Denmark (2 %). Only one observation was reported from Finland and none from Norway. The distribution of hybrids (Figure 4b) shows large similarities with the resighting data of the ringed male Lesser White-fronted Geese of the mixed pairs (Figure 4a) with absolute majority of reports in Sweden and the Netherlands. Apart from that, we find higher numbers of reports and a more scattered distribution for hybrids compared to their parents (Figure 4). In Sweden, clusters of reports and/or high number of hybrids were mainly found along the Bothnian Sea coast, close to the cities of Gävle and Söderhamn and an area near Krokom in the province of Jämtland, reflecting the known breeding sites of mixed pairs (Figures 2, 4). Swedish clusters also include three stopover sites: Lake Hjälstaviken in the province of Uppland, the coastal bay Bråviken in the province of Östergötland and the island Gotland in the Baltic Sea (cf. Figure 2). Further south, clusters of reports are found along the Wadden Sea coast in southern Denmark and the coasts of Schleswig-Holstein and Niedersachsen in Germany (during spring migration, cf. Figure 2) and in the Wadden Sea area and Delta area of the Netherlands (autumn and winter). Lake Hjälstaviken was the single most important site in Sweden, with 554 (64 %) of 864 reports, and confirmed records of all but one of the five known mixed breeding pairs. In the Netherlands, most sightings of hybrids were made at the West Frisian island of Schiermonnikoog in Wadden Sea (106 of 330 reports, 32 %, cf. Figure 2). At Lake Hjälstaviken, the maximum count was eight hybrids at a single occasion, at Schiermonnikoog Island the corresponding number was six individuals.

ESTABLISHMENT OF LESSER WHITE-FRONTED × BARNACLE GOOSE PAIRS AND HYBRID DISPERSAL
Our analyses indicate increasing numbers of hybrid LWfG × BG from 2004/05 until 2013/2014, especially in Sweden (breeding, post-breeding, and staging) and in the Netherlands (wintering). These two countries supported the most important staging and wintering sites for both Barnacle Goose and Lesser White-fronted Goose from the Swedish breeding population during this period (Ganter et al. 1999, Koffijberg et al. 2006. Even if we need to be careful about the precision in our annual estimate of numbers of hybrids, as our data mainly rely on non-systematic observations, we find that the increase of hybrids coincided with the establishment of six mixed breeding pairs in Sweden, all consisting of Lesser White-fronted Goose males and Barnacle Goose females (Table 2). Our finding that only males raised by foster parents established mixed breeding confirms earlier studies (see ten Cate (1985) for review). Fabricius (1991) studied partner preference in geese using an experimental set-up where Greylag Goose goslings were imprinted on Canada Goose parents. In this experiment, 14 % of the Greylag goslings-all males-later paired with Canada Geese. Hence, we assume that imprinting of the released Lesser White-fronted Goose males, raised by foster parents, have played a key role to explain the establishment of mixed pairs in Sweden. Notwithstanding these assumptions, we find that additional circumstances may have further enhanced the establishment of mixed pairs between the two species.
The observation that all males involved in mixed pairs originated from two specific years calls for an explanation. Conditions in Swedish Lapland and methods used by the project during the release in 1996 and 1997 were similar to other years (Andersson 2019). Hence, we cannot find any details in the performance of the release projects per se that may have induced a change of behaviour of the released birds.
We speculate that contemporary use of sites by non-breeding birds of the two species during summer likely initiated social bonding for Lesser White-fronted Geese males to Barnacle Goose flocks. During late 1990s, when mixed pairs were established, the Barnacle Goose population was expanding its breeding  ± distribution in Sweden (Bengtsson 2007), at least partly due to naturalized populations of the species (Kampe-Person 2021). Breeding pairs settled along the southwest coast of the Bothnian Sea (e.g., around Gävle and Söderhamn), but also at inland sites (BirdLife Sweden 1998-2005. As a result, there were several new and small, but growing, populations of Barnacle Geese in these regions. At the same time, an increasing numbers of non-breeding Lesser White-fronted Geese (including previously released birds) were found to use the same region in the Bothnian Sea during summer. Because of these parallel changes in spatial distribution of the two species, the likelihood for initiation of pair bonding across the species barriers increased. Even if the pairs were not established during their first summer, the Lesser White-fronted Goose males may have established social bonds to the Barnacle Goose flocks, followed by true pair bonds forming in later years. At least for Barnacle Geese, it is known that more than half of the pair bonds are established during summer (Black et al. 2014). In addition, the period of primary moult is known to facilitate exchange of geese from different flyways, probably due to males being attached to females and thereby also adopting their migration traditions (Koelzsch et al. 2019). As female site fidelity is high in goose species and typically determine the future breeding site (Black et al. 2014), pair bonds established during moulting may explain the breeding distribution of mixed pairs as found in this study. All five Lesser White-fronted Goose males (all with individual codes of coloured leg rings) that later paired with Barnacle Goose females, and produced free-flying hybrids, were observed at sites in the northern part of the Netherlands during their first winter after being released. Even if our data do not allow a precise determination of when the winter distribution of these males changed, we still find that mixed families with young concentrated in the province of Friesland whereas sites earlier used by the males were abandoned. The regular wintering flock of Lesser White-fronted Geese, on the other hand, continued to use their own network of wintering sites (Koffijberg et al. 2006, Koffijberg & van Winden 2013. Especially the Wadden Sea island of Schiermonnikoog (Friesland) stands out in numbers of reported hybrids (32 % of all observations in the Netherlands, Figure 4). On this island, the proportion of Barnacle Geese originating from the Baltic (Swedish) breeding sites was larger than birds of Russian origin, as demonstrated by colour ring sightings (van der Jeugd et al. 2001). Thus, when paired with female Barnacle Geese, the male Lesser White-fronted Geese merely became a functional part of the Barnacle Goose flocks and adopted their network of stopover and wintering sites as well as breeding area.
This quick change in site use may explain the low degree of overlap of mixed pairs (and later hybrids) in relation to sites used by Swedish Lesser White-fronted Geese. In Sweden, hybrids have only been reported in three out of nine stopover sites regularly used by Lesser White-fronted Geese (Lake Hjälstaviken, Alnön near Sundsvall, and the Bråviken area, cf. Figure 2). Of these sites, both Lesser White-fronted Geese and hybrids (perhaps even all mixed pairs and hybrid offspring) frequently used only Lake Hjälstaviken. In the breeding area (and release site) of Lesser White-fronted Goose in the mountains of Arjeplog at 66°N, no hybrids or mixed pairs have been found despite high observer effort in the field during all years. The same applies to known Lesser White-fronted Goose pre-breeding areas like Ammarnäs and the important moulting site in Hudiksvall (cf. Figure 2). The northernmost site where LWfG × BG hybrids occurred was Krokom in Jämtland, at 63.2°N (cf. Figures 2, 4). Shared stopover sites were also found in the German Wadden Sea (important for Barnacle Geese as well; Ganter et al. 1999), but not at the important spring staging site for Lesser White-fronted Goose at Lolland/Falster (Roden Fed) in Denmark. From the seven regular wintering sites of Lesser White-fronted Goose in the Netherlands (see Koffijberg et al. 2006), mixed pairs or hybrids were only reported from Anjumerkolken and Ferwoude in Friesland and from Korendijksche Slikken in Zuid-Holland. Hybrids were never observed in Oudeland van Strijen in Zuid-Holland or Petten in Noord-Holland, which represent the core wintering sites in the Netherlands for Lesser White-fronted Geese (Koffijberg & van Winden 2013).
We found surprisingly few reports of LWfG × BG hybrids in Norway (0) and Finland (1), which suggest a high fidelity to migration routes and staging sites and hence a low degree of natal dispersal. In this context it is also important to note that Norway is within the Svalbard flyway of Barnacle Geese, which shows little overlap with the Russian-Baltic-North Sea flyway of the species (Black et al. 2014). Occasional sightings of Anser hybrids (likely LWfG × BG) are also known from the Russian breeding areas of Barnacle Geese (H. van der Jeugd, pers. comm.), but it is unknown whether these hybrids stem from mixed breeding pairs in Russia, Sweden, or elsewhere. Scattered reports in the more interior parts of Germany (Figure 4), where Barnacle Geese were rare in the past decades, may originate from captive populations. Both Lesser White-fronted Goose and Barnacle Goose are known to hybridize with other species in the wild and in captivity (McCarthy 2006, Kampe-Persson & Lerner 2007, Ottenburghs et al. 2016b. Hence, observations of hybrids are not necessarily linked to the Swedish conservation program for Lesser White-fronted Goose but may originate from captive or naturalized populations.

PRODUCTION AND DEPLETION OF HYBRIDS
Our data indicate that from 1999 to 2013, at least 49 LWfG × BG hybrids were produced in Sweden ( Figure  1). Even if a relatively high number of hybrids were produced over the years, this was not followed by a subsequent increase in the number of birds reported in the field. Instead, numbers remained at a rather stable level, followed by a decline after the last confirmed breeding in 2013 (Table 2, Figure 1). This can, at least partly, be attributed to management actions, as three of the five Lesser White-fronted Goose males in mixed breeding pairs were culled in 2007 (Table 2). however, our data suggest that this only had minor effect since a single male (ID 841; Table 2) was the father of most hybrids both before and after 2007. Furthermore, culling of 12 hybrids probably accelerated the depletion of hybrids, but the active removal of hybrids alone cannot explain the lack of coherence between the cumulative and reported numbers of hybrids (cf. Figure 2).
The observation effort and coverage by ornithologists in the Netherlands and Sweden are high, as well as the readiness to report rare taxa as hybrids (Snäll et al. 2011). Therefore we assume that the presented data mirror a representative development over time. We find only weak evidence for dispersal of hybrids outside the main migration route (see above). The annual survival in migratory Barnacle Geese is estimated to about 0.90 (Ebbinge et al. 1991). For Lesser White-fronted Geese, annual survival estimates in the 1990s and early 2000s ranged from 0.62 (first-year birds) to 0.89 (2 nd year; Schekkerman & Koffijberg 2019). If survival rates of any of the parent species would apply to the hybrids, the annual number of observed birds would have kept more in pace with the cumulative number of offspring produced. Consequently, this suggests that survival of hybrids may have been low.
Hybrids of species with high genetic distances may show low survival and fitness due to outbreeding depression (Allendorf et al. 2001). Outbreeding depression occurs when breakdown of co-adapted gene complexes, often related to immune system, or loss of species-or population-specific adaptations, put negative pressure on the individual (Todesco et al. 2016). Since Branta and Anser species show relatively high genetic distances (e.g. Ottenburghs et al. 2016b), the LWfG × BG hybrids may show lower survival rates compared to birds of the parent species.

CONCERNS FOR CONSERVATION
Observations in Sweden 2017-2019 suggest that 1-3 hybrids may still be alive, being recorded in Barnacle Goose flocks. In the Netherlands, at least one well-documented report from a single hybrid was made in autumn and winter 2017/2018. It is not known if these birds are linked to the former mixed breeding pairs in Sweden or are of other origin. Mixed breedings in Sweden have not been reported since 2013 and the method of using foster parents was abandoned in 2000. Hence, the risk of establishment of new mixed breeding pairs in the wild is very low.
In retrospect, even when hybrids were most numerous, we find no evidence that they ever posed a serious risk to the small breeding populations of Lesser White-fronted Geese in Fennoscandia, as suggested by Jones et al. (2008). This conclusion finds support in a recent genetic study where no signs of introgression, from any goose species, were detected in Lesser White-fronted Geese breeding in Sweden and Norway (nor Russia; Díez-del-Molino et al. 2020). Consequently, the hybrids in this case never influenced the genetics of the endangered taxon in focus of conservation efforts. Nonetheless, in theory, the outcome and scenario may have been different should no actions have been taken, and/or if survival and fitness of the hybrids had been higher. Our study shows that any translocation program using foster parents should seriously take into account the risk that hybrids between the target and foster species may be produced and pose a problem for conservation.
particularly tricky goose hybrids. BirdLife Denmark helped us retrieve data from DOFbasen. Stefan Wolf (OAG Schleswig-Holstein) helped out with access to the ornithological reports of Schleswig-Holstein in Germany. Erik van Winden (Sovon) made data available for the Netherlands. Two anonymous reviewers and the subject editor of Ornis Svecica provided helpful input to our manuscript.