Her er der lidt interessant læsning.
https://frontiersinzoology.biomedcentral.com/articles/10.1186/s12983-016-0153-1
Denne artikel udtrykker tydeligt, at der mangler forskning på området, men det lader til at det anderkendes at gåsehybrider oftest er fertile, selvom der ved stor genforskel ses sterilitet ved hunner. Her nævnes Canadagås og grågås som to arter med stor genforskel, men også som den hybrid der oftest forekommer. Så den egentlige faunaforurening afhænger naturligvis af, hvor ofte hybrider opstår, hvilket i følge artiklen lader til at være relativt sjældent, og om disse fortsat avler. Som jeg forstår artiklen lader det til at hybriderne fortsat avler inden for den ene af de to arter den kommer fra. Så generne bliver blandede med ikke helt på kryds og tværs og genforskellen vil jo så udviskes, efterhånden som hybridens afkom parrer sig med racerene dyr. Men som artiklen tydeliggør er der lav evidens på området. Og så er det jo meget muligt at mine engelsk kundskaber ikke slår til. Så læs hellere artiklen selv. Her er nogle uddrag, som dog ikke er helt dækkende for indholdet:
Generelt:
"Most hybrid geese are fertile; only in crosses between distantly related species do female hybrids become sterile."
"The incidence of hybridization varies among bird orders, with the Anseriformes (waterfowl: ducks, geese and swans) showing the highest propensity to hybridize. Over 60 % of waterfowl species has hybridized with at least one other species(...)"
"The knowledge of goose hybrids is clearly lagging behind. Several studies reported goose hybrids [18–21] or provided a description of local records of hybrid geese [22–24], but no study has been dedicated to the incidence of goose hybrids or their fertility."
"However, two surveys in Great Britain monitored the frequency of hybrid geese in 1991 and 2000 when occurrence of the most common hybrid (Canada Goose x Greylag Goose) was quantified. These hybrids represent less than one per cent of the British population of Canada Geese and Greylag Geese (0.33 % in 1991 and 0.11 % in 2000) [28, 29], falling in line with previous estimates from other bird groups"
Årsager til hybrider:
"Intraspecific nest parasitism (Når en art udruger en anden arts æg, såvidt jeg forstår) has been documented for several goose species (Table 3), but only three goose species are known to show interspecific nest parasitism, namely Greylag Goose, Snow Goose and Canada Goose [23, 41]. Interspecific nest parasitism could facilitate hybridization because hatching by a heterospecific foster parent might lead to sexual imprinting on the foster parent’s species and this may in turn lead to interspecific mate choice in the future"
"Fabricius [42] placed eggs of Greylag Geese in the nest of Canada Geese. The young Greylag Geese followed their foster parents to their wintering grounds. On return, all females (16) paired with Greylag Geese, whereas 5 out of 19 males paired with Canada Geese. Furthermore, some Greylag Goose males that lost a partner remated with a female Canada Goose, showing that these males were sexually imprinted on this species."
"Forced extra-pair copulations (often called “rapes”) have been reported in several species of waterfowl"
"Males of several goose species engage in forced extra-pair copulations, such as Greater White-fronted Goose [51], Brent Goose [52] and Canada Goose"
"Although interspecfic extra-pair copulations can potentially result in hybrid offspring, this behavioural mechanism seems of minor importance in the origin of hybrid geese, because of its low frequency and the low fertilization rate of such extra-pair copulations."
"Hubbs’ Principle or the Desparation Hypothesis states that the rarer species is more likely to mate with heterospecifics (...) Some birds will “make the best out of a bad job” and pair with a heterospecific mate"
Fertilitet og faunaforurening:
"At high genetic distances, for most species only male hybrids are fertile"
"For geese, the fertility of male birds at high genetic distances suggests the possibility of interspecific gene flow between distantly related species (e.g., Greylag Goose and Canada Goose), but if consequent backcrosses are sterile or inviable, then the possibility of interspecific gene flow is greatly reduced. On the other hand, the fertility of hybrids at low genetic distances (e.g., Greater White-fronted Goose and Greylag Goose) provides the opportunity of interspecific gene flow between closely related species."
"Most goose hybrids are fertile; only at high genetic distances do female hybrids become sterile."