Just a note that ducks provide many great examples of hybridization dynamics, particularly invasive ducks. This recent paper on geese by Jente Ottenburghs and colleagues (“Hybridization in geese: a review”) shows that they are much the same. Lots of geese species, lots of hybridization, including cross-generic hybridization.
Most hybrid geese are fertile; only in crosses between distantly related species do female hybrids become sterile. This fertility pattern, which is in line with Haldane’s Rule, may facilitate interspecific gene flow between closely related species. The knowledge on hybrid geese should be used, in combination with the information available on hybridization in ducks, to study the process of avian speciation.