WHY DO WATERFOWL HYBRIDIZE?

Here we describe three possible causes of hybrids in waterfowl.

RAPE AS A SOURCE OF HYBRIDS

Drake Pintails attempting to force copulations on the harassed hen.
Photo by Terrence Dini

Many hybrid waterfowl are thought to result from forced copulations (rapes) occurring between species.  Forced copulation in waterfowl is a tactic by which males sire offspring outside their pair bound, a Darwinian benefit when additional young are sired with females of their own species.  But female waterfowl vigorously resist forced copulations, making haste, aggression, and an explosive penis key components of successful rapes.  Presumably the haste and aggression of forced copulations (photo above) sometimes results in males attacking females of other species, thus generating hybrids.  

All hanging out: Pintail drake chasing
a female just after a likely rape attempt.
Photo: Terrence Dini

For a dorky overview of sex in ducks, check out this Modern Farmer video.  The serious content of this video derives from Patricia Brennan’s excellent scientific work on the coevolution of the male and female reproductive anatomy of waterfowl.  She is the scientist that demonstrated the explosive erection of waterfowl penises.  This video from Patricia Brennan shows that male Muscovy ducks can erect their 20cm penis in less than 1/3 of a second and ejaculation occurs almost instantly thereafter.  Clearly the penis and its explosive erection facilitate forced copulations in waterfowl.  

Mallard x Northern pintail   
Photographer unknown.

Rape has been little tested as an explanation for the high frequency of hybrid waterfowl.  Modern genetic analyses, coupled with differences in behavior and size of the parental species, now make critical tests of this idea possible.  The parental species of hybrids can be determined from nuclear genes and the mother species of hybrids can be determined from mitochondrial genes.  Then, differences between species pairs that produce hybrids can be used to predict their sire (the father species).  

Males of only some ducks and geese use rape as a supplemental reproductive tactic.  Thus, mallards, pintails, gadwalls and green-winged teal are raping species, while shovelers, and Blue-winged and Cinnamon Teal seldom, if ever, pursue force copulations.  If hybrids are sired in interspecific rapes, then we predict the father species of hybrids will be a raping species.  Thus, we predict Mallard x Northern Shoveler hybrids will be sired by Mallards.  But, when hybrids occur between two species characterized by rape, predicting the father species is a bit more complicated.  When there is a large size difference between the parental species, then the sire is likely to be the larger species, because hens of the smaller species are less likely to escape forced copulation attempts from much larger males (as in male Gadwall x female Green-winged Teal hybrids).  But predicting the sire for hybrids between raping species that are more similar in size (as in Mallard x Northern Pintail hybrids), is less clear.  Mallards are larger, but are they enough larger to sire all the Mallard x pintail hybrids? On the other hand, male Northern Pintails are more motivated to force copulations than male Mallards, suggesting pintail sires.  Evaluating these alternatives will require a good sample of Mallard x pintail hybrids. 

For a more detailed treatment of using genetics to test the importance of rape as a source of hybrid waterfowl, check out this study of albatrosses.  Unlike waterfowl, male albatrosses have no penis, but paired males so aggressively attack females of their own and other species in rape attempts that these forced copulations sometimes result in the production of hybrids.  Genetic samples were available for only six hybrid albatrosses.  With hunter cooperation we should be able to do much better for waterfowl!

BROOD PARASITISM CAN ALSO GENERATE HYBRIDS

The other well-recognized source of hybrid waterfowl is interspecific brood parasitism, which occurs when one species lays eggs in the nest of another species.  The big assumption here is that parasitic young become at least partly sexually imprinted on their host species, giving them some propensity to mate with that species.  

The potential importance of brood parasitism can be addressed in a predicted contrast between Barrow’s x Common Goldeneye hybrids and Canvasback x Redhead hybrids.  Neither goldeneye is a raping species, but both are brood parasites, frequently laying eggs in the nests of other goldeneyes, sometimes of their own species and sometimes of the other species.  

Barrow's Goldeneye  Photo: Steve Mlodinow

Common Goldeneye  Photo: Steve Mlodinow

Barrow's x Common Goldeneye  Photo: Steve Mlodinow

The face mark and white on the wing are intermediate in hybrids and the dark
bar in front of the wing seen in Barrow's is missing in hybrids.

Because both goldeneyes sometimes lay in nests of the other species, we expect parasitic young from both species to be inclined to pair with their host species.  Thus, their parasitic habits may help explain the hybrids that are occasionally observed between them.  That both species are brood parasites means that no siring asymmetry is expected in goldeneye hybrids because females raised in nests of the other species should be equally inclined to pair with males of their host species. 

Note the contrasting prediction for Redhead x Canvasback hybrids.  Redheads frequently lay parasitic eggs in Canvasback nests, but Canvasbacks do not parasitize Redhead nests.  This means that Redhead chicks raised by Canvasbacks will be imprinted on Canvasbacks and inclined to mate with them.  Now ask yourself what happens when those parasitic chicks choose mates, but remember that adult sex ratios are male biased in North American ducks. 

A female Redhead raised by a Canvasback probably has a reasonable chance of mating with a male Canvasback simply because the sex ratio bias will mean than many Canvasback males will go unmated.  But the reverse is not true: A male Redhead raised by a Canvasback probably has little chance of mating with a female Canvasback simply because Canvasback females will all have plenty of Canvasback males to choose from.  
Thus, we expect Redhead x Canvasback hybrids to be sired by male Canvasbacks and not by male Redheads.  This is a neat contrast with the goldeneye prediction, and could be tested with a sample of hybrids between both of these species pairs.  Pretty ambitious, but doable with the help of hunters!

HYBRIDS GENERATED BY A SHORTAGE OF MATES

Rape and brood parasitism are interesting as sources of hybrids between species with broadly overlapping ranges.  Unlike in waterfowl, hybrids are unknown or very rare in most birds with broadly overlapping ranges.  But when the ranges of related species show small overlaps, it is often the case that one of the species will be rare in the area of overlap, making it difficult for individuals to find conspecific mates.  For a female forming a pair bond outside of her normal range, a scarcity of conspecific males can be a source of hybrids.  There is little controversy about this idea because it is a predictor of hybrids in many species.

Blue-winged x Cinnamon Teal.  Photo: Steve Mlodinow

See the hybrid?  Photo: Steve Mlodinow

Mate shortage seems like a reasonable cause of hybrids between American and Eurasian Widgeon.  Eurasian Widgeon are regular on both coasts of North America, but far less common than American Widgeon. Thus a shortage of conspecific mates may sometimes result in female European Wigeon wintering in North America pairing with male American Wigeon.  Hybrids between these species, including backcrosses are often reported, but only F1 hybrids can be used to test this prediction.

A shortage of potential mate has also been suggested as the source of hybrids between Blue-winged and Cinnamon Cinnamon Teal. Cinnamons are primarily distributed in the western U.S., while Bluewings are most abundant in the Midwest.  But, where one species is abundant and the other rare, a bird of the rare species that looses its mate may be forced to pair with a mate of the other species. Thus, when the first nest of a presumptive Cinnamon Teal female was found in North Dakota, three young from this nest that were hatched and raised in captivity proved to be Cinnamon x Blue-wing hybrids.  A shortage of conspecific mates may have forced this female to pair with a male Blue-winged Teal.  Teal form pair bounds on their winter range where they should have plenty of time to find conspecific mates.  But a female Cinnamon Teal that looses her mate might find no conspecific replacement if she was nesting at the edge of the breeding range in the midwest, and be forced to pair with a male Blue-winged Teal.