Nancy Newfield Nancy L. Newfield has been watching hummingbirds at her Louisiana home and lots of other places since 1975. Nancy lost her amateur status years ago, and now writes and lectures on hummers. She is co-author of Hummingbird Gardens, reviewed elsewhere on this site. Nancy is also a licensed hummingbird bander and a recognized authority on hummingbird distribution, behavior, and taxonomy.

March 1997

Mixed Marriages?

"Oh, it is the biggest mix-up that I have ever seen...," goes the refrain from one of my favorite Irish ballads, which recounts the woes of a child whose parents are of different religions. "me father, he was orange and me mother, she was green." In biology, the offspring of such a "mixed marriage" is called a hybrid, a term we hear bandied about far too much.

Every time someone sees a bird that doesn't look quite the same as the ones they are used to seeing, the word "hybrid" seems to come up. Often, it is a matter of the observer noticing a characteristic that he has not seen before. Other times, the observer notices a difference in plumage that is within the range of variation for the particular species. Yet again, sometimes the angle from which light strikes the bird causes our eyes to play tricks on us. Don't forget, hummingbirds are masters of illusion!

Hybrids really are very rare. In order for hybridization to take place, a male of one species and a female of another species must be in the same locality at the same time. The two species must be closely related. You won't see a hybrid of a duck with a crow, for instance! And, both male and female must be in breeding condition.

Generally, it is thought that hybridization occurs when a member of one species is at the edge of its breeding range and cannot find a mate of its own species. Appropriate nest sites for the female must be available as well. There may be logistical difficulties as well. The relative sizes of the two parents could also affect the outcome. And, for some species normal nesting season is in winter, while others breed in spring or summer.

Nevertheless, in spite of all the natural obstacles that are supposed to prevent members of one species from breeding with members of other species, hybridization does take place. Most of the hummingbirds that inhabit the United States and Canada belong to closely related species. Members of the genera Archilochus, Selasphorus, Calypte, and Stellula are genetically quite close and a number of hybrid specimens are well-documented. But none of the species in this group would likely be capable of mating with more distantly related species such as members of the genera Amazilia, Cynanthus, or Anthracothorax. For them, the genetic distance is too great so that no offspring would be produced even if copulation between a male of one species and a female of another species occurred.

These closely related species exhibit strikingly different colors, but plumages follow a similar pattern. Likewise, courtship displays are often variations upon a single theme—elaborate dives and closer, more intimate "shuttle displays."

If you view the almost-350-member hummingbird family as you would look at a family tree, you will see that all of the widespread North American species are tiny twigs descending from the same large branch. These birds are, however, separate and distinct species, not just the same hummers dressed in different feathers.

In the field, we distinguish birds by differences in their color patterns, but the real differences are much more fundamental than that. Because females wear more subdued plumages, distinguishing among them can be quite a challenge.

In the hand, we look for distinctive shapes to wing and tail feathers. Bills show subtle differences to the shape and the placement of the nostril. Banders take careful measurements of the bill, wing, and tail, and of the widths of certain tail feathers to make sure of their species diagnoses. There is a range of measurements that is normal for each age and sex of each species. These characteristics are far too subtle to be determined by just watching a bird in the wild.

Suspicion of hybridization is usually based upon the presence of characteristics and measurements that are typical of two different species, or the presence of characteristics and measurements that are intermediate between two species. Because some characteristics are variable within a species, it is not always apparent whether an individual is exhibiting hybrid characteristics or simply showing a variation that is normal for its species.

The labelling of an individual as a hybrid is somewhat speculative, too uncertain to be accurate without a thorough "in-hand" examination. Unless that individual bird resulted from a mating that occurred in an aviary, it is often difficult to be certain of its parentage. In many other types of birds, we can see both parents tending a nest, but with hummingbirds the male parent is not evident. We just cannot be sure!

Most of the hybrid combinations that have been described involve species that inhabit the western United States. Ruby-throated Hummingbirds have fewer opportunities to mate outside their species. Yet, hybridization with the closely related Black-chinned Hummingbird has been documented on a couple of occasions.

I have heard speculation about the possibilities that hybridization between Ruby-throated Hummingbirds and some of the western species—especially Rufous Hummingbirds—that overwinter in the southeastern United States might be occurring. It is true that many of the wintering birds are still present when the Ruby-throats arrive back in their natal regions. And male Rufous, Black-chinned, and Allen's have been noted performing their towering dive displays on their wintering grounds.

But, other important elements seem to be lacking. Dive displays are used in an aggressive context as well as for courtship. I have not seen males of any of the western species performing the "shuttle display" that is supposed to be an immediate precursor to actual copulation. And, by the time female Ruby-throats begin arriving in the southeastern states, there is no shortage of displaying male Ruby-throats, so the temptation to accept a less optimal mate would seem unlikely. So far as I know, hummers do not engage in recreational sex!

It also seems unlikely that a female of any of the western species would mate with a male Ruby-throat before she had come into breeding condition. Supposedly, readiness for mating in female hummers is stimulated by nest-building activities. I have not heard of nest-building by females away from their own breeding areas. And, sperm-storage in female hummers is not known, so the notion that a female Rufous might mate with a male Ruby-throat and then return to the Pacific Northwest to build her nest seems to be without foundation!

All of the hybrids I've seen and read about are males. Females, because they are so similar to each other, are nearly impossible to detect. Surely there are as many female hybrids as there are males. But, they will be much more difficult to distinguish.

According to everything I've read about hybridization, offspring of mixed matings are supposed to be sterile, that is, incapable of producing offspring themselves. But, some hybrids of other types of birds are known to reproduce. We really don't know anything about reproductive capabilities of hybrid hummers, and surely recognizing the progeny of a hybrid parent—if such an event should occur—would be a considerable challenge.

Not all oddities are hybrids, however. In the years I've been studying hummers, I've seen a Ruby-throat that was smaller than normal, female Rufous and Black-chinneds that were much larger than most of their kin, and a melanistic Ruby-throat that had a brown back and purple gorget feathers. Recently, I handled an adult male Ruby-throat on which the shafts of his tail feathers were a bright buff, instead of the usual inky black. And I banded a Black-chinned that had wing bars. "Oh, it is the biggest mix-up that I have ever seen..."

Baton Rouge (LA) area readers: Barbara Nielsen and I, along with landscape architect Steve Schurtz, will present a program "Living with Hummers" for the Hilltop Arboretum on Saturday, 22 March 1997. The program is scheduled for 9:00-11:30 AM in the L.S.U. College of Design Auditorium, with registration beginning at 8:30 AM. Registration costs $10.00 for members; $15.00 for non-members. Telephone 504-767-6916 to preregister or for more information. Last year's program sold out quickly! Y'All come!

Happy Hummingbirding!

Copyright © 1997
Nancy L. Newfield
Casa Colibri
Metairie, LA

Previous Hummer Notes columns: July 1996 | August 1996 | September 1996 | October 1996 | November 1996 | December 1996 | January 1997 | February 1997