Sometimes the terms associated with birds and animals can seem quite confusing. I hope this list may help clarify some misconceptions for us. The following definitions are taken from a number of sources and hopefully will make clear the distinction of various classifications of birds (and animals).
Species: A single distinct class of living creatures sharing specific DNA and physical features that differentiate them from other species. Species are uniform in appearance (unless other factors, described later, apply) and produce offspring with the same characteristics, appearance and DNA structure. An example of a species would be a Gouldian Finch.
Subspecies: A division within a species usually created by geographic isolation from the main (nominate) species. A population within a population (usually) sharing most of the same physical characteristics and DNA structure. A physically distinct sub-unit within an otherwise identical group of birds or animals. A subspecies can be though of as a "race" within a species. Most subspecies distinctions are visible. An example would be a (Southern race, smaller) European vs. a (larger, Northern race) Siberian Goldfinch. Other than the size, they are the same bird.
Intergrades: A bird, animal or other organism which shows mixed characteristics caused by interbreeding of various subspecies. This usually happens where the ranges of subspecies overlap and are often referred to as intergrade zones. An example of an intergrade would be where the ranges of the Siberian and European Goldfinches overlap. The birds in those areas are of a medium size.
Hybrid: The result of a cross-breeding between two entirely different species. Most hybrids display visible characteristics of both parents. Some hybrids are uniform in appearance though others can vary considerably from one individual to the next. An example would be a Zebra finch crossed with an Owl finch. Some hybrids are sterile and are referred to as "mules". Others are fertile and can reproduce with another hybrid or with either of its original parental species.
Mutation: A visible effect of a change in the DNA structure of a bird or animal resulting in an offspring that display a one or more characteristics which are different that of the parent's. Mutations do occur in the wild though the phenomenon is fairly rare and so (except for certain isolated groups) not often seen. A mutation is a permanent change in DNA coding and can be consistently reproduced by selective breeding. It is a DNA alteration that can be transmitted and therefore inherited in subsequent generations. An example of a mutation is a Yellow-bodied Gouldian vs. a Green-bodied bird. They are the same species but there is a change in DNA structure represented.
Sport: "Sport" is a very old term which refers to a one-time visible change in an animal or bird which differs from a hybrid because it cannot be reproduced in subsequent breeding. It can be thought of as a "genetic glitch" and there seems to be no real explanation for the appearance of this type of bird or animal. An example of a "sport" would be the so called "half sider" Zebra finch where by one half of the bird appears male and the other appears to be female. The gender of the bird can be either male or female but the trait cannot be reproduced.
Breed: Breed is a term that can only be properly used to fully domesticated animals. Dogs, chickens, cattle and sheep occur in many various breeds by selective breeding over a period of many centuries. Breed means a genetically pure line of animal with consistent and predictable traits with little or no variation between individuals. An example would be a Great Dane vs. a Beagle. Both are dogs but are distinct breeds developed over an extremely long period of time. Both are certainly dogs and share over 99% of the same DNA but their physical appearance is extremely distinct. Canaries are probably an exception to the use of this term among cage birds because they have been domesticated for many centuries.
Albino (Albinism): Albinism in an inherited DNA defect whereby an animal or bird can not produce color pigment or melanin. Albinos are characterized by red eyes. I included this definition because not all pure white animals or birds are albino though many people assume that they are. If a white animal of any species has dark eyes, it is not an albino but rather a mutation. Albinism is a mutation as can be the so called "dark-eyed white" depending on what species you are referring to. For example, Shaft Tail finches can be bred as albinos or (more commonly) "dark-eyed whites".
I hope that these definitions are helpful to you. Having a basic command of the different terms helps reduce confusion when "talking birds" with our friends or customers!
On the subject of mutations among finches, I though you might find the following list to be interesting. Some of these cross-breedings occur in the wild though most have been recorded in captivity. Where there is documented evidence of wild bird hybridization, the listing will be followed by a (W), meaning that it has been documented in the wild.
This is only a partial list as there are literally thousands of hybrids recorded but I thought it would be fun to see some of the 'goings on' in collections around the world!
These are not in any particular order:
Cut Throat finch/European Linnet
Cut Throat/BH and WH Nuns
Gold Breasted/Black Rumped Fire Finch (W)
Gold Breasted/Senegal Fire Finch (W)
Green Singer/Grey Singer (W)
Red Cheeked Cordon Bleu/White Fronted Canary (W)
RC Cordon Bleu/Blue Capped (W)
Gouldian/Blue Faced Parrot Finch
Abyssinian Crimson Winged/Green Twinspot (W)
Crimson Winged/Swee waxbill (W)
*Many crosses between various Parrot Finch species*
Pin Tailed Parrot finch/Diamond Sparrow
Red Headed Parrot Finch/Society
Blue faced Parrot Finch/Society
*Many crosses between Orange cheeked, Red Eared, Swee, Black Rumped, Rosy Rumped waxbills, many (W)*
Red Headed Finch/Diamond Sparrow
Plum Headed/Star Finch (W)
Cuban Melodious/Zebra finch
*Many crosses of Fire Finch species in captivity and (W)*
Senegal Fire Finch/RC Cordon Bleu (W)
Shaft Tail/Plum Headed
*Many crosses of various Nuns and Mannikins with each other and Societies also*
Crimson Finch/Star Finch
Star Finch/Shaft Tail
Shaft Tail/Diamond sparrow (W)
Masked Grassfinch/Shaft tail (W)
*Many crossings of Melbas and other Pytillias with one another)*
Zebra/Red-billed Fire Finch
Zebra Finch/European Goldfinch
RC Cordon Bleus/Purple Grenadier (W)
*Many crosses of Siskins, Linnets, Gold and Green finches with each other and with Canaries*
This is only a fraction of recorded crosses but certainly shows how creative Nature can be when She wants to be! Enjoy your birds! KL