We have seen them in our neighborhood.
My son wonders if they mated, would the offspring be purple?
Why not?
And why couldn’t they mate?
Different breeds of dogs can.
Yeah, but bluebirds and cardinals aren’t different breeds of birds. They’re different species of birds. They’re both members of the same order, but they’re entirely diferrent genuses (geni?). It’s a little bit like asking “Why can’t people and chimps mate?” Because we’re different animals.
It’s not that they COULDN’T mate, it’s that they won’t mate.
Not sure why, but birds only mate with birds of the same kind. My mom is a bird FREAK, I’ll ask her and get back to you!
Genera.
Most species will only attempt to mate with their own simply because it’s not evolutionarily profitable to do otherwise. Since odds are very low that cross-matings will produce viable offpsring, it’s rather a waste of energy.
Cardinals are in the Finch (Fringilidae) family (just like me!), along with chickadees, buntings and…er…finches.
Bluebirds (Sialia sp.) are in the Turdidae Family, along with the familiar American Robin.
Both are membersof the Passeriformes order.
Matings (or attempts thereat) within a genus are more likely (though still rare) than matings across genera. Matings across families are even more unlikely.
To the second question (which may not be possible in the specified case, but could be sought in some other single species with multiple solid colors):
The colors of the feathers are not determined by mixing colors (or pigments) they are determined by specific gene combinations for colors. When the offspring is created, one set of genes (for red or for blue) “beats out” the other color’s genes to be inherited, so only one of the colors is passed on.
This is similar to hair color in humans. When a blonde viking weds a raven-tressed Greek, their children are not generally born with uniformly brown hair. Since hair color (and feather color and eye color, etc.) is actually controlled by different sets of genes doing different things, their children may actually have black hair or blonde hair (or possibly brown or even reddish).
Human skin color is slightly different in that the “color” is actually an expression of the amount of melanin in the skin. The genes control the amount of melanin, so two people with different skin colors may produce children that appear to be a blend of their colors. Since there are not separate genes for separate colors, the control of melanin production acts more like a rheostat (dimmer switch) controlling the amount of melanin rather than like a lighting board with separate colors being selected.
Occasionally, birds of different (closely related) species mate and produce offspring. However, those offspring are sterile. (Same process as a horse and a donkey producing a mule.)
(Of course, this doesn’t apply to bluebirds and cardinals, for the taxonomic reasons stated above.)