The importance of nature versus nurture pervades psychology and biology. Do humans or other animals behave the way they do because of conscious, learned choice or because behaviors are encoded in their genes?

In flycatchers, we know their calls are innate; an Eastern Phoebe male does not need a tutor to learn how to sing “Fee-bee”. Sparrows, wrens and other songbirds must be taught their songs. Without a tutor like their father or other singing males, a young bird will never learn to sing a “proper” song and will have no luck attracting a mate.

The role of nature versus nurture is not always so clear.  As a case in point, we can consider the White-throated Sparrow. This species is a common breeding bird in the North Woods. The cadence of its beautiful song can be captured by the mnemonic, “Poor Sam Peabody Peabody Peabody.

In the spring and fall, White-throats frequently visit our feeders and some even overwinter. Observing White-throats at your feeder will confirm that there are two distinct morphs. Some have white stripes alternating with black stripes on the top to the head while others have tan and black stripes.

Tan or white stripes have nothing to do with the sex or age of the bird. Instead, the striping pattern is determined by a single gene that comes in two variants. One causes tan striping and one causes white striping.

This gene is adjacent to several other genes that are inherited together (a supergene). These linked genes influence different behaviors as we shall see.

Here’s a cool bit of information. White-striped females mate only with tan-striped males and tan-striped females mate only with white-striped males.

We have nature and nurture interacting here. A white-striped female is impelled by her genes to only be attracted to tan-striped males, a point for nature. However, the choice of the tan-striped male she will pair with is her choice, a point for nurture.

Elaine Tuttle and colleagues have been studying White-throated Sparrows in this regard for over 20 years. Of 1,116 pairs, only 18 pairs were white-white or tan-tan pairings.

So, why do females choose a mate with different striping colors? It all comes down to the supergene, the linked genes adjacent to the gene that determines head stripe color.

White-striped males are genetically coded to be aggressive, a behavior that can deter nest predators. However, they do spend a lot of time away from the nest, looking to cheat on their mate. They provide little parental care.

Tan-striped males are much less aggressive than white-striped males. They rarely try to cheat on their partner. Tan-stripers contribute significant parental care.

Tan-striped females are the least aggressive of all White-throated Sparrows. They are excellent foragers.

Female white-stripers are similar in aggressiveness to male tan-striped birds and are less efficient at finding food for their young that their tan-striped counterparts.

A successful White-throated Sparrow pair requires enough aggressive behavior to protect a nest and adequate foraging skill to meet the demands of hungry nestlings.

In a white-striped male x tan-striped female pairing, the male provides the aggression (with a fair amount of cheating on his mate) and the female does most of the parental care.

In a tan-striped male x white-striped female pairing, equality reigns. Both contribute creditable efforts at finding food for their young and both are mildly aggressive to deter predators.

A white x white pair would be very aggressive but inadequate at provisioning their young. A tan x tan pair would provide abundantly for their nestlings but would be too timid in nest protection.

Both morphs are maintained because the two pair types have equal lifetime reproductive success. They arrive at that success by splitting the parental duties in different ways, behaviors that are encoded in their genes.