It is ironic that many of us view tropical birds as strange, and perhaps even bizarre, when they vastly outnumber temperate-zone species. The paradox hits home when many aspects of birds we take for granted are, instead, products of a temperate-zone bias—due to the geographic distribution of people who study birds.
My premise here is that temperate-zone birds are atypical and their unbalanced contribution to knowledge about birds has led us astray. Much conventional wisdom applies only to a select group of birds from temperate regions, birds that do not represent general adaptations of birds. Let’s begin by describing a true avian generality.
Most birds nest as monogamous pairs but young may, or may not, be fathered by the male of the pair. When the young are his, the pair bond is said to be genetically monogamous but, if some are not his, the pair bond is socially monogamous but not genetically so. When some young are regularly fathered by males other than the laying female’s mate, the species is said to have an extra-pair mating system.
Premise number two: This simple distinction between genetic and social monogamy has enormous consequences that underlie latitudinal differences in bird evolution as well as many familiar traits of birds in your backyard. Indeed, it underlies the temperate-zone bias.
To understand these consequences, consider the causes of an extra-pair mating system. This system will be favored by selection when two criteria are met, one for each sex.
For males, the dangers of leaving the home territory and inviting attacks by defending males must be outweighed by the increase in the number of young he fathers over his lifetime that results from these off-territory forays.
For females, an extra-pair mating system provides her with better genes for her babies than her social mates does; for example, genes that may offer more variation for immune system responses to parasites.
Male costs and female benefits of an extra-pair mating system depend upon having neighboring pairs at the same stage of the nesting cycle. A key requirement for males is to have many females nearby in the egg-laying stage because a female that has laid her eggs is no longer fertilizable. If a male has a low likelihood of encountering laying females in neighboring territories, the costs of encountering males defending these sites is too high.
For females, the best way to identify males with good genes, indirectly because genes cannot be seen, is to compare males at the same stage of nesting. For example, a female may assess a male’s viability by observing how much time he spends singing, But a male without a mate can sing more than a male feeding nestlings, regardless of their relative health. She needs to compare apples with apples and oranges with oranges to make her assessment of possible extra-pair sires. Thus, for both males and females, synchronous breeding must occur before an extra-pair mating system can be favored by selection.
Breeding synchrony is, of course, related to latitude. The temperate-zone climate forces synchrony upon breeding birds so we would expect to find extra-pair mating systems more prevalent there. DNA fingerprinting studies have indeed shown that these systems are so common that recent reviews emphasize that extra-pair copulations and fertilizations outside the pair bond occur routinely.
But are extra-pair mating systems the norm? Almost all studies of parentage are of temperate-zone breeders with high breeding synchrony. One result of this system is that males have relatively large testes. Male tropical perching birds have much smaller testes, often one-tenth the size of those of temperate-zone birds of similar body sizes, which predicts few extra-pair mating systems in tropical birds.
Direct tests have shown that breeding synchrony, not latitude per se, predicts the occurrence of extra-pair mating systems. Synchronously breeding Clay-colored Robins in Panama have high numbers of extra-pair young whereas asynchronously breeding Dusky Antbirds, Buff-breasted Wrens, and Mangrove Swallows do not. Close relatives often differ too. Blue-headed Vireos breed asynchronously and have a genetically monogamous breeding system whereas Red-eyed Vireos in the same woodlot breed synchronously and have an extra-pair mating system. The tropics show us that this system is not typical of perching birds, as thought by most biologists.
The temperate-zone bias is exemplified by the purported function of the male hormone, testosterone, in territorial behavior. Testosterone is generally believed to underlie singing and aggression in territorial birds. Recent research on tropical birds suggested that testosterone has little to do with territoriality or its evolution.
Instead, we predicted that testosterone functions to enhance male performance in extra-pair mating systems, which are normal in temperate zone birds but uncommon or absent in permanently territorial tropical species. This prediction for the extra-pair function of testosterone is supported by recent studies of a tropical bird and a temperate-zone species lacking an extra-pair mating system.
The contrast between temperate and tropical avian territoriality has important implications for the evolution of avian mating systems. Text books promote the idea that testosterone in males induces territorial behavior, singing and, conversely, its absence in females explains why they do not sing or defend territories. This is not true for most tropical species, where testosterone is not elevated in breeding males, females sing as much as males, and there is no seasonality to territorial behavior.
The temperate-zone bias suggests that sex roles differ consistently. Males defend the territory, sing, and feed nestlings whereas females nest build, incubate, and generally stay out of territorial defense. Now we know that these "classic differences" in sex roles are really based upon the extra-pair mating system and are not true of most perching birds.
Females of tropical species sing and defend territories. Female singing in tropical birds and singing by males often results in spectacular duetting, something absent in temperate-zone springs no matter how melodious the birds may be. With similar duties in breeding, tropical females are also often as bright as males. I hope you begin to see now why tropical birds may be different, but temperate-zone birds are definitely odd!