Authored by Phil Doerr. The female goldfinch has by now had over a month of “nesting”, including the demands of nest construction, egg laying, incubation, feeding chicks, and finally perhaps taking a break to fuel up for the fall molt and winter flocking with her “mates”….or she might start all over again with another male, in pursuit of her optional“bet hedging” reproductive strategy. This is the option where she produces a second brood with her genes plus the genes of a second male. Consequently, there are more young and there’s more genetic diversity among those young, all pluses, providing she can pull it off.
Curiously, American goldfinches may utilize a strategy peculiar to a relatively small number of bird species. This strategy involves invoking asynchronous hatching of the brood. So, one might reasonably ask just what is asynchronous hatching, and why does it happen?
The great majority of birds have evolved the reproductive habit of laying the “usual” number of eggs before the hen (usually!) begins to sit, and incubate the eggs. This continous attentive behavior keeps the eggs warm, and the embyoes growing until hatch time. And because the incubation of the eggs by the warmth of the hen began simultaneously the eggs all reach developmental maturity simultaneously and hatch at the same time. Why is this synchronous hatching such a commonly evolved reproductive strategy? Certainly it’s a plan that works well for waterfowl or game birds that all need to leave the nest at once and head to the water, or the bush! These chicks are precocial, meaning immediately mobile and able to feed for themselves. It also works well among birds for which nesting season food supplies are reasonably reliable. When the young are suddenly present all at once, the adults are motivated to bring home lotsa groceries to keep all the demanding “gapes” happy. Adults are, in fact, programmed by evolution to keep stuffing food into the always open gullets! This behavior also explains why a tiny common yellowthroat may be observed attempting to satisfy a much larger (and parasitic) brown-headed cowbird chick! And as long as there is always plenty of food, this system works well. Sometimes, however, adults may experience difficulty finding enough food and then all the chicks are likely to die, resulting in a missed breeding season. One might speculate that if an adult bird could “decide” to feed just one or two of the several chicks in the nest, they might survive while the others would die. An unhappy consequence for some, but at least the adults would produce some young that year.
Asynchronous hatching is an alternative strategy that may result when a female begins incubating before all her eggs have been laid…this is a “decision” that may happen by natural selection when variabity occurs. That is, while most females lay all their eggs before initiating incubation, sometimes a female may start to incubate when the second or 3rd egg is laid.
The outcome is that some embryoes start to develop a few days sooner, and hence hatch a few days before the others. The early hatching chicks get a real jumpstart on their nest mates, often monopolizing food at the expense of smaller mates to the extent they may not survive, and this may be particularly advantageous during a food shortage.
Asynchronous hatching is a strategy that has evolved in a number of raptors and other large birds, as an effective way for relatively long lived birds to produce at least one young in most years. Consequently, many hawks and falcons are among the most familiar groups displaying ansynchronous hatching, given that prey abundance and availability can be notoriously unpredictable. A first hatched, first served, strategy that awards advantage to those first in line!
An important question concerning American Goldfinches is “are there summer seed shortages dramatic enough, or frequent enough to provide a selective advantage?” There’s not much evidence for this, only the fact that because asynchronous hatching does occur regularly, there might an advantage often enough, that the trait persists in the population. Such uncertainty suggests an opportunity for more research to explore asynchrony in goldfinches.
Asynchrony allows some female goldfinches to favor their “first born”, but just why seems an elusive answer!
Thanks for reading, and thanks for all you do for birds. Remember that all those seeds in your “weeds” are important groceries to birds, and to the critters birds eat!
Phil Doerr ([email protected])
Wake Audubon Board