Many of the birds that nest in Maine are feeding nestlings now. Some have already fledged young. This time of year is a good occasion to consider the striking variation in the reproductive biology of birds.
Ornithologists use the term life history to describe reproductive traits of a bird species. Aspects of the life history include number of eggs per nest, number of nesting attempts per year, age at first reproduction and life expectancy. To understand some of the variation in life histories of birds, we can start with a discussion of a household budget.
Warren Buffett and other extremely wealthy people don’t have to bother with keeping a budget for their families but most of us do. The first budget items we have to pay each month are our necessities: mortgage or rent payment, groceries, heat, electricity, transportation. Let’s call these budget items maintenance.
If we have money left over at the end of the month, we can invest that money. We may choose to save for retirement, for a bigger house, for education, for a birding trip to Antarctica.
Birds, like all living things, have an energy budget to manage. Every bird has a limited amount of food it can find and eat and therefore its energy budget is limited. A bird must spend much of its energy budget on maintenance. In this case, maintenance means the costs of staying alive (basic metabolic needs). Any remaining energy is then available for investment. The investment for birds is a genetic investment. By raising young, the parents get their genes represented in future generations.
A person just joining the workforce is likely to have a starting salary lower than she can expect later in life, leaving little to invest in the future. Some birds follow the same pattern. Albatrosses may not reproduce until they are ten years old. Presumably, these oceanic wanderers become proficient over time at finding their patchy prey. These birds only reproduce after their energy budget becomes larger.
A local example of delayed reproduction is the Bald Eagle, reproducing only in the fourth or fifth year of life.
Some people are willing to be mobile to make more money at the beginning of their careers. White-winged Crossbills provide an avian parallel. These birds depend on conifer seeds for their energy. Conifers produce huge bumper crops of seeds every few years. The crossbills move throughout the northern portions of the northern hemisphere to find areas where the local conifers are heavy with cones. With large amount of energy available, the crossbills immediately start nesting. In fact, White-winged Crossbills have been reported nesting in very month of the year! I remember seeing White-winged Crossbills on nests in northeastern Vermont one year in January with the temperature 30 degrees below.
A high salary is not the sole consideration for employment. Some people find great satisfaction in their job even though the salary is modest. Let’s travel to the tropical oceans. These oceans are usually nutrient poor and have low fish abundance. Fish-eating tropical birds like the Sooty Tern have little energy to invest in reproduction. They produce a single egg per nesting attempt. Compare that to the typical clutch size of three eggs for temperate species like the Common Tern.
If you have some money to invest, where do you put it? Some cautious investors will invest in treasury bills, certificates of deposit or a savings account. Investors willing to take a risk may choose to invest in several speculative stocks.
In birds, egg size determines the amount of risk. Some birds, like American Woodcocks, lay fairly large eggs for their size. These eggs contain large amounts of yolk that provides nutrition for the developing embryo. When the chick hatches, it is feathered and capable of sight. The chick can start looking for food soon after it breaks free from its shell. This sort of development is termed precocial.
On the other hand, songbirds take a riskier approach. The egg of a Red-eyed Vireo or Gray Catbird is relatively small and has only a modest amount of yolk. When the young hatch, they are blind and unfeathered. They are utterly dependent on the parents for warmth and food. This development type is called altricial. Altricial birds typically are not capable of finding their own food until at least 11 days after hatching.
Precocial development, in which the mother makes a high investment in each egg, usually results in some offspring surviving. Altricial development is riskier because the parents must gamble on finding sufficient food after the chicks hatch to complete development.
[First published on July 24, 2011]