Many of the birds that nest in Maine are feeding nestlings now.\u00a0 Some have already fledged young.\u00a0 This time of year is a good occasion to consider the striking variation in the reproductive biology of birds.<\/p>\n
Ornithologists use the term life history to describe reproductive traits of a bird species.\u00a0 Aspects of the life history include number of eggs per nest, number of nesting attempts per year, age at first reproduction and life expectancy.\u00a0 To understand some of the variation in life histories of birds, we can start with a discussion of a household budget.<\/p>\n
Warren Buffett and other extremely wealthy people don\u2019t have to bother with keeping a budget for their families but most of us do.\u00a0 The first budget items we have to pay each month are our necessities: mortgage or rent payment, groceries, heat, electricity, transportation.\u00a0 Let\u2019s call these budget items maintenance.<\/p>\n
If we have money left over at the end of the month, we can invest that money.\u00a0 We may choose to save for retirement, for a bigger house, for education, for a birding trip to Antarctica.<\/p>\n
Birds, like all living things, have an energy budget to manage.\u00a0 Every bird has a limited amount of food it can find and eat and therefore its energy budget is limited.\u00a0 A bird must spend much of its energy budget on maintenance.\u00a0 In this case, maintenance means the costs of staying alive (basic metabolic needs).\u00a0 Any remaining energy is then available for investment.\u00a0 The investment for birds is a genetic investment.\u00a0 By raising young, the parents get their genes represented in future generations.<\/p>\n
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.\u00a0 Some birds follow the same pattern.\u00a0 Albatrosses may not reproduce until they are ten years old.\u00a0 Presumably, these oceanic wanderers become proficient over time at finding their patchy prey.\u00a0 These birds only reproduce after their energy budget becomes larger.<\/p>\n
A local example of delayed reproduction is the Bald Eagle, reproducing only in the fourth or fifth year of life.<\/p>\n
Some people are willing to be mobile to make more money at the beginning of their careers.\u00a0 White-winged Crossbills provide an avian parallel.\u00a0 These birds depend on conifer seeds for their energy.\u00a0 Conifers produce huge bumper crops of seeds every few years.\u00a0 The crossbills move throughout the northern portions of the northern hemisphere to find areas where the local conifers are heavy with cones.\u00a0 With large amount of energy available, the crossbills immediately start nesting.\u00a0 In fact, White-winged Crossbills have been reported nesting in very month of the year!\u00a0 I remember seeing White-winged Crossbills on nests in northeastern Vermont one year in January with the temperature 30 degrees below.<\/p>\n
A high salary is not the sole consideration for employment.\u00a0 Some people find great satisfaction in their job even though the salary is modest.\u00a0 Let\u2019s travel to the tropical oceans. These oceans are usually nutrient poor and have low fish abundance.\u00a0 Fish-eating tropical birds like the Sooty Tern have little energy to invest in reproduction.\u00a0 They produce a single egg per nesting attempt.\u00a0 Compare that to the typical clutch size of three eggs for temperate species like the Common Tern.<\/p>\n
If you have some money to invest, where do you put it?\u00a0 Some cautious investors will invest in treasury bills, certificates of deposit or a savings account.\u00a0 Investors willing to take a risk may choose to invest in several speculative stocks.<\/p>\n
In birds, egg size determines the amount of risk.\u00a0 Some birds, like American Woodcocks, lay fairly large eggs for their size.\u00a0 These eggs contain large amounts of yolk that provides nutrition for the developing embryo.\u00a0 When the chick hatches, it is feathered and capable of sight.\u00a0 The chick can start looking for food soon after it breaks free from its shell.\u00a0 This sort of development is termed precocial.<\/p>\n
On the other hand, songbirds take a riskier approach.\u00a0 The egg of a Red-eyed Vireo or Gray Catbird is relatively small and has only a modest amount of yolk.\u00a0 When the young hatch, they are blind and unfeathered.\u00a0 They are utterly dependent on the parents for warmth and food.\u00a0 This development type is called altricial.\u00a0 Altricial birds typically are not capable of finding their own food until at least 11 days after hatching.<\/p>\n
Precocial development, in which the mother makes a high investment in each egg, usually results in some offspring surviving.\u00a0 Altricial development is riskier because the parents must gamble on finding sufficient food after the chicks hatch to complete development.<\/p>\n
[First published on July 24, 2011]<\/p>\n\n
Many of the birds that nest in Maine are feeding nestlings now.\u00a0 Some have already fledged young.\u00a0 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.\u00a0 Aspects of the life history […]<\/p>\n","protected":false},"author":146,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_s2mail":"","ngg_post_thumbnail":0,"footnotes":""},"categories":[433],"tags":[],"builder_content":"","_links":{"self":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/417"}],"collection":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/users\/146"}],"replies":[{"embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/comments?post=417"}],"version-history":[{"count":1,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/417\/revisions"}],"predecessor-version":[{"id":418,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/417\/revisions\/418"}],"wp:attachment":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/media?parent=417"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/categories?post=417"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/tags?post=417"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}