Life History 1 We envy birds their ability to soar above the mundane concerns of our daily lives



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Life History 1

  • But the lives of birds are anything but free of care

  • Birds face a constant struggle against overwhelming odds to keep themselves alive and raise their young

  • Rreproductive potential of birds is tremendous, and populations with no limits will grow exponentially

  • But birds rarely achieve this exponential ideal, because of the limits imposed by their environment

  • Populations of most species of birds are remarkably stable over time

  • Density-dependent factors, like resources and predation, and density-independent factors like sunshine and rainfall combine to regulate avian populations

  • To understand how these forces act on populations of birds we have to understand how bird populations are structured

  • A population’s age structure is determined by the number of individuals of each sex in each age class

  • One of the simplest ways to keep track of how a population is growing and changing is to construct a life table of that population

  • Life tables record births and deaths (vital statistics)

  • Life tables can be constructed from many types of information, such as:

  • population census

  • skull measurements on dead animals

  • birth and death dates on tombstones

  • The original use of life tables was to build “actuarial tables” for the insurance industry

  • Insurance agents needed a reliable way to estimate the probability that a potential client would die within a certain interval of time

  • R = lx bx , summed over all age intervals:

  • R = ∑ lx bx (discrete form) ...or..

  • R = ∫ lx bx dx (continuous form)

  • Humans as a species should fall under the logistic model of population growth

  • Logistic model assumes

  • Continuous breeders - like bacteria, humans…

  • Growth is limited

  • Life tables reveal many interesting things about how populations are structured

  • Show us that birth and death rates for most species are different for different age classes, and differ between males and females

  • Show us natural selection can act differently at different ages, or between the sexes

  • Pattern of age-specific births and deaths also varies from species to species

  • One way to visualize these different patterns is to construct a survivorship curve, a plot of lx against time (remember, lx=ax/a0)

  • Survivorship curves for various sheep and deer show Type I survivorship

  • Notice that these curves can be different for males and females (a and d), and can vary from one habitat to the next (c - shrub vs. chaparral)

  • This diagonal survivorship curve, shown here for the lizard Sceloporus, represents a different pattern of survival

  • For these animals there is a constant probability of death at all age intervals

  • This type of curve, called a Type II curve, is also typical of birds

  • Birds have a very high mortality rate in their first year of life, and a fairly constant rate after that

  • The final type of survivorship curve, Type III, is typical of many fish and most invertebrates

  • Juvenile mortality is extremely high, but the survivors of this initial blitz enjoy a low mortality later in life

  • The crawfish, for example, has several hundred young at one time, but only a few survive the first few weeks

  • A single fish can lay up to one million eggs, but most of these perish as juveniles

  • Life tables show us that different species have different life histories

  • Life history traits are the pattern of age-specific birth and mortality of a population or species

  • These traits are ultimately under genetic control, so they not only vary to some extent within a population, they can be inherited

  • Because they can vary and are inherited, they can also evolve

  • Life history traits include:


  • Size (large or small body)

  • Rate of growth and development (fast or slow)

  • Age at first reproduction (early or late)

  • Reproduce all at once OR in a series of events

  • Have many small offspring OR few large offspring

  • Reproductive Effort (the total energy allocated to repr.)

  • Longevity (short or long life)

  • Dispersal ability (near or far)

  • All of these traits affect survival, so natural selection should favor certain combinations over others in certain environments

  • Making one choice precludes others (ex. many young = small young)

  • And certain traits are always associated with other traits (ex. longevity, iteroparity)

  • If an organism puts too much energy into reproduction, it might not have enough energy to maintain itself, and might die before reproducing again

  • If it puts less energy into reproduction, fewer offspring might survive, but the effort could be repeated

  • The sum of all the choices an organism makes creates its life history strategy

  • A life history strategy is a set of co-adapted traits designed by natural selection to solve a particular ecological problem

  • Every choice the organism makes on how to allocate its limited lifetime energy involves a trade-off, affecting all other choices

  • Life history strategies represent a compromise between conflicting demands

  • Strategies represent a choice about how to allocate limited time and resources, how to balance reproduction and survival

  • Many strong forces limit the growth potential of bird populations: climate, limited habitat, predation, diseases, accidental death, and starvation

  • These forces and others work to keep bird populations in check

  • Loss of habitat is becoming increasingly critical for the survival of many birds…

  • Climate imposes broad limits on ranges and population size

  • directly through extremes of temperature and rainfall

  • indirectly through the effects of climate on food supply (seeds, fruits, and insects)

  • The passing of El Niño always leaves several million seabirds dead in its wake

  • Changes in salinity and temperature of the seawater affect the anchovy populations they rely upon for food

  • We often see such events at second hand in irruptive invasions

  • During years of poor seed production, large flocks of northern coniferous birds like the Evening Grosbeak and the Purple Finch invade the south, showing up at bird feeders and in roadside flocks

  • Snowy Owls may also invade in large numbers when lemming populations are at a low ebb

  • In the invasion of 1945-1946 over 14,000 Snowy Owls were seen in southern Canada and New England

  • Lack of suitable habitat is a limiting factor for many species

  • In some cases these limitations are natural, but increasingly they are due man

  • Lack of habitat often acts on bird populations through limiting food supply

  • Lack of suitable nesting habitat can also be a major problem

  • The Red-cockaded Woodpecker has become increasingly rare as the old Longleaf pine forests it used to nest in have fallen to the lumberman’s axe

  • Trees must be at least 80-100 years old to be suitable for nesting

  • These older trees must also be infected with red heart fungus

  • The fungus softens the heart wood of the tree just enough for the woodpecker to carve out a nest

  • They then drill additional resin holes, which elude a sticky trap for predatory rat snakes that climb the pines searching for the nests

  • Populations of these woodpeckers are limited by the number of old infected pines

  • Not all habitat changes caused by humans are harmful

  • In New York State, the decline of small family farms has caused widespread secondary succession

  • As these old field reached the shrubby stage, they once more became suitable nesting habitat for Great Blue Herons

  • Species is becoming far more common as it reoccupies its former northern range (McCrimmon 1981)

  • Predators can be a major limiting factor

  • A large Brown Noddy breeding colony of 35,000 birds (Anous solidus) in the Dry Tortugas was plundered by rats for 19 years until only 400 birds survived (Robertson 1964)

  • Predators can include mammals like rats, cats, skunks, foxes and weasels, reptiles like snakes and gators, other birds like hawks, owls, and crows

  • Marra (2011) ~80% of mortality in juvenile Catbirds in Washington suburbs was due to predators, of whom 47% (ironically) were cats!


  • Smithsonian/Fish & Wildlife report 2013 – cats are deadlier than we suspected

  • Cats kill 2.4 billion birds per year (and 12.3 billion mammals)

  • 2-4 times higher than previous estimates!

  • In the Saskatchewan River delta in Canada, about 10% of the ducklings born each year are snatched to a watery grave by the Northern Pike

  • Even predatory birds are themselves limited by predation

  • Lockie’s (1955) study of the Short-eared Owl (Asio flammeus) found that only 2 of 24 nests managed to fledge young due to predation by crows and foxes

  • Diseases and parasites are also a major source of mortality in birds

  • Large numbers succumb to the same types of fungal, bacterial and viral pests that we do, plus scores of nematodes, cestodes, ticks, mites and other invertebrate pests

  • In one sample of 175 scoters of various species, Bourgeois and Threlfall (1982) found that 91% of the birds were infected by one or more of 45 different species of parasites

  • Peters (1936) drew up a list of 198 different ectoparasites known to infect some or all of 255 species of birds found east of the Mississippi


  • These parasites included lice, fleas, various flies and their larvae, mosquitoes, black flies, leeches, ticks, and several species of skin and feather mites

  • Baby birds in the nest are especially prone to parasite infection

  • The rapid development of nestlings might be partly an adaptation to get them out of the nest before parasite loads build up to critical levels

  • In 1973, a colony of Sooty Terns nesting in the Seychelle Islands deserted their entire brood due to a heavy infestation of ticks

  • Leaving all the remaining young behind to starve…

  • We ourselves have become a primary source of avian mortality

  • directly by hunting birds for food or sport

  • indirectly by converting their habitats to farms and factories

  • Over 57 million birds are fated every year to become road kill

  • Another 80 million birds perish every year from colliding with plate-glass windows

  • Food limitation has historically been viewed as the most critical factor limiting bird populations

  • Peter Grant’s study of Darwin’s Medium Ground Finch is a classic example of how closely reproductive success is tied to food availability

  • Biomass of finches was strongly correlated with seed biomass, and the loss of these seeds caused widespread starvation

  • When food is limited, adult birds may be unable to breed

  • When food is abundant, adults have enough food left over to provide an optimal diet for nestlings and fledglings

  • The importance of food supply to reproductive success has been demonstrated through

  • manipulating brood size

  • removal of mates or helpers

  • artificial food supplementation

  • These studies show that parental birds must pay a substantial energetic cost to meet juvenile food demands

  • Studies by Tom Sherry and Dick Holmes have shown a strong relationship between reproductive success among wild passerines and food resources (Holmes et al. 1986, 1991)

  • Reproductive success in Black-throated Blue Warblers is strongly correlated with caterpillar biomass

  • Colonial wading birds are known to be typically food-limited birds

  • Rapid increase in populations of several species of wading birds in Louisiana is due to supplemental food from crawfish farms, new food source peaking during the wading bird nesting season

  • Having considered the many ways in which birds can die, the enormous reproductive potential of birds makes very good sense-it is a highly adaptive trait

  • Birds must make up their population losses each year by recruitment

  • Recruitment of new birds into the population can come from survival of the young-of-the year, or through immigration from other local populations

  • Survival rates vary greatly from species to species, being higher for the larger seabirds, raptors, and wading birds, and lower for songbirds

  • An Albatross only has 3 chances in a hundred of dying in any given year

  • Song Sparrows experience over 70% mortality each year of their relatively brief lives…

  • Birds have a Type II survivorship curve

  • The rate of mortality is roughly the same from year to year for Herring Gulls

  • Notice the sharp dip in the first year, when the loss of eggs and chicks is quite high

  • If gulls make it through the first year, they have a reasonable shot at a long life

  • Juvenile mortality is uniformly high for most species of birds

  • The first graph shows percent survival for Ruffed Grouse, the second for Bronzed Grackles, during their first summer and autumn

  • This poses special problems for breeding birds, who must adjust the pattern of their reproduction to balance the pattern of their mortality

  • Consider these extremes of avian life history traits

  • Small birds, like ducks and robins, have relatively short lives

  • They must mature quickly and put enormous energy into raising several young in each of the few years they will live

  • Large birds, like eagles and albatross, have relatively long lives

  • They’ll be around for a while, so they can mature slowly

  • Once they reach reproductive age, they parcel out their lifetime’s reproduction in a long series of reproductive events, with relatively few young in each brood



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