1 Matt Johnson HSU Wildlife 365 Ornithology ORNITHOLOGY (Humboldt State Univ. WILDLIFE 365) LECTURE 25 - AVIAN DEMOGRAPHY I. Intro to demographics [I skipped this (I) sp 01] A. Demography is the study of populations. B. Central to the study of avian demography is the quantification of lifetime reproductive output. This is the combination of four variables: 1. Age of first reproduction 2. reproductive success, or number of young fledged per reproductive event 3. the average survival of those young 4. longevity (itself a function of adult survival) 5. The term survival refers to the probability of surviving to a particular age, or it can be expressed as annual survival (the probability of surviving from a point in time to one year after that point). 6. The term fecundity refers to the number of young successfully reared. This can be lifetime fecundity, or annual fecundity. II. Life History Patterns A. General patterns - there are three major consistent patterns 1. Mortality is high at young ages, then usually stabilizes for adult birds. Also can be viewed in terms of survival. 2. Reproductive success and effort increase with age and experience 3. Fecundity tends to be inversely related to longevity among species, perhaps within species as well. 4. These patterns result in some very conspicuous changes in the life history patterns of different species. a. A sparrow is short-lived, breeds early, has low survival, but high fecundity b. An albatross is long-lived, breeds only after several years, has high survival, but low fecundity. c. In ecology, we call species having life history patterns like the sparrow as "r-selected", and species like the Albatross as K-selected. The reasons for those letters will become clearer later. The result, is that both species may in fact replace themselves with the same number of young. The sparrow breeds faster, but for not as long; the albatross (or 2 penguin) breeds slowly, but for a longer period. OVERHEAD B. Annual survival and mortality 1. In general, survival tends be higher in larger than small bodied birds, in seabirds than in landbirds, and in the Tropics than in the Temperate zone (your book says there is some debate, and there is, much evidence suggests the pattern is widespread, and I firmly believe that as more and more studies are completed, it will be clearer and clearer than tropical survival rates are high) Karr's study was among species; a better test is within species, or among closely related species, and these suggest t high survival in the tropics. 2. Also, a young bird's chances of survival are roughly half that of an adult. For small songbirds, survival is especially low in two discrete periods. Survival is only about 50% for the first 9 days out of the nest, when the young are poor flyers and even though their diet is still supplemented by their parents feeding them, they are vulnerable to predation. Next, once they finally become fully independent of their parents, survival is only 60% over a two week period, when they are susceptible to starvation before they fully hone their independent feeding skills. 3. Also, females in many species have slightly lower survival rates than males, leading to a sex-biased sex ratio. Several hypotheses exist for this. One, the incubating females are subjected to higher rates of predation than the males. Two, females simply invest more energetically into breeding, and suffer higher mortality accordingly. Third, in species where males are behaviorally dominant, females may be relegated to poorer quality habitats in the non-breeding season, and suffer higher mortality rates. 4. Most small birds live 2- 5 years. 20-30 years for larger waterbirds, such as egrets, and raptors. Ducks are somewhere in between. As noted earlier, rough correlation with body size. C. Fecundity. Remember, lifetime reproductive output is a combination of age of first repro, reproductive success per season, young survival, and longevity. The reproductive success per season is in turn a function of the number of attempts per breeding season, and the success of those attempts. 1. Remember, as we noted when we talked first about clutch sizes, there is a profoundly important trade-off in reproductive effort of success per attempt and probability of future attempts (survival). Many bird adopt strategies to temper investments in reproductive events to maximize their chances of reproducing again, perhaps with greater success. 2. One such trade-off strategy is in delayed breeding. a. Some birds breed at 1 year of age (e.g., their first spring) but others do not. 3 b. Why delay? If ya think about it, the age at first breeding determines the interval between generations, its extremely important in determining population growth. Thus, individuals that breed their first year would quickly outproduce those that delayed breeding, and therefore soon replace the others -- unless the costs of early breeding are severe enough to offset the benefits. c. Thus, for delayed breeding to remain a persistent strategy in birds, it must contribute to maximizing lifetime reproductive success, which may be most evident in long-lived species. For example, in Adelie Penguins, among all ages, mortality is higher (39%) for breeders than for nonbreeders (22%). For young birds this is especially true: birds breeding at 3 years of age (the average age is 4 years) suffer an incredible 75% mortality. Breeding is indeed very costly, and for longlived species, it behooves individuals to delay the investment of breeding until their chances of success are high, which gets better with experience. 3. A similar trade-off exists in differential effort over a birds' lifespan. [I skipped C- 3, 4, 5 sp. 01] a. If a birds lifetime reproductive effort is partly dependent on future reproduction, then it should temper its efforts to maximize survival especially when it is young, that is, when it has many potential opportunities in its future. As a bird ages, it should slowly increase its effort, for future opportunities become less likely with increasing age. b. Support for this predicted pattern lies in differential effort for nesting California gulls. OVERHEAD 4. Nesting success per season. a. The success of a nesting effort is affected by many things. The most important factors, listed in order of decreasing importance are: predation, starvation, desertion, hatching failure, and adverse weather. b. In general nesting success tends to be higher in high than in low latitudes, in cavity nesters than in cup nesters, and in larger species with hardy young than in smaller species with more vulnerable young. 5. Number of broods attempted per season. a. Depends on the length of the breeding season -- which is dependent not only the length of favorable climate and food conditions in the place of breeding, but also on how long the birds are there to exploit that season (usually longer for residents than migrants). b. Short growing seasons like those in the high temperate areas or the arctic have short breeding seasons with few broods. c. Birds with extended parental care also usually have only 1 brood. 4 d. Tropical birds usually have more broods than do temperate birds because of a longer breeding season. However, tropical birds usually require longer intervals between broods. Also, they often lay only every other day, have long incubation periods, and slow nestling growth rates. All of these patterns point to a hypothesized shortage of food for tropical birds, but this has proven very difficult to test rigorously. LECTURE 25 cont’d – BIRD POPULATIONS I. Population Dynamics. A. Intro. [I skipped A and B sp. 01] An understanding of how populations of animals change is central to the conservation of wildlife. We conserve species by maintaining viable populations. For several reasons, birds have played a dominant role in our understanding of animal population dynamics, or changes. 1. First, birds are diurnal and easily distinguished in the field. (in fact many actually announce their presence and species identity during the breeding season via bird song.) 2. Second, many birds are relatively easily trapped and marked with bands. The enables the study of their population via effective mark-recapture studies. 3. Third, most birds raise their young in discrete nests, making quantification of reproduction possible, whereas it is often nearly impossible to do so in other animals. 4. Fourth, the highly developed and conspicuous social structure of birds (pecking order) makes it possible to study the role of dominance and territoriality in regulating population numbers. 5. For these reasons, the population ecology of birds is better known than for any other group of organisms -- they form a pillar of the science of ecology. B. Bird numbers. 1. Population sizes. a. Population sizes range from the tiny -- especially common in top predators on islands. The Mauritius Kestrel and Chatham Island Black Robin populations have probably never been above 50 in their entire history..... b. To the grotesquely abundant House Sparrow and European Starling (500 millions) -- widely spread species...... c. To the very concentrated, Adelie penguins nesting in colonies in Antarctica can reach density of up to 1 million per colony. 5 d. But these pale in comparison to the extinct Passenger Pigeon, was once the most abundant bird on the planet. Nesting colonies in the mid-west and east were up to a billion strong. Audubon watched a flock fly over his house, at times at a rate of about 300,000 per hour, literally blocking out the sun and darkening the sky for....for 3 days. The young pigeons, called squabs, were easily collected in the colonies, their fat bodies used for meat for both humans and livestock. Hunting stopped when populations got too low for it to remain viable. This occurred in the 1870's....when the population still numbered in the tens of thousands. But the bird still went extinct, the last one dying in the Cinci Zoo in 1914, illustrating that it is not always necessary to kill the last pair of a species to drive it to extinction. Some species need relatively large populations just to remain stable. 2. Counting birds. a. It is rarely possible for a researcher to study the whole population of a single species. Instead, we study localized populations in smaller, defined areas. We then extrapolate these results to the population over a wider area, the degree of which we extrapolate with precision is dependent on how "representative" the study population and study site are to the species' population and geographic range. b. The annual cycles of most birds fall into discrete breeding and non-breeding seasons, over which their numbers change dramatically, even if among years the number fairly stable. OVERHEAD c. Some birds are easiest to count in the breeding season (most songbirds because they are territorial and vocal), whereas some birds are most easily counted in winter (e.g., waterfowl in winter flocks) or migration (e.g., raptors). The important point is that to examine population changes of a species over time, called dynamics, we need to count them with consistent methods and during consistent periods of their annual cycles. C. Population trends. Bird populations are rarely stable over time; instead, they fluctuate from year to year as conditions change. Some examples of long-term trends: OVERHEAD 1. Steady increase or decrease. 2. Irregular annual fluctuations, but overall trend. 3. Irregular crashes followed by recoveries in an otherwise (more or less) stable population. 6 II. 4. A trend whose direction is influenced by frequency of irregular occurrences. 5. Cyclic populations. Population Growth. A. Equations. [I skipped all equations sp 01] There are many forms of population growth equations, called growth models. The three major forms are the exponential growth model, the geometric growth model, and the logistic growth model. 1. The exponential/geometric equation models the growth of a population in the absence of any limiting factors -resources (food, cover, mates, etc.) are unlimited. a. Exponential - used for continuously increasing populations. Nt = No ert, where N equals population size (# individuals at beginning of study, 0, ands some late time t), e equals base of natural logarithm (2.78), and r equals the per-capita rate of population growth. That is, the per-capita birth rate - per capita death rate. (per capita meaning the average per individual in the population....so that if 100 birds produce 1000 chicks in a year, the per capita birth rate is 10 chicks per bird). b. Geometric. The same model, essentially, except that is applies to populations that breed in discrete intervals. Nt = No t , where = the geometric per capita growth rate. It is the same as er. It is calculated as a ratio of population sizes between years (e.g., N1/N0). BOARD GRAPHS c. If r = 0 or if = 1, the population is stable. If r<0 or <1, the population declines, if r>0 or >1, the population increases. These make perfect sense from the equations above. 2. Logistic growth rate. Obviously, many populations cannot increase forever; resources become limited and the rate of population growth slows. a. Equation. dN/dt = rN (1-N/K) or Nt+1 = Nter(1-Nt/K), where K equals the "carrying capacity" of the population in a particular area. K is determined by the availability of resources (food, mates, cover, etc.). Thus, as the population N get larger, it gets closer an closer to K. So, the value of N/K gets closer and closer to 1 as the N increases and approaches K. Thus, the value 1-N/K gets closer and closer to 0 as N increases. Thus, the population growth (dN/dt) approaches 0 as N increases and approaches K. BOARD GRAPHS b. Thus, the logistic growth at first resembles that of the exponential/geometric models.....but as N gets larger, the population growth begins to slow, and eventually, 7 III. when N reaches K, population growth stops altogether, and the populations remains stable at K. c. Overall then, species or population that are very good at competing for resources are likely to eventually reach their K, although it may take them a long time. In contrast, species that are not very good competitors may increase at first very rapidly with a fast reproduction rate, but they will then be limited by better competing species. 3. Now we see the reason for the notations r and K selected. OVERHEAD, r-selected species are good reproducers, Kselected species are good competitors. Remember this is a continuum. Population Limitation/Regulation. – [resume here sp. 01] A. Populations do not increase without end....they eventually are limited by one or more factors. There are 6 major ones 1. Food-supply. a. Lots of circumstantial evidence that food limited many species of birds: their pops are largest where food is most abundant, pops are largest when food is most abundant, or natural changes in food-supply are followed by corresponding changes in pop size. OVERHEAD (Newton 162) b. Some experimental evidence -- food supplementation has shown that for some species, they are held at low population sizes due to limited food-supply. Give them more, and their population increases. OVERHEAD (Newton 185) 2. Nest sites. Very good experimental evidence here suggests that some species, especially secondary cavity nesters, are limited by the available of suitable nest sites. OVERHEAD (Newton 201). 3. Predators. a. Food and other resources like nest sites can put a ceiling on bird numbers. However, some birds may be held below this ceiling by predators or parasites. b. In some cases, where predators have been removed, populations increase indicating the pop is limited by predators. c. These predators may act on adults and/or nests. Much evidence suggests that as humans fragment forests, predators such as raccoons, jays, house cats, etc. have an easier time accessing and depredating songbirds' nests. Thus, habitat fragmentation increases rates of nest predation, which can limit songbird population sizes. 8 d. An interesting aspect of predation on adults, is the distinction between additive and compensatory predation. i. In the absence of predation, many species are probably limited by food-supply, and many individuals would starve to death every year. In the presence of predation, some individuals are killed before they remove much food from the environments, which effectively diminishes the intensity of competition among the survivors. Thus, predation in many cases takes a portion of the population that might otherwise die anyway, and in such cases predation has little overall effect on the population. This form of predation is called "compensatory." That is, predation compensates mortality that would occur as starvation in its absence. ii. But in other cases, predation removes individuals from the population that would have otherwise been able to survive, and thus it adds to the factors limiting the population. This form of predation is called "additive." e. In our macaroni experiment, predation was at least partially compensatory. In the absence of predation (green and yellow) many foragers starved...but in the presence of predation (blue and red), fewer starved. Falcons were killing foragers, some f which surely would have died of starvation anyway. This fact, that predation is compensatory, permits hunting species without detrimental effects on their populations. f. Also, because predation removes individuals from the population that would otherwise remove resources if they were alive, population can be stabilized by predation if food is finite and not replenished (as is the case in winter). If all individuals remain alive and compete for food by scrambling to gather what they can, then eventually food will be so scarce that most individuals will die, and the population will crash. However, by removing some foragers from the population, predation can ensure that enough food is available in the environment to maintain at least some individuals. This was the case in our macaroni experiment. In the absence of predation (yellow and green), the foragers scrambled for macaroni until it was really scarce, at which point their numbers crashed due to starvation. In the presence of predation (blue and red) some individuals were getting picked off early, 9 ensuring enough resources later for more foragers to survive. 4. Parasites. a. Parasites can hold bird population below ceilings set by food supply n the same way as predators. Most damaging to many birds is the parasitism impose by brood parasites -- such as the brown-headed cowbird. b. Also like predation, high rates of parasitism are exacerbated by human forest fragmentation. Cowbirds are birds that food in fields, but look for host bird nests in forests. So they need forest "edges" between forest and field. As we chop up our forests into smaller and smaller pieces, we effectively create more forest edge, even if we don't reduce forest areas all that much. c. Thus, songbird reproduction is severely limited by increased rates of nest predation and brood parasitism in forest edges -- and forest fragmentation is leading to a rapid increase in the spread of forest edge at the cost of a loss of forest "interior" habitat. OVERHEAD 5. Weather a. Weather can occasionally directly limit bird populations, such as when ice storms or hurricanes kill birds... b. But more often, weather limits birds indirectly, by operating through some other factor. Often, weather influences the availability of food. In fact, a three-way interaction that limits bird numbers is probably commonplace in winter: weather influences food supply such that harsh winter make less food available. In turn, the less food available, the more time birds spend foraging for it, and the more risks they take in finding it, which increases the effects of predation. Thus, weather, food supply, and predation all work in concert to limit the birds' wintering numbers. 6. Human impacts: we'll talk more about these when we discuss avian conservation, but briefly, here are the pathways by which human activities can limit bird populations. a. Habitat destruction is clearly the primary way by which humans negatively affect bird populations. We convert natural forest and other habitat types to agriculture, urban lands, flood them for reservoirs, etc....this greatly reduces the total area which supports bird. In many cases, bird species are dependent on certain, and naturally rare or sensitive, habitats. It is these with which we must be especially careful -- e.g., old growth forests, mangrove swamps, arctic tundra. 10 b. Habitat degradation -- fragmentation is an example. Not much actual area of habitat is lost, but it is reduced in quality such that it no longer becomes suitable for certain species. c. Overhunting d. Incidental killing - wires, plastics at sea, pesticides, etc. B. Overall, although few population remains perfectly stable, MOST remain within observed upper and lower bounds, or limits. This pattern is called population regulation. 1. In essence, as a regulated population declines below its long-term average, its population growth rate increases, causing it to rise back toward the average. Conversely, as a regulated population rises above its long-term average, its population growth rate decreases, causing it to come back down toward the average. 2. This characteristic behavior suggests that a regulated population's growth rate depends on its current population size, or density. Thus, we call this pattern densitydependence. In essence, population regulation means that there is density-dependence somewhere in the factors affecting its numbers. 3. Competition for food or nest sites is usually densitydependent -- it gets more severe as density rises. Predation can also be density dependent if a greater proportion of the population may be taken as the population rises. 4. One of the most influential forms of density dependence regulating bird numbers is that of intra-specific competition for habitat. a. If habitat is limited in supply (it usually is, and is becoming increasingly so as human alter the environment), then there is intense competition within species for the best habitat. b. In birds, we see dramatic dominance hierarchies. The dominant individuals get the best habitats, and relegate poorer quality habitats to more subordinate birds. c. Therefore, when a populations is low, most individuals may have access to high quality habitat. But as the population size grows, competition intensifies for those high quality habitats, and more and more birds are relegated to poorer quality habitats. Consequently, the average reproductive success of individuals in the population is highest when the population is low. In contrast, the average reproductive success of individuals in the population is lowest when population size is high. Pop size low, reproduction high; pop size high, reproduction low -- that IS density dependence. 11 d. Of course, if there were more habitat overall, or if it was all high in quality, the population size would be increase. e. Thus, is easy to see that populations are limited in a density dependent manner by the interaction between the quality and quantity of habitat. f. In many cases, this "pre-emptive" occupation of habitats is so extreme that some birds actually do not obtain a habitat (i.e., a territory) at all. Instead, they simply "float" over the rest of the population, hoping for a vacancy to overtake, or perhaps looking for EPCs. These birds are called "floaters."