Demography and Populations Survivorship Demography is the study of fecundity and survival Four critical variables Age of first breeding Number of young fledged each year Juvenile survival Adult survival % Surviving Age Bird % Surviving Mammal Age Avian life history patterns Extremes of avian life history patterns Ecological factors influence life history patterns in predictable ways Mortality rates high in first year of life, then drop off Reproductive success improves with age and experience Species Survival before breeding Age of first reproduction Fecundity Adult mortality rate Long-lived species tend to have low reproductive rates and take longer to reach maturity compared to shortlived species From Ricklefs. 2000. Condor 102:9-22 Albatrosses and eagles Ducks quail and small passerines Moderate (30%/year) Low (15%/year) Late (8 10 years) Early (1 year) Low (0.2 young/year) Moderate (3 young/year) Low (5%/year) High (50%/year) 1
Components of a life table Σ 1.01 Σ 0.52 Sx = annual survivorship Lx = probability of surviving to age x Bx = age-specific fecundity (number of female young produced each year by adults in this cohort) (Lx) (Bx) = expected annual fecundity R 0 = (Lx) (Bx) = net reproductive rate Equal lifetime fecundity Expected annual fecundity = (Lx) (Bx) Lx = Probability of surviving to age x Bx = Age specific fecundity Lifetime fecundity = Area under curve Survival rates Adult vs juvenile survival Seabirds > Landbirds Large > Small Tropical > Temperate Survival in Florida Scrub Jays Fledging weight and survival in Great Tits 2
Tropical vs temperate survival rates Karr (1990) found no differences between Maryland and Panama Oatley and Underhill (1993) found survival in African species higher than similar European species Johston et al. (1997) found Trinidadian birds survive better than N. American birds Maximum ages (wild birds): Small passerines: 10-15 years Waterbirds & raptors: 20-30 years Longevity records: 65 year old Laysan Albatross 36 year old Eurasian Oystercatcher 34 year old Great Frigatebird Captive birds live longer than wild birds Some captive parrots >80 years Avian longevity Life expectancy = (2-m)/2m Where m = annual mortality AMOY (m) =? m = 5% (2-m)/2m = 19.5 Fecundity Number of young raised to independence Annual fecundity Number of nesting attempts Nesting success Clutch size Lifetime fecundity Age at first breeding Breeding frequency Lifespan Fecundity age at first breeding Recall that: dn/dt = rn Where: N = population size t = time r = instantaneous population growth rate Alternatively: (Log e R 0 )/T = r Where: R 0 = net reproductive rate = Σ L x B x (R 0 > 1 increase, R 0 < 1 decrease) T = generation time = Σ xl x B x / R 0 = 2.6 years for Screech Owl What to do? Note: the age at first breeding has a disproportionate effect on the potential growth rate of a population (r). For example, doubling Ro (via higher fledging success) increases r by 31%, But.. Reducing T by 50% increases r by 100%. Therefore individuals that can breed earlier should (all other things being equal) because that will give them the higher payoff in lifetime fecundity. (all other things are not equal). 3
Fecundity age and experience Older California Gulls spend more time attending their nests and young and are more successful than young birds Delayed breeding is favored in long-lived species and where cost of breeding is high Cost of first breeding may be reduced by males of some species that retain cryptic female plumage Fecundity number of nesting attempts Number of attempts reflects: Length of nesting season Food supplies Predation pressure Tropical species: Two to six broods/season Longer renesting intervals Fecundity nesting success Fecundity clutch size Temperate > Tropical Large, well tended > Small, precocial Hole-nesters > Open-nesters Primary causes of nest failure Predation Starvation Desertion Hatching failure Adverse weather Passerines, 2-12 (usually 2-6) Precocial species, up to 20 Hummingbirds and doves, always 2 Most pelagic seabirds, 1 4
Evolution of clutch size: 4 major hypotheses 1. Lack's Food Limitation Hypothesis: parental ability to care for nestlings dictates clutch size. 2. Tradeoff Hypothesis: future survival dictates maximum possible clutch sizes. 3. Predation Hypothesis: nest predation selects for smaller clutches because they are less conspicuous and minimize short-term losses. 4. Seasonality Hypothesis: clutch size reflects the seasonal availability of resources relative to population size. Lack s food limitation hypothesis Birds raise maximum number of young they can feed Clutch size manipulation experiments confirm this, but there are many exceptions Alternative hypotheses Tradeoff hypothesis Predation hypothesis Seasonality hypothesis In the population of Great Tits in Wytham Wood, broods of 10 12 chicks were the most productive. The average clutch size is 8.5 Why don't birds lay more eggs? Relationship between reproductive effort and parental fitness is a major goal of life history theory Lack (1947) submits that clutch size has been shaped by natural selection to reflect (on average) the number of young adults can feed 25 year cycle of interest in topic Early experiments focused on chick stage 97 studies to date Modification of Lack's theory to include trade-offs Counting young fledged doesn t accurately measure parental fitness Trade-offs in parental and offspring survival, and lifetime reproductive performance may be critical Current evidence conflicting Daan et al. (1996) parental survival reduced when Kestrel brood enlarged Orell et al. (1996) brood enlargement did not affect adult survival in Willow Tit Life history attributes depend on "state-dependent" variables McNamara and Houston (1996) 5
Estimating the true cost of reproduction Estimates of egg composition indicate that the energy required for egg production range from 13-41% BMR in passerines to 200% BMR in waterfowl - Need more direct measures. Ward (1996) finds 5% BMR in doubly labeled water studies of Barn Swallows Incubation costs are also not trivial, 19-50% above BMR. Experiments show low nestling growth rates or fewer young fledged by birds that incubate extra eggs Full cost studies of Collared Flycatcher, Goldeneye, Gannet, and Common Tern are revealing. Estimating the true cost of reproduction future work How are costs of egg production, incubation, and chick rearing stages partitioned? How do stages of nesting cycle interact? How important are state-dependent variables, especially the quality of individuals at fledging? How important are factors other than the parent s ability to produce and provision a brood, such as predation and the length of nesting season? The predation hypothesis Three ways predation could select for small clutch size 1. Larger clutches take longer to lay (longer exposure to predators) 2. Larger broods are noisier and more conspicuous (more likely to attract predators) 3. Longer breeding seasons could favor more nesting attempts with fewer eggs per attempt Age-related mortality explains life history strategies of tropical and temperate songbirds Thomas E. Martin Science 2015;349:966-970 Conceptual framework for life history strategies. Over the circle of life, mortality risk varies across life stages (gray ring) and exerts selection on growth strategies (orange boxes) and parental strategies (yellow boxes). Blue arrows reflect positive selection; red arrows reflect negative selection; black arrows reflect the influence of life history traits on each other. Nest and fledgling predation exert opposing selection on length of the nestling period. Fledgling predation risk favors longer nestling development to enhance locomotor traits (i.e., longer wings), but longer periods increase nest predation risk. Increased parental investment and steady mass growth allow enhanced wing growth without extending the length of the nestling period and increasing predation risk. Parental investment is a function of total parental effort (total provisioning rate) partitioned among young, where total parental effort is a result of adult and nest mortality. The higher parental investment that facilitates the longer wings favored in tropical birds is achieved by small clutch sizes. 6
The seasonality hypothesis Fig. 2 Growth and nest predation on three continents.(a) Peak growth rate is faster in species with higher nest predation risk but is slower in tropical species with the same level of risk as temperate species, while controlling for mass (table S1A). Growth rate is the conventional peak rate of growth, K i (see Fig. 3A). (B) Nestling period covaries with growth rate, but tropical species have shorter nestling periods for the same growth rate as temperate species (table S1B). Magnitude of seasonal flux in resources shapes clutch size (Ricklefs 1980) Clutch size increases with A/E ratio Northern Flicker clutch size increases by 1 egg per 10 0 of latitude Rates of population growth Recall that R 0 = (L x ) (B x ) = net reproductive rate R 0 > 1 = population growth R 0 < 1 = population decline ln R 0 /T = r = intrinsic rate of natural increase (where T = generation time) r > 0 = population growth r < 0 = population decline Exponential growth can be described by: dn/dt = rn R 0 = 1.25 r = 0.23 House Finch introduction to Eastern U.S. Growth potential differs among species K selected species 10% 30%/year R 0 = 1.1 1.3 r = 0.09 0.26 r-selected species 50% 100%/year R 0 = 1.5 2.0 r = 0.41 0.70 7
Eventually, negative feedback slows growth Population Regulation Density dependence the tendency for the death rate in a population to increase, or the birth rate to decrease, as population density increases Population regulation maintenance of average population size via density dependent processes K = Carrying capacity Population limitation ceiling on population growth (carrying capacity, K) Factors that limit populations Food is the factor that limits most bird populations Irruptions of Snowy Owls and northern finches is directly tied to food supplies (small mammals and conifer seeds). Habitat Weather Parasites & Disease 8
Density-dependent population regulation in Great Tits Density dependent juvenile survival limits population size from year to year. Fecundity varies as a function of population density A = Deciduous B = Mixed C = Pine Populations are limited during the winter where juvenile survival is closely tied to the abundance of seeds from Beech trees. Populations are limited during the winter where juvenile survival is closely tied to the abundance of seeds from Beech trees. Population management requires good demographic data Summary Variations in demographic parameters (breeding success, mortality, immigration, and emigration) determine population size Biotic (habitat, food, parasites, predators) and abiotic factors (weather) can set limits on population size Bird populations are regulated primarily by density dependent processes 9