James N. M. Smith

Impact of food and predation on the snowshoe hare cycle.

Snowshoe hare populations in the boreal forests of North America go through 10-year cycles. Supplemental food and mammalian predator abundance were manipulated in a factorial design on 1-square-kilometer areas for 8 years in the Yukon. Two blocks of forest were fertilized to test for nutrient effects. Predator exclosure doubled and food addition tripled hare density during the cyclic peak and decline. Predator exclosure combined with food addition increased density 11-fold. Added nutrients increased plant growth but not hare density. Food and predation together had a more than additive effect, which suggests that a three-trophic-level interaction generates hare cycles.

Immigration and the ephemerality of a natural population bottleneck: evidence from molecular markers

Population bottlenecks are often invoked to explain low levels of genetic variation in natural populations, yet few studies have documented the direct genetic consequences of known bottlenecks in the wild. Empirical studies of natural population bottlenecks are therefore needed, because key assumptions of theoretical and laboratory studies of bottlenecks may not hold in the wild. Here we present microsatellite data from a severe bottleneck (95% mortality) in an insular population of song sparrows (Melospiza melodia). The major findings of our study are as follows: (i) The bottleneck reduced heterozygosity and allelic diversity nearly to neutral expectations, despite non–random survival of birds with respect to inbreeding and wing length. (ii) All measures of genetic diversity regained pre–bottleneck levels within two to three years of the crash. This rapid recovery was due to low levels of immigration. (iii) The rapid recovery occurred despite a coincident, strong increase in average inbreeding. These results show that immigration at levels that are hard to measure in most field studies can lead to qualitatively very different genetic outcomes from those expected from mutations only. We suggest that future theoretical and empirical work on bottlenecks and metapopulations should address the impact of immigration.

Balancing food and predator pressure induces chronic stress in songbirds

The never–ending tension between finding food and avoiding predators may be the most universal natural stressor wild animals experience. The ‘chronic stress’ hypothesis predicts: (i) an animals stress profile will be a simultaneous function of food and predator pressures given the aforesaid tension; and (ii) these inseparable effects on physiology will produce inseparable effects on demography because of the resulting adverse health effects. This hypothesis was originally proposed to explain synergistic (inseparable) food and predator effects on demography in snowshoe hares (Lepus americanus). We conducted a 2 × 2, manipulative food addition plus natural predator reduction experiment on song sparrows (Melospiza melodia) that was, to our knowledge, the first to demonstrate comparable synergistic effects in a bird: added food and lower predator pressure in combination produced an increase in annual reproductive success almost double that expected from an additive model. Here we report the predicted simultaneous food and predator effects on measures of chronic stress in the context of the same experiment: birds at unfed, high predator pressure (HPP) sites had the highest stress levels; those at either unfed or HPP sites showed intermediate levels; and fed birds at low predator pressure sites had the lowest stress levels.

Stability, regulation, and the determination of abundance in an insular song sparrow population

The population dynamics of the Song Sparrow, Melospiza melodia, were studied for 15 yr on Mandarte Island, in southwestern British Columbia, Canada. This population exhibited a high median density (7.8 females/ha) and fluctuated strongly (10—fold or more) from year to year. The population received few successful immigrants, even at low densities, and its dynamics were thus driven by local events. Two strong density—dependent regulating mechanisms were detected. First, reproductive output was strongly depressed at high densities because of an increased rate of nest failure and a decline in mean clutch size. Greater nest failure at high density was due to increased predation on eggs and nestlings. Nest failure increased with the rate of nest parasitism by Brown—headed Cowbirds (Molothrus ater) and was lowest when cowbirds were absent from Mandarte, suggesting that cowbirds either cause or facilitate nest failure. Second, the rate of juvenile recruitment was inversely related to the density of adults, becaus...

FOOD-SEARCHING BEHAVIOR OF TITMICE IN PATCHY ENVIRONMENTS'

The food-searching behavior of titmice (Paridae) was investigated in laboratory and field to determine how effective individual birds were at locating patchy sources of hidden food. A series of three experiments was performed using tame great tits, Parus major. In Experiment 1, individual birds were offered a choice among six densities of hidden food in discrete patches. The birds learned to concentrate their search strongly on the more dense patches, and five of the six birds preferred to search at the highest density. In Experiment 2, the food distribution was altered by interchanging patches of the highest and lowest density gradually. The birds initially found fewer prey per trial, but soon recovered to near their original performance by switching their search effort to the area containing the second highest prey density. Most birds failed to respond to prey appearing where they had previously been absent. Experiment 3 varied the size of prey items among four areas containing equal numbers of hidden prey. A group of six birds learned to search selectively in areas containing larger prey and, as a result, a greater total quantity of prey. In all experiments, there were marked differences in performance by individual birds. The experiments support Royamas (1970, 1971) hypothesis that great tits distribute their search effort in relation to spatial differences in the profitability of feeding sites. Field observations on a pair of great tits and a pair of blue tits, Parus caeruleus, showed that searching behavior of wild tits was also very selective in space, at least when the birds were feeding nestlings. Birds often returned to previous capture sites and were more likely to do so when they found prey there quickly. These observations suggest that the laboratory experiments have real relevance to the food searching of wild tits. The searching capacity of great tits is discussed; it is suggested that they can approach ideal responses in stable but patchy feeding environments. Some factors leading to deviations from ideal responses are discussed.

Population changes of the vertebrate community during a snowshoe hare cycle in Canada’s boreal forest

We measured the density changes of 22 species of vertebrates during a snowshoe cycle in northern Canada. Hares were the dominant herbivore in the system and changes in their numbers were correlated with changes in numbers of arctic ground squirrel, spruce grouse, ptarmigan, lynx, coyote, great horned owl, goshawk, raven and hawk owl. Hare numbers were not correlated with numbers of red-backed vole which showed peaks during the low, increase, and early decline phases of the hare cycle. Hawk owls were the only predator whose numbers correlated with changes in red-backed voles while boreal owls and weasels were correlated with densities of Microtus. Red squirrel, American kestrel, red-tailed hawk, northern harrier, wolverine, magpie, and gray jay showed no correlation with hare or vole numbers. We conclude that species in the boreal forests of Canada do not exhibit the strong synchrony found between voles and other members of the vertebrate community in northern Fennoscandia. We discuss some of the possible reasons for these differences.

Natural selection on beak and body size in the song sparrow

We documented temporal patterns of natural selection on beak and body traits in a song sparrow population. We looked for evidence of selection in association with reproduction and overwinter survival in order to identify the conditions under which size in beak and body traits is adaptive. We also attempted to identify the specific traits most closely associated with fitness under these conditions.

Can the solar cycle and climate synchronize the snowshoe hare cycle in Canada? Evidence from tree rings and ice cores

Dark marks in the rings of white spruce less than 50 yr old in Yukon, Canada, are correlated with the number of stems browsed by snowshoe hares. The frequency of these marks is positively correlated with the density of hares in the same region. The frequency of marks in trees germinating between 1751 and 1983 is positively correlated with the hare fur records of the Hudson Bay Company. Both tree marks and hare numbers are correlated with sunspot numbers, and there is a 10-yr periodicity in the correlograms. Phase analysis shows that tree marks and sunspot numbers have periods of nearly constant phase difference during the years 1751-1787, 1838-1870, and 1948 to the present, and these periods coincide with those of high sunspot maxima. The nearly constant phase relations between the annual net snow accumulation on Mount Logan and (1) tree mark ratios, (2) hare fur records before about 1895, and (3) sunspot number during periods of high amplitude in the cycles suggest there is a solar cycle-climate-hare population and tree mark link. We suggest four ways of testing this hypothesis.

Seasonal Variation in Feeding Habits of Darwin's Ground Finches

We investigated the effects of seasonal variation in food abundance on food partitioning among Darwins ground finches of the genus Geospiza. Finch populations, the availability of seed of fruits, and finch feeding habits were measured during 2 visits to each of 4 study sites on 3 Galapagos islands. Our 1st visit was made near the end of the 1973 rains at a time of seasonal food abundance and the 2nd, 6—8 mo later, when food was scarce. If interspecific competition prevails, finch populations should decline from wet to dry season in response to declining food availability. At 2 sites where food supplies declined sharply, so did an index of finch numbers; at the other 2 sites, both food supplies and finch numbers remained high. Total finch biomass at 5 dry—season study sites was correlated with food abundance but was not at 8 wet—season study sites. If interspecific competition is stronger, diets of finch species should diverge as food becomes scarce, but if intraspecific competition is stronger, diets should expand and overlap among species increase. At all 4 sites, finch species diverged in their diets and took a narrower range of foods in the dry season. All species shifted from a common wet—season diet of soft, easy—to—handle seeds and fruits to different diets reflecting the morphological specializations of each species. The results are consistent with interspecific competition for food occuring at all sides, but other explanations cannot be ruled out. Even where food remained relatively abundant, species diets changed and overlapped less. Intense competition was not, therefore, a necessary condition for seasonal diet changes; these may have been simple proximate responses to food availability. Existing foraging theory does not predict these results or others, because the assumption that available foods do not change qualitatively between seasons is violated. Where food greatly decreased, inter— and intraspecific competition may have caused the death or dispersal of most of the finches in the wet—season populations and diet divergence minimized interspecific competition are probably complemented by year—to—year differences caused by the unpredictable Galapagos rainfall.

DOES HIGH FECUNDITY REDUCE SURVIVAL IN SONG SPARROWS

Several life history models (reviewed by Stearns, 1976) incorporate the intuitively attractive assumption that animals forego maximum annual fecundity to increase their subsequent survival. Birds are among the most convenient animals to test this assumption, since fecundity and survival can be measured readily. Descriptive studies by Kluyver (1963) and Campbell (Lack, 1966, p. 109), and a recent experimental study by De Steven (1980), do not support the assumption. On the other hand, a descriptive study by Bryant (1979) and experimental studies by Kluyver (1971) and Askenmo (1979) do support the assumption. Further empirical information is needed to establish how often, and under what circumstances, survival and fecundity are negatively associated. In this paper I report on a positive association between reproductive performance and female survival in the song sparrow, Melospiza melodia. This study of an island population provided detailed life history information because individuals were sedentary and immigration rates low. I addressed the following questions: (1) Did females which disappeared between breeding seasons differ in fecundity and reproductive success from surviving females? (2) Are fecundity and reproductive success of individual females similar in successive years? (3) Are differences in fecundity and reproductive success heritable? (4) Is the probability that an individual will be recruited to the breeding population influenced by the size of clutch in which it was raised, or by the total fecundity of its mother in the year it was hatched?