Harry M. Tiebout

Dynamic Spatial Ecology of the Water Snake, Nerodia sipedon

The harmonic mean method was used to calculate activity areas. Mean home range size was 5.4 ha, with a very concentrated core area of only 7.7% of the home range. Recause most animals tested exhibited shifting activity centers, and because home range size was positively correlated with the number of days a snake was tracked, it was concluded that N. sipedon at this site did not maintain a home range in the traditional sense. A dynamic methodology for quantifying areal utilization is proposed. N. sipedon was found to be very selective in its habitat utilization. Cattails (Typhus latifolia) and flooded meadow (primarily Phalaris and Carex spp.) were generally prefet ed in excess of availability, while open water was avoided. Dead cattail clumps were the most utilized substrate. Water (surface and submerged) and grass (dead and live) were also preferred to a lesser extent. Seasonal and daily trends are discussed in terms of responses to exogenous and endogenous cycles. Diversity of substrate use peaked during morning hours and showed an overall increase as the season progressed. Mean perch height was 10.9 cm above water, without strong differences among daylight hours. Perch heights were greatest during May; in habitats along open water; and on branches, dead cattails, and dead logs. The mean insolation level was 50.9%. On a daily cycle snakes received more sunlight during the midday hours, but it appeared snakes were not responding passively to decreased shade availability. Perch height and insolation level were positively correlated, thus high substrates were also associated with greater insolation. Three measures of snake activity were analyzed: incidence of movement (s = 27.7%), total rate of movement (? = 5.2 m/h), and conditional rate of movement (x = 18.7 m/h). Snakes were generally more active during April while they dispersed from the hibernaculum. From May-July the animals were equally active at all times of the day. Snakes exhibited greater rates of travel across open and exposed habitat types. Individual snakes differed significantly in all three measures of activity and also had variation in home range size spanning four orders of magnitude. Nevertheless, more active snakes did not utilize a larger area than their less active conspecifics.

Daytime Energy Management by Tropical Hummingbirds: Responses to Foraging Constraint

Because hummingbirds are extremely sensitive to energy stress, yet often face conditions when energy intake is reduced or energy expenditures must be increased, they should have flexible energy budgets. I experimentally measured the ability of two syntopic tropical hummingbird species to manage daytime energy budgets under conditions simulating natural foraging constraint, defined here as low nectar secretion rate, low flower density, or both. I tested a territorial species, Amazilia saucerottei, and a traplining species, Chlorostilbon canivetii, individually in a large flight cage at two rates of food delivery (HIGH = ad libitum, LOW = 64% of ad libitum) and two perch-to-feeder distances (NEAR = 4 m, FAR = 20 m). Both species increased flight time when distance was increased (FAR > NEAR by 44%) and when rate of food delivery was reduced (LOW > HIGH by 23%). However, birds on the HIGH food delivery rate did not increase food intake to compensate for increased flight expenditures to the FAR feeder. Birds on LOW food maintained total rates of energy expenditure that were less than birds on HIGH food, and LOW food birds experienced no effect of feeder distance on expenditure. These results appear to be due primarily to short-term reductions in perching metabolic rates (PMRs) by birds on LOW food or with the FAR feeder. Reduced daytime PMRs helped to mitigate the potentially negative impacts of foraging constraint: rates of mass loss of the LOW food birds were an estimated 48% less than if birds had maintained normal PMRs. Reduced PMRs also enabled birds visiting the HIGH FAR feeder to maintain rates of mass gain equal to unconstrained birds (HIGH NEAR feeder). Relative differences between species in energy management reflected their foraging modes. The low-reward trapliner paid the greatest energy costs at LOW food, regardless of distance, due to its overall tendency for high flight time. In contrast, the territorialist did poorly in the HIGH FAR treatment, due to flight expenditures as- sociated with defending a rich but dispersed resource. Although individual birds exhibited considerable flexibility in energy management, the foraging mode represented by each species appeared to be energetically specialized for particular levels of food availability and dispersion.

MECHANISMS OF COMPETITION IN TROPICAL HUMMINGBIRDS: METABOLIC COSTS FOR LOSERS AND WINNERS'

The goal of this study was to measure directly the acquisition and allocation of energy for competing hummingbirds. In flight-cage trials providing an easily defensible food source, complete energy budgets (intake, expenditure, and storage) were obtained for two sympatric Costa Rican hummingbirds: a territorial (Amazilia saucerottei) and a low- reward traplining (Chlorostilbon canivetii) species. Behavior and energy responses of solitary birds (controls) were compared to the responses of conspecific and heterospecific pairs competing for a single feeder. Relative to controls, birds in pairs generally spent more time in flight, due primarily to increased rates of visiting the feeder or to agonistic encounters, with a consequent increase in energy expenditure. Ability of birds in pairs to compensate for increased expenditures by increasing intake, thus maintaining a constant body mass over 24 h, depended upon species and dominance status. The greatest asymmetries in energy effects occurred in heterospecific pairs, in which A mazilia aggressively interfered with access to the feeder by Chlorostilbon. In these trials, Amazilia boosted intake sufficiently to maintain adequate rates of energy storage; Chlorostilbon did not increase intake and thus suffered significant loss of energy stores. Conspecific pairs of each species experienced energy effects inter- mediate to the extreme responses of heterospecific pairs. Individuals of either species had greater energy success when paired with a Chlorostilbon rather than with an Amazilia. For both species, individuals that were relatively more aggressive than their cagemates expe- rienced the greater energy success, though absolute frequency of aggression by an individual was a poor indicator of its energy success. Differential energy response to competition by Amazilia and Chlorostilbon appears to be one mechanism that contributes to local species abundance and guild composition.

Mesocosm Experiments on Habitat Choice by an Endemic Lizard: Implications for Timber Management

We investigated the impacts of various logging practices on habitat choice by the endemic Florida scrub lizard (Sceloporus woodi) in the Florida scrub of Ocala National Forest (ONF). We used large outdoor mesocosms as a novel means to evaluate lizard preferences for habitats with different structural features produced by standard forestry practices. Captive lizards were offered a choice between two adjoin- ing habitats (= sides of a mesocosm) created using one of two substratum treatments (SAND = 75% open sand; WOOD = 75% coarse woody debris (CWD)) coupled with one of two insolation treatments (LIGHT = ambient sunlight; DARK = 45% ambient sunlight). The mesocosms proved to be an effective technique for evaluating lizard habitat preferences. Lizards were easily observed and remained active and healthy throughout the experiment Sighting frequencies differed significantly among the four mesocosm sides, yielding an overall preference ranking of DARK SAND > LIGHT SAND > DARK WOOD - LIGHT WOOD. Analysis of sighting frequencies by treatment factors (substratum and insolation) and of dissimilarity ma- trices both indicated that habitat choice was based primarily on substratum composition and only weakly determined by insolation level. In addition, size- and gender-specific preferences suggest that social inter- actions may help shape patterns of habitat used in conjunction with individual preferences. We conclude that the least favored mesocosm side (LIGHT WOOD) represents a habitat type that could potentially serve as a population sink for scrub lizards and recommend several methods to reduce the accumulation of CWD or to ameliorate its potential thermal stress on lizards. In addition, the most favored mesocosm side (DARK SAND) represents a shaded sandy habitat type not currently found in ONF timber stands. We present several alternative harvesting and site preparation methods that could produce such habitats and recommend further research on their potential value for enhancing populations of scrub lizards and other open-habitat scrub

Comparative energetics of divergent foraging modes: a doubly labelled water experiment on hummingbird competition

Two tropical hummingbird species with divergen foraging modes and competitive strategies were tested for energetic and behavioural responses during food competition. In a large flight cage, heterospecific pairs competed for limited food (sucrose solution) delivered to five artificial flowers (feeders) arranged in two food-density treatments. When feeders were tightly clumped, the dominant territorial species (the steely-vented hummingbird, Amazilia saucerottei) actively defended them, thereby reducing access by the subordinate ‘traplining’ species (the fork-tailed emerald, Chlorostilbon canivetii). As a consequence, the dominant species maintained relatively high mass-specific rates of energy intake (J/g/h) and a balanced energy budget; whereas the trapliner maintained low food intake, high energy expenditures, and suffered negative energy balance. When feeders were widely dispersed and infefensible, the trapliner obtained food at a higher mass-specific rate than the territorialist, but again suffered negative energy balance as a result of excessive energy expenditure. In contrast, the territorialist had low intake and expenditures, thus maintaining energy balance. Regardless of treatment, both species changed their behaviour in similar ways over the feeding day, reducing (1) visit rate to feeders, (2) diversity of feeders visited, (3) overlap between cagemates in feeders visited, and (4) time spent flying. After repeated interactions, each cagemate eventually concentrated its foraging on a disjunct subset of feeders, thus enhancing foraging profitability while partially mitigating the negative energetic impacts of competition for limited food.

Costs and benefits of interspecific dominance rank: are subordinates better at finding novel food locations?

Abstract Dominance status in birds may involve a trade-off in foraging costs and benefits. This study tested the hypothesis that the cost of being a subordinate species may be offset by the benefit of being better at finding novel food locations compared to their dominant competitors. In a pair of flight-cage experiments, the ability of two tropical hummingbird species to find novel locations of food was measured. A dominant species (steely-vented hummingbird, Amazilia saucerottei ) and a subordinate species (fork-tailed emerald, Chlorostilbon canivetii ), tested both as solitary birds and in competing heterospecific pairs, did not differ significantly in (1) the time required to first discover novel food locations, (2) the number of feeding visits made to such sources or (3) the number of novel food locations one bird could discover before its cage-mate. The dominant Amazilia , however, was twice as fast as Chlorostilbon at finding novel food locations previously discovered by a competing heterospecific cage-mate. This behaviour pattern could facilitate the pre-emption of newly found food sources from Chlorostilbon in the wild. Other possible mechanisms that could confer an advantage on the subordinate species are discussed: (1) ability to locate cryptic food sources, (2) ability to recognize as food a novel item that has never been used before and (3) specialization on finding food sources that dominant species cannot pre-empt.

Monitoring Plethodon Cinereus Populations

Recently there has been a rapidly growing interest in monitoring populations of plethodontid salamanders which stems from three related areas of research. First, worldwide declines and extinctions of anurans,raise the possibility that other amphibian taxa may also be at risk now or in the near future. Indeed, it has been predicted that growing human impact on ecosystems may induce even higher rates of decline in native amphibian and other herpetofaunal populations in the future(Beebee, 1996; Pechmann et al., 1991;Seigel and Simons, 1995). Second, the pronounced sensitivity of amphibians to environmental changes makes them particularly valuable as indicator species, even for species not facing any immediate risk of extinction. For example, studies have shown that survival of many amphibian species is strongly dependent upon factors that can be influenced by human activities, including habitat fragmentation and edge effects,UV radiation levels (Grant et al., 1992), soil and water pH (Frisbie and Wyman, 1991; Sugalski and Claussen, 1997), and percent organic matter and percent moisture of soil (Grant et al., 1992; Droege et al., 1997). Finally, because of their abundance, total biomass, and role in food chains, amphibians and other herpetofauna are ecologically sifinificant members of their communities (Hairston, 1987). In particular, the redback salamander (Plethodon cinereus is one of the most important mid-level predators in eastern North American forest ecosystems.