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Feeding-Time Minimization and the Territorial Behavior of the Willow Flycatcher (Empidonax Traillii)

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Feeding-Time Minimization and the Territorial Behavior of the Willow Flycatcher (Empidonax Traillii) FEEDING-TIME MINIMIZATION AND THE TERRITORIAL BEHAVIOR OF THE WILLOW FLYCATCHER (EMPIDONAX TRAILLII) DAVID R. C. PRESCOTT AND ALEX L. A. MIDDLETON Departmentof Zoology,College of BiologicalScience, Universityof Guelph,Guelph, Ontario N1G 2W1, Canada ABSTRACT.--Westudied breeding male Willow Flycatchers(Empidonax traillii) to testa model of optimalterritorial behavior for feeding-timeminimizers proposed by Hixon (1980).Specific predictionswere that feeding time, defense time, and territory size should decreasewith increasingfood availability and increasewith increasedpressure from both conspecificand heterospecificcompetitors. The constantreproductive output of 4 and 3 eggsfor firstand secondclutches, respectively, and the large componentof uncommitted(sitting) time (62.6 + 3.2%) over the breeding seasonconfirmed that the Willow Flycatcherconformed to a time-minimizationstrategy. Data analyzedover three stagesof the breedingcycle, and over all stages,however, showed that only 5 of the 36 possiblerelationships were significantas predicted by the model.In general, food availability and competitorpressure were not important influenceson the territorial behavior of thesebirds. Variations in territory size could not be attributed to constraintson feeding time, but correlatedclosely with the energeticrequirements of all birds occupying the territory. We believethat breedinginsectivorous passerines, including Willow Flycatchers,maintain a large componentof uncommittedtime, as well as a larger than necessaryterritory, to minimize the impact of short-termvariations in competitorpressure and food supply.Such birds need not conformto the predictionsof modelsthat optimize foragingtime. ReceivedI December1986, accepted16 July 1987. SINCEthe conceptof economicdefendability tion of time and energy by the territory holder was introduced to the study of territoriality to various territorial activities. Territorial be- (Brown 1964), numerous models, based on costs havior then can be viewed as a consequenceof and benefitsaccrued to the territory holder,have the balancebetween the time (or energy)spent been proposed to account for variations in the acquiringbenefits (e.g. food) from the territory size of the area defended by territorial animals versus the time allocated to territorial defense. (Carpenter and MacMillen 1976, Dill 1978, Ko- Hence, models that predict changesin territory dric-Brown and Brown 1978, MacLean and Sea- size basedon proximate constraintson energy stedt 1979, Tullock 1979, Ebersole 1980, Hixon acquisitionare also capableof predicting con- 1980,Wittenberger 1981, Schoener1983). In an- comitant changesin the allocation of feeding imals that defend exclusivefeeding areas,cost- and defensetimes by the territory holder (e.g. benefitratios are most often thought to be prox- Hixon 1980, Schoener 1983). imately influencedby food availability. This is Schoener (1971) identified alternative strat- supportedby an inverse relationship between egies in the allocation of time and energy by territory size and food abundancefor a variety foraging animals. Foragers that realize no re- of vertebratepredators (e.g. Stenger 1958, Smith productive gain by increasing foraging effort 1968,Holmes 1970,Slaney and Northcote1974, (i.e. have a fixed reproductiveoutput) Schoener Simon 1975, Dill et al. 1981, Davis 1982). Com- termed feeding-time minimizers. Animals petitor pressurealso may exert an effect on ter- whose fecundity is enhanced by energy gain ritory size (Yeaton and Cody 1974, Myers et al. were termed energy maximizers. These con- 1979, Ewald et al. 1980, Norton et al. 1982), al- ceptshave been integrated into models of ter- thoughthe presenceof territorial intrudersneed ritorial behavior (Dill 1978,Pyke 1979,Ebersole not be independentof food availability (Myers 1980, Hixon 1980, Schoener 1983). Predictions et al. 1979, Norton et al. 1982, Schoener1983). of territorial dynamics and time allocation for Both food and intruders apparently influence energy maximizers have often been contradic- territoriality by causingchanges in the alloca- tory (Schoener 1983). However, Hixon (1980) 17 The Auk 105: 17-28. January1988 18 PRESCOTTAND MIDDLETON [Auk,Vol. 105 and Schoener (1983) predicted that for time following season,apparently because of agricultural minimizers, territory size, feeding time, and de- development of the peripheral areas.Therefore, an fense time should decrease with increased food additional site at Sayer'sMills, 19 km eastof Guelph, availability, increase with increased pressure was chosenfor study in 1985. This was the driest of from conspecific intruders against which the the three study areas,with standing water restricted to a small ephemeralcreek on the westernside of the territory is defended (intruders that do not sup- site. Three pairs of Willow Flycatcherswere present press food abundance), and increase with in- at Sayer'sMills in 1985. creasingpressure from heterospecificintruders An assumptionof the model of optimal feeding that forage on the territory and hence reduce territoriality is that territories are noncontiguous food availability. At present empirical testsfor (Hixon 1980, Schoener1983). We consideredonly ter- the specific predictions of the time-minimiza- ritories where < 50%of the boundary was sharedwith tion models are lacking. conspecificsand that bordered on suitable, unoccu- We studied the territorial behavior of the Wil- pied habitat (to permit territorial expansion).During low Flycatcher (Empidonaxtraillii) to test the late May (territory-establishmentstage) in 1984 and 1985, 5 territories were selected for observation at above predictions. Previous time and energy Badenoch. In 1984, 2 additional territories were cho- budgetanalyses of breedingWillow Flycatchers sen at Guelph Lake and, in 1985, 2 at Sayer'sMills. led Ettinger and King (1980) to classify this Although 3 territorial maleswere color-bandedearly speciesas a feeding-time minimizer. Several as- in the 1984 season, efforts to mark birds were soon pects of the ecology of the Willow Flycatcher abandonedwhen it became apparent that individual are consistentwith assumptionsof the model birds confined their activities to well-defined areas and render it a convenient speciesfor testing and could be distinguishedreadily from their neigh- the models (see Hixon 1980 and Schoener 1983 bors. Each territory was visited randomly at least 4 for discussionof assumptions).First, it occupies times during each breeding season (May-August). a relatively open and homogeneous habitat Eachvisit was consideredan independent repetition. (King 1955, Walkinshaw 1966) such that prey Abandonment of territories by the flycatchersearly in the season(twice in 1985) occasionallyprevented items are presumably evenly distributed and repeatedobservations on a specificterritory. In such behavioral observations facilitated. Second, like casesadditional noncontiguous territories were found other Empidonaxflycatchers, it confines its ac- asreplacements; data from the new territoriesand the tivities to a small feeding territory defended ones they replaced were treated separately. The se- vigorouslyagainst conspecific intruders (Davis quence of observation of these territories followed 1954, 1959; Davis et al. 1963). Finally, it feeds the order previously determined for the abandoned almost exclusively on invertebrate prey (Bent territories. 1942) that are readily quantified in the field. Observations on all territories were made between 0700 and 1200 from a 2.5-m portablestepladder. Be- STUDY AREA AND METHODS causeweather factorsprobably influence the foraging behavior of insectivorous birds (Lederer 1972, Grubb Most data were collected from Willow Flycatcher 1979), as well as the distribution and availability of territories in the BadenochSwamp, 14 km southeast invertebrateprey (Digby 1958,Taylor 1963),data were of Guelph, Ontario (43ø32'N,80ø13'W). The studyarea collectedonly on rainlessmornings with little or no consistedprimarily of densethickets of 3-4 m high wind (< 15 km/h) and seasonaltemperatures. willow (Salixsp.), interspersedwith grassyclearings Territorysize.--Territory size was determined by re- and sparsesecondary growth (< 1.5 m height) of red- cording the position of singing perches,flight paths, osier dogwood(Cornus stolonifera), hawthorn (Cratae- and territorial disputesof both male and female Wil- gussp.), and trembling aspen(Populus tremuloides) sap- low Flycatcherson 5-m grid maps of each study area lings. White elm (Ulmus americana)and black ash constructedby ground mapping with compassand (Fraxinusnigra) up to 25 m in height were scattered tape.The positionof foragingbouts was alsoused to throughoutthe wetter areasof the swamp.Areas of determine territory boundaries, although birds ap- standingwater supportedstands of densecattail (Ty- parently never fed beyond the limits imposedby ad- pha sp.). During the breeding seasonsof 1984 and vertising perches. In September 1985 aerial photo- 1985, 9-11 pairs of Willow Flycatcherswere present graphs were taken of each study area from an at the Badenoch site. approximatealtitude of 1,000 m. The exactscale of During 1984 additional observationswere made at eachphotograph was determined from ground-mea- a site 5 km northeastof Guelph. This site ("Guelph sured distancesbetween prominent physiognomic Lake") had vegetation similar to Badenochbut con- features of the habitat. Field data were then super- tained only 3-4 breeding pairs of Willow Flycatchers. imposedon the photographs,and territory size was This site
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