Social Displays in Red-Winged Blackbirds (Agelaius phoeniceus): Sensitivity to Thermoregulatory Costs

Abstract

ABSTR•CT.-Breeding male Red-winged Blackbirds (Agelaius phoeniceus) engage in territorial advertisement and agonistic displays that have ordered benefits. We studied the frequency of these behaviors in relation to thermoregulatory cost by comparing plots of activity vs. microclimate with plots of energy and water cost vs. microclimate, using standard operative temperature, Tes, to quantify microclimate. We used a multiple regression model to control for nonthermal environmental factors covariant with Te• to clarify the behavior-thermoreg-ulation relationship. We found that male Red-winged Blackbirds shifted from display to shelter seeking and foraging in proportion to increasing thermoregulatory energy cost. Water costs appeared unimportant. Higher-value agonistic display appeared less sensitive to ther-moregulatory cost than did lower-value territorial advertisement. ENDOTFiERMY incurs food and water costs that can be minimized by appropriate behavior (Bar-tholomew 1964, Morse 1980). King and Murphy (1985) questioned whether food availability is a major limiting factor driving behavior and population dynamics, citing compensating mechanisms and apparent periods of idleness. Maintenance (basal plus thermoregulatory) metabolism , however, accounts for 40-60% of the total avian energy budget (Walsberg 1983a, Weathers et al. 1984). Most small birds probably regulate their food energy budget over periods of, at most, one or a few days because the rate of energy use is large compared with storage capacity (Walsberg 1983b). The time allocated to foraging and shelter seeking vs. other activities thus should respond to thermoregulatory energy and water demands on a short time scale (hours to days). Higher costs do appear to reduce activity (Morton 1967, Morse 1970, Grubb 1975, Evans 1976, Searcy 1979, Grubb and Greenwald 1982). Temperate Zone breeding display presents a paradox, however, as it is concentrated early in the day and early in the year, when temperatures are low and thermo-regulatory costs high. We used male Red-winged Blackbirds (Age-laius phoeniceus) breeding in a cattail marsh in • Present address 413 west-central Indiana to test the hypothesis that the allocation of time to two general classes of breeding behavior, territorial advertisement and agonistic display, should be sensitive to ther-tooregulatory cost and to the potential consequences of the behavior for reproductive success. The null hypothesis was that there is no sensitivity to thermoregulatory cost, possibly because periods of idleness are reduced to support increased foraging effort (Ettinger and King 1980) or because the heat increments of social display activity may substitute for thermoreg-ulatory heat production (Walsberg 1983b, Pala-dino and King 1984), or both. MATERIALS AND METHODS Experimental desl•n.-The experimental design used a reverse optimality approach (McFarland 1977; but see Maynard Smith 1978). Briefly, an optimally adapted animal will have an "objective function" (the an-imal's internal representation of costs and benefits, measurable by the resources allocated to the activity) identical to a "cost function" (the actual energetic or reproductive costs and benefits of the activity in that environment). Reverse optimality assumes optimization , and seeks the function being optimized by comparing behavior patterns with possible cost functions. We assumed that time resource allocation inversely proportional to a thermoregulatory cost function indicates that the cost function may enter into the time allocation decision. Similarly, time allocation directly proportional to a benefit suggests that the benefit enters the decision.