Parental investment strategies determined by expected benefits

Methods Redwinged Blackbird (Agelaius phoeniceus) 99 provide almost all of the parental care required to fledge the altricial young, including nest building, egg laying, incubating, feeding, and defending the nest. l'he normal clutch size of Redwings is 3 or 4 eggs, with an occasional 2 or 5. We manipulated clutches in 43 nests during the laying and early incubation periods so that each of approximately equal numbers of nests (8 to 10) contained from 1 to 5 eggs. The average initial clutch size of groups of 99 with varying ultimate clutch size was not significantly different, varying from 3.2 to 4.0 eggs (F = 2.69; d.f. = 4.38; p >.05), with an average of 3.6 (Table 1). The average length of time with eggs in the nest (the laying and incubation period) was similar for 99 with different final clutch sizes, with an average duration of 13.6 days. Nest building also required similar investment by 09 with different final clutch sizes. Although energy requirements to incubate larger clutches may be slightly greater, incubation energy requirements are only a fraction (1/6-1/8) of the energy required by the parent to care for nestlings in passerine species (RICKLEFS 1974). The most detrimental aspect of incubation for the parent can be considered to be the increased exposure of the parent to possible predation, a risk that would be independent of clutch size. Thus, the cost to each 9 to bring her clutch to hatch can be considered to be nearly equivalent amongst the 99. In contrast, the expected benefit of a 9 with a larger clutch will be greater than that of a 9 with a smaller clutch SO that according to the current hypothesis of parental investment decisions, a parent with a larger clutch would be expected to increase the effort (cost) that she would be willing to incur to maintain the present nesting attempt. This was tested by measuring the nest defense aggression of 99 with the varying clutch and brood sizes as a measure of parental investment. Nest defense aggression was measured throughout the nesting period for nests of all final clutch sizes. A nest predator was simulated by a 70 cm long black and green rubber snake wrapped around the nest-supporting vegetation with its head in the nest cup. The behavior of the 99 upon returning to the proximity of their nest was recorded for the first five min after their return. A scoring scheme was devised to rank the 99’ aggression toward the snake, incorporating both the behavioral acts performed and the duration of the acts since the intensity of aggression is a function of both the acts elicited and the amount of time spent on each act. The scoring scheme is similar to that used by ROBERTSON and NORMAN (1977), with behavioral act scores as follows: distant (> 5 m) silent observation = 1; close (> 1, < 5 m) silent observation = 2; distant ‘check’ calling = 3; close ‘check’ calling = 4; circling nest in large circles or fly-by investigations = 5; nest attentiveness (perching near or on nest, silent) == 6; close harsh calling = 7; nest attentiveness with calling = 8; circling the nest closely (< 1 m), silently = 9; circling the nest closely with calling = 10; attadring silently = 15; Attacking with calling = 20. Duration scores, with the scale designed to reflect what we felt were differences in aggression are: act given briefly or only once = 1; gives several times or continuously up to 1 min = 2; given for 1 to 3 min = 4; given for 3 to 5 min = 6; given for 5 min = 8. The score for a given trial was computed as the sum of the products of behavioral act scores and duration scores obtained during the 5 min trial period. For example, a bird which displayed distant ‘check’ calling for 2 min, close harsh calling for 2% min, and briefly attacked with calling would score (3x4) + (7x4) + (20X 1) = 60. More aggressive acts were thus given higher scores. The scores were used as a measure of nest defense aggression. Aggression toward a predator is a very risky undertaking. The greater the aggression and the closer the parent is to the predator, the higher the probability that the parent will be wounded or killed, Thus, more aggression incurs a larger cost in re reproductive attempts, and the aggressiveness of responses may be used as a measure of relative parental investment.