Title. Evolution of maternal investment strategies for the American Bald Eagle, Haliaeetus. leucocephalus, based on environmental risk factors.

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1 Title Evolution of maternal investment strategies for the American Bald Eagle, Haliaeetus leucocephalus, based on environmental risk factors. Author Bethany Levenson,

2 Abstract The American Bald Eagle (Haliaeetus leucocephalus) can be found in Canada, Alaska, the contiguous United States, and Mexico. Bald Eagles typically lay between one and three eggs per year. The juvenile eagles fledge when they are between eight and 14 weeks old and are sexually mature when they reach between four and five years old. Egg quantity and quality was plotted to determine the risk management strategy used by the balk eagle. We conclude that starvation is a far greater threat to offspring survival than predation.

3 Introduction The American Bald Eagle (Haliaeetus leucocephalus) is a sea eagle and a bird of prey. It can be found in Canada, Alaska, throughout the contiguous United States, as well as Mexico. They mostly feed on fish, although they will also eat other birds and mammals. An adult female Bald Eagle will weigh about 12 pounds, and is 25% larger than the male birds, which average 9 pounds. The birds build giant nests, which they will use year after year to lay their eggs. Average clutch size is three eggs. The largest nest reported weighed 2.7 metric tons. The Bald Eagle reaches sexual maturing within four or five years and has a life span of about 28 years in the wild. Starvation is a great threat to nestlings, but not a great threat to adults. The lifetime fitness goal of each sexually mature female, like the American bald eagle, is the survival of two sexually mature offspring to replace her and her mate. Given that most offspring perish before reaching maturity, how do mature females of any species clam, insect, fish, frog or elephant reach this goal? According to the maternal risk management model (Cassill, D. L., 2013), utopian environments favor investments in a few, low quality offspring, predation environments favor investments in offspring quantity, seasonal environments with periods of scarcity favor investments in offspring quality and multi-risk environments with high predation and periods of scarcity favor investments offspring diversity usually a few highquality offspring and many-low quality offspring. In this study, we measured offspring quality and quantity for the saltwater crocodile and then used these metrics to predict the environmental risk factors that shaped the evolution of maternal investment strategies.

4 Method Offspring number and relative offspring body-size were plotted on the inner x and y axes. The outer x and y axes are qualitative probabilities of predation or starvation. The relative body size of offspring at independence and thus the probability of starvation was estimated as S = m / M where S = expected probability of offspring mortality based on cycles of food scarcity; M = mass of mother at the time of offspring independence; m x = mass per offspring at the time of its independence. The expected probability of offspring mortality by predation was estimated as P = 1 (2/N) where P = expected probability of predation; 2 = expected lifetime fitness per mother; N = the number of offspring produced by a mother per clutch or lifetime. The expected probability of offspring mortality in multiple-risk environments is estimated as PS. Results Offspring quality was estimated using 12 lbs. as the average weight of offspring at independence (25 times a mean birth weight of 8 oz.). Maternal weight was also estimated as 12 lbs. Relative offspring quality represents the probability of offspring mortality by starvation, which was calculated as 12/12 = 1.0. Offspring quantity was estimated at 3 eggs per clutch. The probability of offspring mortality by predation was calculated as 1 [2/3] = To summarize here, the percent of offspring that will die of predation (33%) is far less than the percent of offspring that will die of starvation (100%; Fig. 1).

5 Figure 1: The maternal investment strategy by the American bald eagle. P: Predation environments favor investments in offspring quantity rather than quality. S: Seasonal environments that cycle between abundance and scarcity favor investments in offspring quality rather than quantity. R: Multi-risk environments with high predation and periods of scarcity select for a few high-quality offspring and many-low quality offspring. U: Utopian environments favor few, low quality offspring. Discussion The Bald Eagle is the national bird of the United States, and is not really bald, but rather has a white head. The Bald Eagle is an opportunistic feeder who feeds mostly on fish. An average eagle s diet is composed of 56% fish, 28% birds, 14% mammals, and 2% other prey. Eagles are aggressive competitors and can outcompete animals such as coyotes and foxes, which target the same prey. They can reach speeds up to 70 km/hr when soaring or flapping, and speeds

6 up to 160 km/hr when diving for prey. Hatchlings rely on their parents to bring them food when they are first born. Bald eagles are not at much risk from predators, but it is highly likely that some of the nestlings will die from starvation, nest collapses, sibling aggression, or inclement weather. Based on our calculations, we can deduce that the saltwater crocodile s maternal investment strategy, investing more in offspring quantity than quality, evolved in an environment in which offspring are helpless and would die of starvation as infants, but not predation as an adult.

7 References Bird, D.M. (2004). The Bird Almanac: A Guide to Essential Facts and Figures of the World's Birds. Ontario, Canada: Firefly Books Ltd, Richmond Hills. Brown, L. (1976). Birds of Prey: Their biology and ecology (pp. 226). London: Hamlyn. Del Hoyo, J., Elliott, A., & Sargatal, J. (1996). Handbook of the Birds of the World (3rd ed.). Barcelona: Lynx Edicions. Erickson, G. M., Gignac P. M., Steppan S. J., Lappin A. K., Vliet K. A., Brueggen, J. D., Inouye, B. D., Kledzik, D., & Webb, G. J. W. (2012). Insights into the ecology and evolutionary success of crocodilians revealed through bite-force and toothpressure experimentation. PLoS ONE, 7(3), e doi: /journal.pone Erickson, L. (n.d.). Bald eagle. In Journey North (about bald eagle nests). Retrieved from Greer, A. E. (1974). On the maximum total length of the salt-water crocodile (Crocodylus porosus). Journal of herpetology, 8(4), doi: / Virginia Department of Game and Inland Fisheries. (n.d.). Bald Eagle Facts. Retrieved October 15, 2013, from