drosen's blog

Ruffs and Reeves:

As with nearly all evolutionary concepts, the deciding factor that determines behavior is the return on investment regarding behaviors, morphology and reproductive success. Evidence of this is reinforced by the dichotomous mating systems observed in Reeves. Males of is species fall into one of 2 categories depending on their allele distribution. Homozygous males have darker feathers and are territorial while the heterozygous males are lighter colored and act as satellite wanders. As seen in Leks and other group settings, species who control territories often have a higher reproductive success rate. However, they also expend more energy by protecting their territory and engaging in displays to reinforce their dominance. Conversely, the wandering males expend far less energy during the breeding season, but have a reduced reproductive success rate.  This dual-reproductive system has been conserved as the ratios of resource allocation: reproductive success are roughly equivalent between the 2 phenotypes. This shows that breeding systems are not only complicated, but can even vary between individuals in a single species.

Monogamy: As previously mentioned, sexual selection is most intense in polygamous relationships as seen in leks and due to these intense pressures this is seen in less than 10% of all avian species. The alternative, monogamy, revolves around a bonded pairing of individuals that cooperate to copulate and raise their young as this is occasionally essential for the offspring to survive.. Males typically hunt, defend their territory and even assist with incubation in some scenarios and the females will determine the viability of their partner based on these efforts. Once formed, the duration of these bonds can vary with some relationships lasting only for the breeding season and other lasting for life. Despite the superficial appearance of monogamy, there are frequent episodes of “extra-pair copulation” by females who willingly accept advances from other males. On average, only 14% of the offspring in a nest are the offspring of the tending father. This is done by females to ensure variability and viability of their young and males participate in this behavior so as to pass on their genetics as frequently as possible.

Lek Displays:  While many courtship displays occur in pairs, the most extreme cases of sexual dimorphism occur when courtship displays are done in large groups with multiple members of the same species. These large masses of courting displays are known as Leks and due to the extreme competition among the males in these groups there is additional emphasis on dominance and attractive morphology. Typically, a small number of males are selected for and sire the vast majority of the upcoming generation. As previously noted, natural selection favors those with the highest reproductive success so one must question why a subordinate male would continue to reside in these Leks if unsuccessful.  Studies have shown that females typically favor larger groups of males and subordinate males do have a minor chance to reproduce simply by being in the presence of a dominant figure. Of note, leks are typically more closely related to each other than other individuals in other leks and theories proposed by Paris Hamilton suggest that a subordinate role is tolerated more easily if the dominant male has similar genetics as this qualifies as reproductive success in a sense.

Good Gene Hypothesis: Courtship displays are ancient reproductive strategies that have been tightly associated with elaborate plumage. This elaboration is meant to convey superior fitness to females, however, the mechanism of this is unknown. The prevailing theory detailing this correlation is the “Good Gene Hypothesis”.  This theory suggests that exotic or more elaborate plumage serves as a handicap by making them less optimal for flight or running as well as more easily identifiable by predators. However, if a male is able to survive with these additional stressors then a female can infer that they are superior in terms of stamina or survival skills.  Furthermore, this theory suggests that plumage can reveal parasite load and hormonal levels as they have both been shown to affect brilliance or color scheme. Due to the high associated cost associated with developing, maintaining and surviving with such elaborate feather schemes, many species have developed post-breeding molting cycles to eliminate these handicaps during the non-breeding seasons.

Mates: Different investments between females (few # of costly eggs) vs males (high number of low cost sperm) naturally create both sexually dimorphism and behavioral differences between the genders. Natural selection favors those with the highest reproductive success and typically, birds form lifelong mated pairs that cooperate to raise their young as they require assistance to do so. However, as males will have higher success by finding multiple mates they often engage in cross pair copulation with other females. Furthermore, females have ways to protect their larger investments by selectivly choosing their "partner" as well as secondary partners. It is this "choosy" behavior that has driven the extravagant courtship displays seen in the wild. In addition, females will often breed with several neighboring species to ensure that at least some offspring are viable.

Social Rank: Social conflict increases stress hormone levels, such as glucocorticoid, and has other associated costs on the body’s function. To mitigate this effect,  birds have developed a system that is dependent on the dominance of alphas and the subordinate social status of the subordinate individuals. These rankings are often established through physical contest and reinforced by threatening behavior as well as submissive posturing of the inferior specimens. Several factors, including age, parasite load,  physiology and sex all determine status and these figures typically have greater access to food or mates as well as decreased risk of predation. Furthermore, as evidence suggests, birds are able to differentiate individuals based on heritable features such as plumage, size, behavior and voice.

Flocks: Range from temporary, less orderly aggregations to organized foraging associations involving multiple species. These groups can be simple and focus on localized nesting areas, or opportunistic feeding grounds, as well as more intricate social system. Flocks are flexible systems that are defined by several tradeoffs. Benefits include predation risk secondary to statistical improvement as well as additional alerting neighbors as well as increased foraging efficiency as birds are able to share information to utilize their specialties. Flocks can utilize the “Beater Effect” , where decoy predators will flush out prey for the rest of the flock, as well as the more classic rounding up of prey seen in sea birds. However, this system is not without drawbacks as there are increased demand for food supplies, higher rates of disease and increased levels of aggression to maintain a personal space and metabolic needs.  In addition, due to their intrinsically selfish,self fulfilling nature, there are “scrounger” individuals who will benefit from the “producer” birds without contributing. These individuals are a net negative and offer no additional benefit to the flock, further increasing the demand of the flock.

Agonistic Behavior: Interactions between 2 individuals are inherently selfish however, their desire to succeed can lead to aggressive or cooperative interactions. Questions concerning the morality and truthfulness of these interactions as well as the presence of “cheating” the system. The behavior between rivals, i.e aggressive (attack or threaten) as well as passive (flee or submit) are known as agonistic behaviors. Conflict typically peaks at threatening gestures due to the risk of injury when fighting. This includes use of wings, bills and claws as potential weapons that will eventually lead to actual battle if one does not give. To avoid higher frequencies of injury, communication of aggressive and submissive behavior is a large part in social structure. This has lead to submissive behavior and an advanced, ranked social structure to mitigate the stress of a social system.  

Territorial Behavior: This is a pattern of aggressive behavior or spacing. A territory is typically a  fixed area that is defended in both breeding and non breeding season, however, a range can gradually shift locations if the resource that is being defended is mobile. To defend this territory, acts of dominance, songs, or aggressive pursuit discourage other from entering or using the space that is being guarded. The primary use of the territory’s resources are the defender, its mate, and occasionally its progeny.

While a simple territory may solely focus on simple food supplies; more all purpose nesting grounds serve to reduce predation and competition for mates as well. Although the benefits are apparent, there are certain costs associated with territoriality as well. Resources must be spatially and temporally viable in order for the territory’s maintenance to be beneficial.. Birds spend a large amount of calories defending their zones and this expenditure linearly increases with territory size. Furthermore, an abundance of resources can attract an overwhelming amount of outsiders that are unable to be driven away.

Social Behavior: Birds span wide to short ranges and are found in both isolated to large clusters. The determinants of their lifestyles are secondary to what is most effective to meet their requirements of both protection and food. Typically, there are established territories that are defended and utilized with various methods that are flexible in the setting of a cost benefit relationship. Various species typically have a personal zone of space that they prefer. However, this can be reduced in group settings such as while flying in a V formation or when huddling for warmth.  This behavior is understandably more common in more social species and their desire for personal space is species dependent.