Final answer:
The question inquires about the relationship between testosterone levels and plumage brightness in male birds and is a causal biological query. It engages with the idea that energy expenditure in courtship and plumage could influence parental care, which varies across species.
Step-by-step explanation:
The question, "Does increased testosterone during the spring cause the male bird's plumage to increase in brightness?" is indeed causal. It explores the relationship between hormone levels, in this case, testosterone, and a physical trait of the birds, which is the brightness of their plumage. This inquiry is primarily concerned with biological functions and how they influence behavior and physical characteristics in animals.
Regarding the statement that males of species carrying out courtship rituals do not assist in parental care, whereas those using colorful plumage to attract mates have energy available to assist with parental care, it's important to consider that energy allocation for reproductive strategies may vary widely across species. Some birds may focus on courtship displays, expending a significant amount of energy, which might leave less available for parental duties. In contrast, those that invest in features like colorful plumage may not engage in complex courtship rituals, potentially conserving energy that could be used for supporting offspring. The distinction between these reproductive strategies can affect the amount of energy available for parental care.
Thus, while testosterone and other sex hormones do play a crucial role in the physical development and maturation processes, including the development of secondary sex characteristics and the drive to engage in mating behaviors, the exact relationship between hormone levels and specific behaviors or traits, such as plumage brightness, can vary.