Over the past few decades, researchers have come to recognize that a mothers physiological response to her environment can have transgenerational effects. Females transfer testosterone to their young prenatally and this maternal effect makes offspring more aggressive. Aggression can strongly influence an animals success depending on environmental context, and maternal effects are an important source of variation in aggressive behavior; yet, we do not fully understand what shapes maternal hormone responses or the proximate mechanisms that mediate its effects. While it is generally assumed that females in competitive environments allocate more testosterone, adaptively creating more aggressive offspring, not all bird species (in which maternal effects are best studied) respond by allocating more testosterone to egg yolks, making it hard to predict how different bird species behaviors and egg components will respond to environmental change. Additionally, we do not know what mechanisms mediate the effects of prenatal testosterone on offspring behavior, preventing us from understanding how it fits into larger ecological and evolutionary frameworks. Hence, my dissertation addressed 1) the causes of interspecies variation in yolk testosterone allocation in competitive environments and 2) the molecular mechanisms facilitating behavioral plasticity in offspring exposed to prenataltestosterone. For Aim 1, I performed a meta-analysis to identify species-specific traits influencing yolk testosterone allocation (Chapter 2) and experimentally tested the findings(Chapter 3). I found that colonial species do not allocate more yolk testosterone in competitive environments, unlike solitary species. This work challenges a widely held assumption that this maternal effect is characterized by a uniform response to competition, showing that it should be contextualized with life-history traits. Next, I explored natural variation in molecular responses to yolk testosterone in a wild songbird (Chapter 4) and experimentally tested the mechanisms in a captive species (Chapter 5). Hundreds of neural genes are differentially expressed in offspring exposed to yolk testosterone, including genes in behavioral pathways (e.g., nitric oxide and serotonin). Additionally, the data suggest epigenetic mechanisms may play a role in mediating the effects of maternal testosterone on offspring phenotype. Ultimately, this work helps us better understand the environmental and molecular causes of phenotypic plasticity.