Why the first year is the decision point
The clearest pattern across these studies is that dispersal is a young animal's problem. In American redstarts, conditions during a bird's first tropical nonbreeding season appear to shape how far it flies on its first spring migration and which direction it settles—and that early experience seems to set the latitude where it breeds for the rest of its life. Adults of the same species, by contrast, showed considerable site fidelity between breeding seasons, meaning conditions no longer shifted their movements after the first breeding attempt [1].
Burrowing owls show the same split from a different angle. Juvenile owls varied widely in when they left home and adjusted their timing to experimental food and parasite treatments, local conditions, and their parents' departure. Adult owls did not change their migratory departure in response to food supplements in either year of the study [5]. In other words, the same environmental lever moved the young but not the old.
What conditions actually tip the decision
Food and parasites are two of the levers with direct experimental support. In burrowing owls, juveniles at nests treated for ectoparasites began dispersing at a younger age than juveniles at control nests, while juveniles that received no food supplement in a low-food year waited more than two weeks after their parents left before dispersing. The authors read this as unfavorable conditions—low food or heavy parasite loads—causing juveniles to delay departure, while prompting adults to leave sooner [5].
For giraffes, the relevant condition is human presence. Calves born farther from high-impact human settlements were more likely to disperse (socially, spatially, or both), moved greater distances, and left at a younger age [2]. That is a correlational pattern from seven years of watching 137 calves, not a manipulation, so it identifies where dispersal is more common rather than proving what causes it.
Sometimes "staying" and "leaving" aren't opposites
Giraffes complicate the stay-or-go framing in a useful way. Females were predominantly philopatric—they stayed—and half of the minority that dispersed kept associating with the adult female community they were born into, which the authors suggest reflects the importance of maintaining female ties from calf to adulthood. Males mostly dispersed (69% of young males), and 84% of those joiners attached themselves to a different adult female social community than their mother's. Yet one in four of these social dispersers barely moved at all [2].
The authors propose that for adolescent males, overlapping female communities may offer a pool of unrelated mating partners without the risks of traveling to unfamiliar areas [2]. That is offered as a possible function, not a demonstrated one. The broader point is methodological: in animals with layered societies, measuring distance moved alone can miss dispersal that happens socially.
Why age patterns can be hard to see at the population level
If young animals move and older ones stay, you might expect age to map neatly onto space—older birds in better territories, for instance. In one wild bird population, researchers found associations between age and aspects of territory quality, but little spatial age structure compared with the spatial structure of territory quality and reproductive output, and little repeatability of that age structure between years. They suggest high breeding site fidelity among survivors, combined with frequent territory turnover from high mortality and immigration, weakens the link between age and territory quality—and that habitat quality may matter more than age for where reproduction happens in this system [4].
Individual social tendency may also color these decisions. A review of sociability as a personality trait notes that whether sociability correlates with other behaviors, and in what direction, may vary with sex, body condition, and ecological conditions, and that the mechanisms behind social tendency vary with species and age. Direct evidence linking sociability to dispersal is described as limited to a few examples [3]. So age and ecology set the stage, but an individual's own social disposition may shift how it responds.
Sources used in this answer
Natal dispersal driven by environmental conditions interacting across the annual cycle of a migratory songbird
In American redstarts, habitat use during the first tropical nonbreeding season appears to interact with spring phenology on temperate breeding grounds to influence first-migration distance and natal dispersal direction, while adults showed considerable site fidelity between breeding seasons.
Leaving by staying: Social dispersal in giraffes
Among giraffe calves, females were predominantly philopatric and males mostly dispersed (often socially rather than spatially), and calves born farther from high-impact human settlements were more likely to disperse, move farther, and disperse younger.
Sociability as a personality trait in animals: methods, causes and consequences
A review finds that sociability varies consistently among individuals, that its correlates may differ with sex, body condition, and ecological conditions, and that direct evidence linking sociability to dispersal is limited.
Age-specificity in territory quality and spatial structure in a wild bird population
In a wild bird population, age was associated with some aspects of territory quality but showed little spatial age structure or between-year repeatability compared with territory quality and reproductive output.
Experimental changes in food and ectoparasites affect dispersal timing in juvenile burrowing owls
Juvenile burrowing owls adjusted dispersal timing to experimental food and parasite treatments, local conditions, and parental departure, whereas experimental food supplements did not affect adult departure decisions.