Why departure is not a simple tug-of-war
Migratory departure is not a single switch but a process with stages. A bird or other migrant must first be in the right internal condition—fueled, hormonally primed, and physiologically ready—before it can even consider leaving. That internal readiness is shaped by the individual's state, and departure timing may depend on it [1]. But being ready is not the same as going. For the actual departure, another external cue is often involved, such as a change in weather or wind [3].
This staging helps explain why the question 'which dominates' has no single answer. Internal state and external cues are not competing forces on the same lever; they operate at different points in the decision chain. The migrant integrates its internal state with perceived external cues, and the outcome depends on which stage of the journey it is at [2].
Internal state often sets the broad window for departure
In many species, the internal state—body condition, hormone levels, and migratory restlessness—defines a window during which departure is possible. For nocturnal migrants stopping over, departure from a stopover site is cued by a suite of innate, internal and external factors [5]. The internal factors include hormonal changes such as diel variation in corticosterone, which in northern wheatears correlates with decisions about whether to resume migration [5]. This does not mean the hormone single-handedly causes departure; rather, it is part of the internal state that makes departure more or less likely on a given night.
The primacy of internal state is clearest at the very beginning of migration. In three long-distance migratory species, the internal state of the individual interacts with external cues, but just when leaving the wintering grounds, the internal state appears to dominate; once migration is triggered, external cues become more influential [2]. That pattern suggests a handoff: internal state gets the animal moving, and external conditions then shape the timing and route.
External cues can trigger, delay, or redirect departure
Once an animal is internally ready, external cues often decide the exact moment of departure. For migrant songbirds facing an ecological barrier, departure timing is associated with both the bird's internal condition and the external environment, including geomagnetic conditions [4]. In that context, external cues can act as the final trigger or as a brake, depending on whether conditions are favorable.
The same logic appears across taxa: for the actual departure, an external cue is often involved [3]. This is why a migrant can be physiologically primed yet remain at a stopover for days, waiting for winds or weather to shift. The external cue does not override internal state; it operates on an animal that is already in a departable state.
What tips the balance when the two conflict
When internal state and external cues point in different directions, the outcome seems to depend on the stage of migration and the species' decision rules. At the start of migration, internal state may win, as suggested by the pattern in three long-distance migrants where the internal state dominates just as animals leave the wintering grounds [2]. At stopover sites later in migration, external cues such as weather or geomagnetic conditions can hold back an otherwise ready migrant [4][5].
The broader synthesis is that migrants use decision rules that integrate internal and external information, and the relative weight of each is context-dependent rather than fixed [1][3]. This is an interpretation of how the system is organized, not a claim that one factor is universally dominant. The evidence supports a staged, conditional answer: internal state often sets the window, external cues often set the moment, and the balance shifts with the phase of migration.
Sources used in this answer
Cues, strategies, and outcomes: how migrating vertebrates track environmental change
Migrating vertebrates integrate internal state data with external cues to make decisions about departure timing, and departure timing may depend on the migrant's state.
The role of environmental factors on the spatial behaviour of three long-distance migratory species
In three long-distance migratory species, the internal state of the individual interacts with external cues, with internal state dominating just when leaving the wintering grounds, but once migration is triggered, external cues become more influential.
Cues and decision rules in animal migration
For the actual departure in animal migration, an external cue is often involved, in addition to internal physiological factors.
Factors influencing the movement biology of migrant songbirds confronted with an ecological barrier
In migrant songbirds confronted with an ecological barrier, departure timing is associated with both internal state and external environmental factors such as geomagnetic conditions.
Diel variation in corticosterone and departure decision making in migrating birds
In northern wheatears, departure from a stopover site is cued by a suite of innate, internal and external factors, including diel variation in corticosterone.