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Flying animals that are adapted to life in cluttered environments require effective and robust guidance mechanisms to avoid collisions. Birds and insects use optic flow cues to avoid obstacles when flying, but these are only generated by self-motion and are likely to be used in conjunction with other cues such as visual contrast between foreground and background objects. Orchid bees use a strategy based on relative brightness to find clear paths through dense environments. To investigate the possibility that birds use a similar strategy, we presented zebra finchesTaeniopygia guttatawith symmetric or asymmetric brightness distributions behind the aperture of a tunnel and recorded their flight through it. The background brightness conditions influenced both the birds’ decision to enter the tunnel and their flight direction upon exit. Zebra finches were more likely to initiate flight through the tunnel if they could see a bright background through its aperture and were more likely to fly to the bright side on exiting the tunnel. We found no evidence of a centring response during gap negotiation; instead, the zebra finches entered the tunnel by turning tightly around its near edge. These results hint at a possible pre-planning of the trajectories before the onset of flight.

A tendency to return to the natal/breeding site, ‘philopatry’, is widespread amongst migratory birds. It has been suggested that a magnetic ‘map’ could underpin such movements, though it is unclear how a magnetic map might be impacted by gradual drift in the Earth’s magnetic field (‘secular variation’). Here, using the International Geomagnetic Reference Field, we quantified how secular variation translates to movement in the implied positions at which combinations of different magnetic cues (inclination, declination and intensity) intersect, noting that the magnitude of such movements is determined by the magnitude of the movements of each of the two isolines, and the angle between their movement vectors. We propose that magnetic parameters varying in a near-parallel arrangement are unlikely to be used as a bi-coordinate map during philopatry, but that birds could use near-orthogonal magnetic gradient cues as a bi-coordinate map if augmented with navigation using more local cues. We further suggest that uni-coordinate magnetic information could also provide a philopatry mechanism that is substantially less impacted by secular variation than a bi-coordinate ‘map’. We propose that between-year shifts in the position of magnetic coordinates might provide a priori predictions for changes in the breeding sites of migratory birds.