Interactions between migrating birds and offshore platforms: conclusions and synthesis

Abstract

Our findings indicate that platforms have both beneficial and adverse proximate impacts on migrant
birds. When migrants unexpectedly encounter poor weather en route across the GOM, many
individuals are forced down and perish in the GOM. During severe weather events, we frequently
observed migrants taking shelter on platforms; in extreme instances, several thousand birds crowded
together onto a single platform to wait out a storm. Although it is impossible to know the fate of
individual migrants after they depart a platform, it is likely that the refuge provided by platforms can
ameliorate high weather-induced mortality, and can theoretically have important population-level
impacts on species with small continental populations or very compressed migration periods.

Platforms offer a second major benefit by presenting foraging opportunities. During our first two field
seasons, we discovered an unexpected abundance of terrestrial insects offshore and expanded our
focus to include insect monitoring via both visual censuses and quantitative sampling with ultraviolet
light traps. During both spring and fall, a large blanket of terrestrial insects—the “aerial plankton”—is transported offshore by north winds. This aerial plankton represents a significant food resource for
birds that stop to rest on platforms. During the spring, 10% of birds on platforms were observed to
forage actively, and 44% of the foragers were successful in obtaining food. During the fall, 16% of
birds on platforms foraged actively, and 46% of the foragers were successful. Estimated energy intake
rates of birds on platforms were sometimes higher than generally observed in “natural” habitats
onshore. The frequent abundance of migratory moths and other insects on platforms was especially
important to fuel-depleted migrant birds forced down by foul weather during the spring, and to the
overshoot migrants during the fall.

Platforms also have adverse impacts on some migrants. During the fall, when migrants are aloft over
the northern GOM at night, birds occasionally die in collisions with platforms. The extent of collision
mortality varied greatly among platforms; the reason for the variation is unknown, but may be related
to differences in platform lighting. The overall probability of a migrant dying in a collision with a
platform is very small; we estimate that total mortality over the GOM is <0.01%. A second adverse
impact involved the influence of platform lighting on the flight behavior of nocturnal migrants. On
some occasions, large numbers of birds from a wide range of species appeared at nightfall and
circulated around a platform in a continuous stream—sometimes all night long. These sudden
appearances and subsequent circulation events occurred even in the absence of any trans-Gulf
migration evident on radar, suggesting that the events involved migrants drawn from a large area. The
frequency of this phenomenon also varied greatly among platforms, and circulations were never
observed at some platforms. Again, we do not know the exact reason for the variation but suspect that
differences in platform lighting are implicated. Circular flight around a platform for long periods of
time represents a significant energy drain on a migrant and may reduce the likelihood of completing
the trans-Gulf flight successfully. Based on the platforms we studied, we estimate that <0.5% of all trans-Gulf migrants become involved in these circulation events.