Interactions between migrating birds and offshore oil and gas platforms in the northern Gulf of Mexico: Final Report

Abstract

From The Impact of Platforms on Migrant Birds: “Platforms have three primary proximate impacts on migrant birds: 1) they provide habitat for resting and refueling; 2) they induce nocturnal circulations; and 3) they result in some mortality through collisions.

The availability of suitable habitats en route where migrants can safely replenish energy reserves is critical to successful migration, especially in the vicinity of major ecological barriers such as the Gulf of Mexico... Differences in stopover durations and rates of mass gain by migrants among different stopover sites are thought to be related to the intrinsic suitability of the different habitats available... The suitability of a habitat is difficult to quantify, but presumably reflects food supply…, competition…, safety from predators…, shelter from severe weather…, and the physical structure of the habitat...

Platforms appeared to be suitable stopover habitats for most species, and most of the migrants that stopped over on platforms probably benefited from their stay, particularly in spring. Many of these migrants were able to feed successfully, and some appeared to achieve rates of mass gain that exceeded what is typical in terrestrial habitats. Platforms thus exert some selective influence on trans-Gulf migrants, since a positive association between fat levels at migratory departure and subsequent survival (estimated via return rates) has been demonstrated... Even the individuals that do not feed probably benefit physiologically from the availability of the platforms. Yapp (1956) suggested that migrants may be affected by sources of fatigue other than total depletion of fat stores, such as excessive accumulation of lactic acid, failure of the nerve-muscle junction, or upset of central nervous coordination. These types of fatigue may be eliminated by simple rest. Many of the migrants that rested quietly on the platforms for hours to days were probably recovering from such sources of fatigue.

Our findings suggest that migrants use platforms in highly nonrandom ways. Migrants selected specific microhabitats on platforms (i.e., used alternative microhabitats nonrandomly), much in the same way that they select specific habitats during terrestrial stopovers. Preferred platform microhabitats were species-specific and generally consistent between spring and fall.

One of the most interesting, albeit speculative, impacts of platforms involves their possible role in the natural selection of migration strategies. Experimental studies of captive populations…as well as analyses of banding recoveries from populations of recently introduced species…indicate that migratory behavior, including preferred orientation, can evolve very rapidly…

Another intriguing possibility concerning selective influences of platforms concerns the life-history ecology of Peregrine Falcons. Although the spectacular recovery of Peregrine Falcons has generally been attributed to the ban on DDT along with extensive reintroduction efforts…, it seems reasonable to consider whether the growth of the platform archipelago in the Gulf may be involved. Indeed, since the first platform was installed in 1942, the period 1975-1985 saw the most dramatic increase in the size of the platform archipelago of any 10-year period, with a net addition of 1,395 platforms (i.e., new platforms installed minus those removed). This period was coincident with the recovery period of exponential population growth of Peregrine Falcons.

The existence of the platform archipelago also has potential selective implications for overshooting migrants... During the fall, many shorter-distance migrants that spend the winter along the Gulf Coast (such as wrens and sparrows) inadvertently overshoot the coastline during nocturnal migratory flights and end up over Gulf waters. These overshoot migrants, which are evolutionarily ill-equipped to deal with the rigors of overwater migration, were among the heaviest users of platforms during the fall, and the availability of platform rest stops probably enabled many individuals to return to land successfully. This alleviation of selection pressure may in turn be implicated in large-scale southward shifts in the wintering distributions of some of these species. Long-term Christmas Bird Count (CBC) data may be useful for documenting trends in winter distributions of overshooting migrants in the Gulf region for tests of the “selection reduction hypothesis.