Birds are commonly used as indicator species as they are important functional components of the ecosystem from the perspective of both trophic ecology and tourism value. Waterbirds in particular are useful because they often occupy higher trophic levels, are highly mobile and can respond quickly to environmental change, and are conspicuous and easy to monitor (Ogden et al. 2014). In practice, the use of birds as bioindicators has been met with both successes and failures, and it is generally recommended to: use a suite of species, look at trends over a large spatial extent, and to tailor the indicator species to specific ecosystem attributes (Stolen et al. 2005). We chose a suite of five waterbird species for the purpose of tracking trends in occurrence rates in the Gulf of Mexico. Three species: white ibis (Eudocimus albus), wood stork (Mycteria americana), and roseate spoonbill (Platalea ajaja), have been used extensively in local monitoring efforts within the region and are well-known to respond to changes in prey abundance induced by hydrological alterations (Brandt et al. 2014). The brown pelican (Pelecanus occidentalis) was chosen as it has been documented to be impacted by human activities, both via competition from human fishing pressure, and also via mortality from entanglement and consumption of marine debris (Ogden et al. 2014). Finally, the magnificent frigatebird (Fregata magnificens) was chosen because its residency is generally restricted to coastal areas of the Gulf of Mexico and it is dependent on coastal habitats such as mangroves for nesting; also, it has a slower reproductive rate and therefore its trends in abundance will integrate a wider spectrum of factors.
To construct relative indices of abundance, we used the Cornell Lab of Ornithology’s eBird database, an extensive, standardized compilation of volunteer and professional bird sighting observations (www.eBird.com, Munson et al. 2014). Further details on methods are available in the full report.