Holding steady: Little change in intensity or timing of bird migration over the Gulf of Mexico

Abstract

Quantifying the timing and intensity of migratory movements is imperative for understanding impacts of changing landscapes and climates on migratory bird populations. Billions of birds migrate in the Western Hemisphere, but accurately estimating the population size of one migratory species, let alone hundreds, presents numerous obstacles. Here, we quantify the timing, intensity, and distribution of bird migration through one of the largest migration corridors in the Western Hemisphere, the Gulf of Mexico (the Gulf). We further assess whether there have been changes in migration timing or intensity through the Gulf. To achieve this, we integrate citizen science (eBird) observations with 21 years of weather surveillance radar data (1995–2015). We predicted no change in migration timing and a decline in migration intensity across the time series. We estimate that an average of 2.1 billion birds pass through this region each spring en route to Nearctic breeding grounds. Annually, half of these individuals pass through the region in just 18 days, between April 19 and May 7. The western region of the Gulf showed a mean rate of passage 5.4 times higher than the central and eastern regions. We did not detect an overall change in the annual numbers of migrants (2007–2015) or the annual timing of peak migration (1995–2015). However, we found that the earliest seasonal movements through the region occurred significantly earlier over time (1.6 days decade−1). Additionally, body mass and migration distance explained the magnitude of phenological changes, with the most rapid advances occurring with an assemblage of larger‐bodied shorter‐distance migrants. Our results provide baseline information that can be used to advance our understanding of the developing implications of climate change, urbanization, and energy development for migratory bird populations in North America.

Bright lights in the big cities: migratory birds’ exposure to artificial light

“…To discriminate contaminated scans (ie with precipitation) from precipitation‐free scans (ie clear or biologically dominated), we designed a random forest classifier (Horton et al . ) using the R package “randomForest” (Liaw and Wiener ). We trained the classifier on 318,047 (spring: 157,279; fall: 160,768) manually classified nocturnal scans, selected from a 2.5‐hour period centered on 3 hours after local sunset on 15 March, 15 April, 15 May, 1 September, 1 October, and 1 November.…”

Environmental effects on flying migrants revealed by radar

“…Knowledge regarding the abundance and distribution of migrants is important for understanding their ecology and could be critical for their conservation (Hüppop et al 2019). Recently, substantial progress has been made with radar-based calculations of transport phenomena involving both migrating insects (Hu et al 2016) and birds (Dokter et al 2018, Horton et al 2019), but such studies are still very rare. Despite the importance of characterizing animal-habitat associations, only a few studies have so far estimated the densities of migrating birds departing from stopover sites using weather radars.…”

Tiger sharks eat songbirds: scavenging a windfall of nutrients from the sky

“…Despite this, the frequency of tiger‐shark–bird interactions reflects the sheer magnitude of seasonal bird migrations across the Gulf of Mexico (in excess of 2 billion birds per season; Horton et al. ). In addition, this seasonal pulse of nutrients benefits a particular portion of the tiger shark population.…”