Global positioning system (GPS) and platform transmitter terminal (PTT) tags reveal fine-scale migratory movements of small birds: A review highlights further opportunities for hypothesis-driven research.

Studying migration is critically important for understanding the full-annual cycle of migrating birds and aids in management and conservation. In recent years, global positioning system (GPS) and platform transmitting terminal (PTT) tags have become miniaturized, revolutionizing migration research by allowing for higher location accuracy and global coverage for smaller species. We reviewed 116 primary research papers from 2006 to 2021 that reported on GPS- or PTT-tagged small birds (<500 g) during migration and assessed research aims, tag efficacy, ecological findings, and future opportunities. Reviewed papers covered 58 unique species (nearly half of which were shorebirds) and 65% of studies tagged birds at breeding grounds. Tags that did not require retrieval had higher success obtaining data, with PTT tags having the greatest success (85% of deployed tags had data). Of studies that reported tag load information, most (70%) reported tag loads -3% body mass, which is a potential concern given that 3% is considered the preferable upper limit. Most papers (69%) focused on presenting spatial information such as migratory routes, stopover sites, and habitat use, and only 33% of papers tested explicit hypotheses, demonstrating that we are in the early stages of understanding small bird migration at fine scales. Almost all tag deployments (93%) occurred in the northern hemisphere, most often in the United States (24%), indicating a deficiency in GPS and PTT research on small bird migration from the southern hemisphere. GPS and PTT tags have revealed new biological insights not possible with other technologies by providing fine-scale movement data and global coverage. There are exciting opportunities for researchers to move beyond descriptive spatial studies and explore hypothesis-driven migratory research, into topics such as intraspecific variation, carry-over effects, and climate-driven movements for irruptive species. [ABSTRACT FROM AUTHOR]