Citizen science photographs indicate different timing and location use of migrating adult and juvenile Whimbrels

Lay Summary center dot It is difficult to collect baseline information for species, like Whimbrel, that breed in remote locations. center dot We aged Whimbrel photographs uploaded to eBird to identify the timing of adult and juvenile Whimbrel migrations along the North American Atlantic coast. center dot We then analyzed all checklists submitted to eBird to identify juvenile and adult Whimbrel migratory hotspots. center dot Juvenile Whimbrel migrated through the Atlantic coast 29-41 days later than adults and used a more diffuse path through the North American coast. center dot The percentage of juvenile photographs collected throughout the study area predicted the percentage of adult photographs 3 years later better than 1 and 2 years later, which is consistent with Whimbrel's delayed breeding strategy. It is imperative to identify factors that influence population trends for declining species, but demographic parameters can be especially challenging to quantify for birds, such as Whimbrels (Numenius phaeopus), that breed in locations that are logistically difficult to access. At least two disjunct Whimbrel populations breed in remote and difficult to access northern latitudes but migrate through the heavily populated North American Atlantic Coast during autumn migration. Here, we capitalize on the Whimbrel migrations through the more populated coastal areas to age Whimbrels in photographs uploaded to the citizen science website, eBird, to identify the timing and location of juvenile Whimbrels staging for trans-Atlantic migratory flights. Mean photograph dates for adult migration were synchronous with reported dates for the Mackenzie Delta population that breeds along the northern coast of Northwest Territories, Canada, and stages in Atlantic Canada and for the Hudson Bay population that stages along the South Atlantic USA coast. However, the mean dates of juvenile photographs were 29-41 days later than adult dates, depending on the region. Space use by juveniles along the coast also differed from that reported for adults. Adults primarily depend on Atlantic Canada and the South Atlantic USA coast during fall migration. The percentage of juveniles was greater outside these two primary staging locations. Region-specific juvenile photograph dates suggest that juveniles may drift farther south than the majority of adults from their respective populations. The percentage of juvenile photos collected better predicted the percentage of adult photos 3 years later than 1 and 2 years later which is consistent with Whimbrel's delayed reproductive strategy and provides validation for using photographs to obtain age ratios. As photograph uploads become more commonplace, this and similar analyses may be used to obtain information that would normally be logistically difficult with traditional field methods.