Introduction

The southern end of Lake Michigan is widely recognized as a migratory funnel during spring. Northbound birds encountering the lake frequently concentrate along the shoreline before continuing their journey. While much migration occurs at night, a substantial component becomes visible after sunrise as birds resume movement from stopover habitat under favorable wind conditions. This morning movement, commonly referred to as longshore flight or morning flight, can produce large and highly concentrated pulses of migrants along the dune and shoreline corridor.

At Indiana Dunes State Park, longshore flights have been documented from the Longshore Flight Tower for more than a decade using a consistent standardized protocol. The elevated vantage point atop a 60-foot dune provides broad views of the Lake Michigan shoreline and adjacent dune and prairie habitats, allowing observers to quantify resumed diurnal migration across a wide airspace. Between 2012 and 2021, monitoring recorded a cumulative total of 2,655,818 birds, with annual totals ranging from 185,180 to 428,374 individuals. These historical counts establish Indiana Dunes as one of the most productive and dynamic visible migration sites in the region.

A major programmatic shift occurred beginning in the 2024 season. The Indiana Dunes State Park Division of State Parks relinquished oversight of survey administration, and a portion of program funding was shifted from State Parks to Indiana Audubon through the Lake Michigan Coastal Program. Indiana Audubon assumed responsibility for survey administration and grant reporting while maintaining the established field protocol.

The 2024 and 2025 seasons therefore represent Indiana Audubon’s first two years of administering the longshore flight count. The purpose of this article is to summarize seasonal abundance, species richness, timing, and flight composition during these two seasons, and to describe how migration intensity is distributed across days and months. By presenting these results under a consistent protocol, this article provides both a recent seasonal summary and a clear baseline for Indiana Audubon’s continued stewardship of longshore migration monitoring at Indiana Dunes State Park.

Study Site

Counts were conducted at the Indiana Dunes State Park Longshore Flight Tower/Observation Tower in Chesterton, Indiana. The observation platform is located atop a 60-foot sand dune and includes a 20-foot observation deck. The site overlooks Lake Michigan to the north and provides unobstructed views over the Dunes Prairie Nature Preserve to the south. The count area consists of all visible airspace surrounding the tower, including movement along the lake edge and over adjacent dune and prairie habitats.

To maintain consistency and avoid inflating totals with birds detected away from the core count area, birds observed on foot more than approximately 200 meters from the tower were not included in daily totals.

The Observation Tower at Indiana Dunes State Park provides elevated vantage point over Lake Michigan and adjacent dune habitats.

Methods

Seasonal Framework

Counts were conducted during the core spring migration period. Rather than beginning strictly on a fixed calendar date, surveys were initiated on the first suitable southerly or southwesterly migratory wind event occurring within one week of March 1. This flexible start date reflects the onset of favorable longshore migration conditions and allows survey effort to align with biologically meaningful flight days. The season continued through May 31. Survey effort was prioritized on days with winds generally from the south through west. Northerly winds were avoided, when possible, as these conditions tend to suppress or alter shoreline morning flight dynamics.

Daily Survey Protocol

Counts began 15 minutes prior to sunrise and continued for at least two hours after sunrise. In most cases, surveys continued for approximately four hours after sunrise. Surveys ended early only when migration activity declined substantially for a sustained 30-minute period. Surveys were extended beyond four hours when active migration persisted, particularly for thermal migrants such as Sandhill Cranes and raptors, which often move later in the morning as conditions develop.

Data Collection and Proofing

A primary counter conducted each survey, often with assistance from volunteer observers. Birds were recorded in real time using pencil and paper, clickers, and/or the eBird mobile app. Secondary observers contributed sightings when appropriate, and additional observer effort was noted in field comments. At the end of each survey day, counts were entered into eBird under the hotspot “Indiana Dunes SP—Observation Tower.” Seasonal proofing was conducted via complete eBird data download at the end of each season.

Observers at the Observation Tower using binoculars and field equipment to conduct standardized counts of migrating birds. Photo by Brad Bumgardner.

Effort Standardization

Because survey duration varied modestly by day, flight intensity was standardized as birds per hour. Checklist-level birds per hour were calculated by dividing total individuals recorded by survey duration in hours. This metric was summarized by year to compare relative flight intensity.

Results Overview

Standardized Flight Intensity (Birds per Hour)

Daily Flight Distribution

Daily totals were strongly right-skewed in both years, with most days producing moderate totals and a small number of days producing exceptionally large flights. Migration volume was highly concentrated into a small number of peak flight days:

• 2024: The five largest flight days accounted for 48.4% of the seasonal total
• 2025: The five largest flight days accounted for 42.9% of the seasonal total

Peak Flight Days

Flight Composition by Species Guilds

Passerines and other landbirds dominated seasonal totals in both years, comprising over 92% of individuals recorded in 2024 and 95.5% in 2025. This is consistent with the early-season dominance of blackbirds, robins, and other songbirds utilizing shoreline migration corridors.

Phenology of Five Focal Species

Five focal species were selected to represent dominant and ecologically distinct components of the longshore flight. Seasonal timing was summarized using peak day counts and cumulative passage metrics (10%, 50%, and 90% of seasonal totals). The charts below show how these species redistribute their migration timing across the spring season, from early arrivals like Canada Goose to later migrants like Tree Swallow and Turkey Vulture.

Turkey Vulture in flight—a thermal migrant commonly observed at Indiana Dunes during spring migration, with passage concentrated in April and May. Photo by Michael Topp.

Detailed Species Data

Discussion