Migratory passerines share localized movement strategies but differ in activity timing and habitat use during post-breeding stopover

Technical Summary: Migratory passerines share localized movement strategies but differ in activity timing and habitat use during post-breeding stopover

Problem Statement
Stopover is a critical, yet poorly understood, phase of avian migration where birds rest and refuel. During post-breeding migration in early autumn, diverse migratory bird species often converge on the same high-quality sites, creating potential for intense interspecific competition for resources. While "Hutchinsonian" niche partitioning (dividing space, time, or habitat) is well-documented in resident breeding populations, it remains unclear if such partitioning occurs during brief visits to novel habitats like migration stopovers. Current knowledge of stopover ecology is largely limited to departure decisions, lacking high-resolution data on fine-scale movement, daily activity timing, and microhabitat selection across multiple species simultaneously.

Methodology
The study was conducted at the McGill Bird Observatory (MBO) on the western tip of Montreal Island, a peri-urban green space serving as a major stopover site for neotropical migrants. Over two years (2022–2023), researchers captured and radio-tagged 101 individuals of five species representing different foraging guilds and migration strategies: Blue-headed Vireo (Vireo solitarius), Hermit Thrush (Catharus guttatus), Magnolia Warbler (Setophaga magnolia), Yellow-rumped Warbler (S. coronata), and Song Sparrow (Melospiza melodia).

The methodology integrated three distinct analytical approaches:

1. High-Resolution Telemetry Tracking: Birds were tracked daily using VHF nanotags and Yagi antennas to record GPS locations. Movement metrics included median step length, net displacement, cumulative distance, and a straightness index (net displacement divided by cumulative distance).
2. Change Point Activity Analysis: Automated Motus tower data were analyzed using change point detection algorithms to determine the precise onset and cessation of daily activity relative to sunrise and sunset.
3. Resource Selection Functions (RSF): Generalized Additive Mixed Models (GAMMs) were used to model habitat selection. The study compared "used" locations against random "available" locations across four spatial scales (25m, 50m, 100m, 200m). Habitat covariates included mean canopy height, canopy height standard deviation (structural heterogeneity), and forest cover.

Key Results

• Movement Strategies: Across all five species, birds exhibited highly localized, tortuous movements (mean straightness index of 0.30), indicative of centralized foraging. There were no significant differences in straightness indices between species. However, step lengths generally decreased as stopover duration increased for Blue-headed Vireos, Yellow-rumped Warblers, and Song Sparrows, suggesting a shift from exploration to restricted foraging once optimal areas were settled. Contrary to predictions, arrival fuel load did not significantly predict movement patterns.
• Diel Activity Timing: Significant species-specific differences were observed in daily activity windows. The aerial insectivore (Blue-headed Vireo) had the shortest activity period, starting ~15 minutes after sunrise and ending ~49 minutes before sunset. In contrast, insectivorous warblers and omnivores (Hermit Thrush, Song Sparrow) began activity before sunrise and continued after sunset. The Hermit Thrush showed the earliest onset, while the Song Sparrow remained active latest into the evening. Activity timing was primarily driven by species identity rather than seasonal trends or days to departure, though some species adjusted timing as departure approached.
• Habitat Use: Habitat selection was best explained at the finest spatial scale (25 m). Species responded differently to vegetation characteristics:
  • Hermit Thrush: Showed strong nonlinear preferences for intermediate forest cover (>50% reduced use) and high structural heterogeneity.
  • Yellow-rumped Warbler: Preferred areas with high canopy height homogeneity (low SD) and peak use at ~75% forest cover.
  • Magnolia Warbler: Preferred ~50% forest cover and avoided areas with mean canopy heights >20 m.
  • Blue-headed Vireo: Showed only marginally significant responses to canopy heterogeneity and forest cover.

Significance and Claims
The authors conclude that while migratory passerines share broadly similar spatial movement strategies (localized foraging) during stopover, they reduce potential competition through niche partitioning in activity timing and microhabitat preferences. The study highlights that:

1. Temporal Partitioning: Differences in foraging guilds (e.g., aerial insectivores vs. ground-foraging omnivores) drive distinct daily activity windows, likely constrained by prey availability (e.g., insect activity vs. fruit/seeds) and visual foraging requirements.
2. Microhabitat Partitioning: Species exhibit specific, non-linear preferences for forest cover and structural heterogeneity at fine scales (25 m), suggesting that habitat quality is not uniform across species.
3. Conservation Implication: The findings underscore the importance of preserving heterogeneous forest structures and forest-edge habitats within peri-urban stopover sites. The authors argue that conservation efforts often focus on breeding habitats, yet the specific microhabitat requirements during the energetically demanding migration period are critical for population sustainability. The study suggests that maintaining structural diversity in forest edges and woodlots is essential for supporting diverse assemblages of migratory birds.