Consistent individual differences in foraging movement tactics drive habitat partitioning in an inland-breeding Great...

Individual variation in animal movement and habitat selection is increasingly recognised as a structurally important component of population ecology, yet the magnitude and temporal consistency of foraging movement specialisation remain poorly characterised in inland-breeding waterbirds. Inland populations of Great Cormorants (Phalacrocorax carbo sinensis) exploit a mosaic of freshwater habitat types in anthropogenic landscapes and generate recurrent conflict with fisheries. Whether individuals consistently differ in foraging movement tactics across habitat types and seasons has not been established.

We deployed solar-powered GPS transmitters on 24 Great Cormorants (5 adults, 19 fledglings) from an inland-breeding colony at Lake Balaton, Central Europe, between 2021 and 2023 (mean deployment duration: 190 days). Across six freshwater habitat types (fishponds, rivers, lakes, wetlands, regulated water bodies, river arms/oxbow lakes), we recorded 249,097 daytime fixes and 20,678 speed-classified foraging locations. Individual variation in seasonal foraging habitat use profiles was quantified using PERMANOVA and ANOSIM on Bray-Curtis dissimilarities; temporal consistency was estimated as adjusted repeatability via linear mixed models.

Individual identity accounted for 73.6% of total variation in seasonal habitat use profiles (PERMANOVA, p < 0.0001), and between-individual dissimilarities significantly exceeded within-individual dissimilarities (ANOSIM R = 0.511, p < 0.0001). Age class explained a negligible fraction of variation (R² = 0.012, p = 0.887). Principal coordinates analysis revealed a structured partition of individual movement space along three largely independent axes - fishpond, river and lake use (r² = 0.957, 0.949, 0.803; all p < 0.001) - jointly accounting for 74.8% of total individual-level variation. Habitat use was significantly repeatable across seasons for four of six habitat types: river (R = 0.722), fishpond (R = 0.642), regulated water body (R = 0.592) and lake (R = 0.575; all p < 0.001).

Inland-breeding Great Cormorants show strong, temporally stable individual differences in foraging movement tactics across seasons, with distinct subsets of individuals consistently targeting fishponds, rivers or lakes - each to the near-exclusion of the others. Habitat partitioning in this population emerges from consistent individual movement decisions, not ontogenetic niche shifts or opportunistic habitat switching. Population-level management will produce disproportionate, habitat-sector-specific outcomes; habitat-explicit, individually targeted approaches are likely to prove more effective in mitigating cormorant-fishery conflicts in inland landscapes.