Vegetation Vigor and Reproductive Activity in Harris’s Hawks. Insights from NDVI Across Five Breeding Seasons

Abstract

We examined how vegetation vigor, measured using the Normalized Difference Vegetation Index (NDVI), relates to nesting activity in Harris’s Hawks (Parabuteo unicinctus) across the lower Rio Grande Valley of Texas. NDVI metrics calculated from Landsat imagery were paired with field-based nest presence data collected across five breeding seasons (2018, 2019, 2020, 2023, 2024). Each surveyed territory (n = 218) was coded as nest present or nest-absent, and NDVI summary statistics (mean, minimum, maximum, median, and range) were calculated for three landcover types: shrub/scrub, cropland, and wetland. A random forest classification model identified mean shrub NDVI, minimum overall NDVI, minimum wetland NDVI, and year as the strongest predictors of nest presence. Partial dependence analyses revealed that higher shrub NDVI increased the likelihood of nesting, while lower territory-wide NDVI minimums and intermediate wetland NDVI minimums were associated with nest presence; these patterns are consistent with selection for heterogeneous habitats that combine productive vegetation with lower-vigor patches. The strong influence of year indicates that interannual climatic variation also shapes breeding decisions. Collectively, this work demonstrates that both vegetation vigor and landscape heterogeneity contribute to nesting activity in Harris’s Hawks and highlights the value of NDVI-based monitoring for understanding and managing raptor habitat in drought-prone regions.