Importance of the 2014 Colorado River Delta pulse flow for migratory songbirds: Insights from foraging behavior

Abstract

The Lower Colorado River provides critical riparian areas in an otherwise arid region and is an important stopover site for migrating landbirds. In order to reverse ongoing habitat degradation due to drought and human-altered hydrology, a pulse flow was released from Morelos Dam in spring of 2014, which brought surface flow to dry stretches of the Colorado River in Mexico. To assess the potential effects of habitat modification resulting from the pulse flow, we used foraging behavior of spring migrants from past and current studies to assess the relative importance of different riparian habitats. We observed foraging birds in 2000 and 2014 at five riparian sites along the Lower Colorado River in Mexico to quantify prey attack rates, prey attack maneuvers, vegetation use patterns, and degree of preference for fully leafed-out or flowering plants. Prey attack rate was highest in mesquite (Prosopis spp.) in 2000 and in willow (Salix gooddingii) in 2014; correspondingly, migrants predominantly used mesquite in 2000 and willow in 2014 and showed a preference for willows in flower or fruit in 2014. Wilson's warbler (Cardellina pusilla) used relatively more low-energy foraging maneuvers in willow than in tamarisk (Tamarix spp.) or mesquite. Those patterns in foraging behavior suggest native riparian vegetation, and especially willow, are important resources for spring migrants along the lower Colorado River. Willow is a relatively short-lived tree dependent on spring floods for dispersal and establishment and thus spring migrants are likely to benefit from controlled pulse flows.

Greenness and Actual Evapotranspiration in the Unrestored Riparian Corridor of the Colorado River Delta in Response to In-Channel Water Deliveries in 2021 and 2022

“…Because EVI2 is not scaled, it is generally expected to be lower than NDVI*. The transition to using EVI2, as opposed to NDVI*, aligns with the evolution of remote sensing applications, leveraging the strengths of EVI2 in distinguishing vegetative signals from background noise, especially in arid landscapes with sparse vegetation [16]. While NDVI* has been proven effective in its specific scaling method to address soil and atmospheric variability, EVI2 offers a more automated and consistent approach for remote sensing studies in the Colorado River Delta [34,35].…”

“…Landsat has been employed to analyze the health of riparian vegetation within the Colorado River Delta through vegetation indexes (VIs) used as proxies of vegetation Remote Sens. 2024, 16, 1801 2 of 36 greenness and water use [12]. The use of remote sensing to estimate riparian vegetation extent, greenness, phenological changes, and water use is especially pertinent in the Delta's narrow and largely inaccessible riparian corridor [13].…”

“…The riparian ecosystem in this arid transborder region produces a unique hydrological setting which supports high biodiversity and primary productivity compared with adjacent uplands [14]. For these reasons, this riparian corridor is a key flyway for migrating neotropical songbirds [15,16].…”