Stopover ecology of landbirds migrating along the middle Rio Grande in spring and fall

Abstract

This research represents the first comprehensive summary of our study of stopover ecology of migratory landbirds in riparian habitats along the middle Rio Grande of central New Mexico. We report results from mist-netting operations conducted during spring and fall migration in 1994, 1995, and 1996. A total of 23,800 individuals of 146 species were captured during the study. Of the 146 species, 53 percent were Neotropical migrants, and 32 percent were temperate migrants. The most abundant species were the MacGillivray's Warbler, Wilson's Warbler, and White-crowned Sparrow in spring and the Chipping Sparrow, Pine Siskin, and White-crowned Sparrow in fall. Migrants were most abundant between late April and the first 2 weeks of May in spring and between the last week of August and mid-October. Temperate migrants passed through the sites earlier in spring and later in fall than Neotropical migrants. About 50 percent of the birds had no visible fat stores upon capture. More birds used riparian habitat along the middle Rio Grande for stopover in fall than in spring. Species richness and relative abundance were lower in spring (108 species, 4,673 birds) than in fall (125 species, 19,127 birds). In addition to the influx of hatching-year birds in fall, differential use of migratory routes by landbirds in spring and fall may explain the seasonal difference in capture rate. We recaptured 2,875 birds (12 percent of the total captures) after the day of initial capture. Energetic condition and migratory status affected recapture probability: birds with low fat stores were more likely to stay overnight, and Neotropical species had shorter stopovers than temperate migrants and residents. The average rate of body mass gain across all species was 7.46 ? 0.10 percent/day in spring and was 4.68 ? 0.05 percent/day in fall, suggesting that migrants were not only able to gain energy for regular metabolic needs, but also for accumulating fat stores for migration. The capture rate was highest in willow habitat (288 birds/1,000 net hour) in spring and in agricultural field/edge habitat (718 birds/1,000 net hour) in fall. Cottonwood with Russian olive understory had the highest species richness in spring (80 species) and in fall (94 species). Saltcedar habitat had lowest species richness both in spring (26 species) and in fall (33 species). Detrended correspondence analysis of spring migration data separated species with high abundance in saltcedar from species with high capture rates in cottonwood overstory habitats, and in agriculture and willow habitats. The same analysis for fall data separated species with high capture rates in habitats dominated by agriculture, willow, saltcedar, and cottonwood. Species composition was most similar between agricultural habitat and cottonwood/Russian olive habitat in spring and among habitats with cottonwood as overstory in fall. During fall migration, about 60 percent of the birds captured were immatures. Body masses of adult birds were generally higher than those of immature birds, and young birds were more likely to be fat-depleted when they captured at our sites. The rate of mass gain was similar between immatures and adults. Our study confirms that riparian habitats along the middle Rio Grande of central New Mexico are important stopover sites during spring and fall migration for birds that breed in New Mexico, its adjacent States, and at a much larger geographic scale. Habitat loss and disturbance along the middle Rio Grande could affect not only local breeding landbird populations, but also many Neotropical and temperate migratory individuals that may use the area for only a few days each migration season.