Breeding Biology of Grassland Songbirds and the Effects of Oil and Natural Gas Development on Their Density and Reproductive Success

Abstract

Understanding the breeding ecology of grassland birds is vital for identifying the mechanisms underlying their widespread population declines. I describe the breeding biology of and quantify the effects of nest predation and brood parasitism on Savannah Sparrow (Passerculus sandwichensis), Baird’s Sparrow (Ammodramus bairdii), Vesper Sparrow (Pooecetes gramineus), Western Meadowlark (Sternella neglecta), and Sprague’s Pipit (Anthus spragueii) in south-eastern Alberta. Nest predation was the primary cause of nest failure, accounting for 75% of all nest losses. For the three sparrow species, daily survival rates were higher during the incubation compared to the nestling stage. For all five species, clutch size, hatching success, and fledging success were within the range of values previously reported for these species in other parts of their range. Brown-headed Cowbirds parasitized nests of all species except Sprague’s Pipit, with 4- 11% of nests containing cowbird eggs. Savannah Sparrow experienced the highest frequency of brood parasitism and was the only species to successfully fledge cowbird young. Parasitized Savannah Sparrow nests experienced reduced hatching success, productivity, and clutch size compared to non-parasitized nests. The overall cost of parasitism to Savannah Sparrow was 1.7 young per successful nest. This information provides a basis for future comparisons regarding the influence of anthropogenic alterations to native grassland habitat on grassland songbird reproductive success and nest survival. Despite dramatic increases in oil and gas development over the last decade, research about the effects of this activity on grassland songbird reproductive success and density is lacking. I assessed how the density and reproductive success of five species of grassland songbirds in south-eastern Alberta varied with distance to oil and gas wells, roads, trails, and patches of introduced crested wheatgrass, along with percent cover of crested wheatgrass. Savannah Sparrow density was 2.5 times higher within 100 m of wells than further away and was almost twice as high in areas without crested wheatgrass compared to those with 60% cover. Sprague’s Pipit nest survival decreased with increased cover of crested wheatgrass. The proportion of Baird’s Sparrow eggs that hatched within 50 m of wells and trails were reduced by 33% and 46%, respectively, compared to areas further from these structures. Hatching success more than doubled for both Sprague’s Pipit and Western Meadowlark as percent cover of crested wheatgrass increased from 0-60%. The number of young fledged from successful Baird’s Sparrow nests was lower near trails. Savannah Sparrow fledging success was higher near wells and in areas with greater cover of crested wheatgrass. The number of young fledged from successful Western Meadowlark nests doubled as distance from crested wheatgrass increased from 0-800 m. There was no relationship between the frequency of brood parasitism and distance to oil and gas infrastructure. Overall, my results indicate that oil and gas development has mixed effects on the density and reproductive success of grassland songbirds. To mitigate potential negative effects, future development by the energy industry should attempt to minimize the amount of edge habitat created, continue to eliminate exotic vegetation in their reclamation protocols, and take precautions to avoid the spread of crested wheatgrass, and other invasive plants, into native grassland.