Effect of field age on diversity, abundance and productivity of birds using Conservation Reserve Program land in Gratiot County, Michigan, USA

Background

The Conservation Reserve Program (CRP) provides economic incentives to US farmers to remove highly erodible and environmentally sensitive cropland from production under 10-year contracts. This study investigated the effect of field age on the diversity, abundance and productivity of birds using CRP fields in Gratiot County, Michigan, northern USA.

Action

In 1992, 19 CRP fields (6–20 ha) of differing ages were selected for study: 1-year-old (n = 3); 2-year-old (n = 3); 3-year-old (n = 3); 4-year-old (n = 3); 5-year-old (n = 6); and 6-year-old (n = 1). The latter two age categories were later combined for analyses. Fields had been planted with alfalfa Medicago sativa, orchard grass Dactylis glomerata, timothy grass Phleum pratense and clover Trifolium spp., and had not been mowed, grazed, burnt or disked since the start of their contract.

The abundance and diversity of birds using fields was assessed using transect surveys carried out every two weeks between 1 May and 15 August 1992. In addition, three 1-year-old, two 4-year-old and four 5-year-old fields were searched for nests during mid-May and mid-June. Nests were checked every 2–3 days to monitor nest survival (which was estimated using the Mayfield method).

Consequences

Overall, 32 bird species were recorded in CRP fields, with the most common being red-winged blackbirds Agelaius phoeniceus, song sparrows Melospiza melodia, bobolinks Dolichonyx oryzivorus and sedge wrens Cistothorus platensis. Average avian diversity in 1-year-old fields was significantly higher than in 5–6-year-old fields.

Average relative abundance was 4.2 birds/ha in 1-year-old fields, 3.7 birds/ha in 2-year-old fields, 3.3 birds/ha in 3-year-old fields, 4.7 birds/ha in 4-year-old fields and 2.1 birds/ha in 5–6-year-old fields. The difference in abundance between the latter two field ages was statistically significant.

In total, 166 active nests were discovered during the study, the majority (83%) of which were of red-winged blackbirds. On average, 10, 22 and 23 nests were found on 1-, 4- and 5-year-old fields respectively. Mayfield estimates of overall nest survival were higher in 4-year-old (28%) and 5-year-old fields (29%) than in 1-year-old fields (14%).

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