Grasshopper Sparrow Study at Wastewater

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On April 22, I initiated the 2012 banding effort for Grasshopper Sparrows at Muskegon Wastewater. Each year since 2008, I have accrued a highly variable amount of hours and seasonal coverage in an attempt to better understand densites, breeding activities, and productivity in this and other birds dwelling in the Wastewater grasslands. A half-hour of netting this evening started this season with two males (including this one below).

Below are the number of males banded each year; the figure in parentheses represents those recaptured during a subsequent year (i.e. "returns").

2008: 43 (19)

2009: 67 (19)

2010: 39 (9)

2011: 57

While the annual totals better reflect differential effort than they do actual densities, the number of returns provide an interesting picture of habitat suitability. When males return in the spring, they will choose the best territories available to them. However, for returning birds, "best" does not necessarily mean the greatest quantity of cover and resources as much as it simply indicates a breeding site that has worked in the past. If a male has successfully reared young at a particular spot, then that spot will usually be chosen the following spring. Conversely, a site that fails to attract a prospective mate or one that has prompted a nest failure (due to predation, exposure, food limitations, etc.), becomes far less appealing. This partly explains why, for instance, a Cerulean Warbler may temporarily sing from a fresh clearcut when he returns the following spring. Since successful sites are so valuable, males do not waste any time when claiming them each spring. With Grasshopper and other grassland sparrows, adult males banded the preceding years generally arrive first, followed by other adult males, and then by young males that will attempt to establish a territory for their first time. While many confounding factors exist, I generally find that a return rate greater than 30% represents high quality habitat.

Over the past three years, the southern fields at the Wastewater have suffered a lot of abuse in failed attempts to eradicate knapweed and restore native grasses. Consequently, the number of returns and the actual populations of Grasshopper Sparrows have declined. Somewhat disturbingly, the first two birds captured this year had not been previously banded. Nevertheless, new habitat plans are in the works, and despite past setbacks, the number of Grasshopper Sparrows at the Wastewater remains quite impressive.
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- Brian Johnson
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