Doolittle, T.C.J. 1992. Status of the eastern Taiga Merlin Falco c. columbarius in the upper midwest. M.S. Thesis, University of Wisconsin-Eau Claire. 132 pp.
Migration data from Upper Midwestern banding stations and migratory counts suggested a strong increase in merlin populations beginning in 1984. Since 1987, one hundred and two territories have been identified within a 1,858 km2 area in Ontario, Minnesota, Wisconsin and Michigan. Merlins have produced 191 young with a mean reproduction rate of 2.9 young/successful nest (83% of the 78 active nests were successful). Merlin breeding densities were low (<1 active pair/100 km2), but aggregated within parts of the study areas (>5 active pairs/10 km2). Eighty-nine breeding adults (57 males and 32 females) were captured and banded. The turnover of adult male merlins was estimated at 86%.
Usually, merlins selected corvid nests in white pine trees (Pinus strobus) as nesting sites. Nest woods with multiple nests were more stable than territories with a single nest. Most nests were within 150 meters of a dominant edge. The edge was usually a lake with an irregular shoreline exceeding 161 km in length.
The lack of periodic fire may jeopardize the long term maintenance of the conifer and boreal communities used as nesting areas by merlins. Other factors such as the deterioration of aquatic habitats due to anthropogenic toxins and a decline in the prey base is, or could be a local problem. However, present merlin populations throughout the region seem to be increasing. One factor augmenting population growth is their ability to nest in urban environments. Urban areas offer a large choice of suitable nests because of abundant crow populations and coniferous trees. Another factor that may contribute to their population increase is their use of artificial nesting structures. Lastly, a significant increase of american crow (Corvus brackyrynchos) populations in the Upper Midwest may have created more available nests on shorelines for merlins to use. This may be the most important factor for the regional increase of merlins.