Hi Cathy,
First, thanks for all the volunteer work that you do. The Cook County Forest Preserve District has a great and growing team of volunteers and staff working to improve the preserves.
Check out our Chicago Wilderness Cafe talk that the Bird
Conservation Network presented last year about managing for
migrant birds if you haven't seen it.
https://www.youtube.com/watch?v=M3_BUN7aj5U
Unfortunately, buckthorn removal can often result in a net loss of migrant birds and I'm sure you're seeing effects from that. It has also been a slow fall in many areas and the dry conditions are resulting in reduced native food sources including both seed-bearing plants that have already died off early and berry plants and trees that haven't produced as much fruit or are producing off schedule. The weather this year has also pushed birds quickly through our area and possibly more than usual through other areas outside of Chicagoland. Especially in fall, many species of migrant birds feed on berries including buckthorn, honeysuckle, and cherry trees (but despite attracting birds, note also that buckthorn berries are believed to have lower nutritional value and may even harm some birds). Other species use the bushes for cover and protection. In some areas where heavy equipment was necessary for mass clearings, the lack of understory cover, compacted soil, and replacement of leaf litter with layers of bark chips isn't great in the short term for ground birds like thrushes either. Hopefully you will start to see at least some saplings and shrubs start to fill in, but If you want to see migrants come back (in the early spring and fall in particular), it is often essential to find opportunities to plant native shrubs, and allow poison ivy, virginia creeper, and other plants to grow as well. A healthy woodland would be full of fire-resistant trees and different height sapling trees that unfortunately take many many years to establish - and even well restored woodlands are not regenerating at a rate quick enough to combat the rate that we're losing trees. Many trees in previously buckthorn infested woods also won't ever regrow low branches that they should naturally otherwise have.
I have seen successful restorations that are great for migrant woodland birds, but it's challenging and also often costly to do on a larger scale. Even in my own yard I still have a large row of buckthorn that I would love to replace with native shrubs, but it might take years for those new shrubs to grow sufficiently to create the same degree of understory. It's hard to know how or where to even start without creating an entire landscape that migrant birds won't want to use anymore. I plan to tackle it slowly. Restoration is incredibly complex and challenging even without throwing in the ever changing impacts of invasive species and global warming. Are the impacts of restoration or weather impacts resulting in the observations you're seeing? Probably some mix of both would be my guess.
Eric Secker