I'm glad that you have had a look at some of the GWCT's work at Loddington. Why you should have thought that I needed telling by you that changes to farming practices had had the greatest adverse impacts on farmland birds, I'm not sure. I have been aware of work undertaken at Loddington since the start of the project and have visited the site several times. I was a one-time trustee of GCT. However, I am concerned that many species have continued to decline over the last decade despite the best efforts of those planning and carrying out agri-environment schemes designed to prevent such declines. The important point being made vis a vis effects predator control on songbirds is as follows: "While predator control may not necessarily be a primary cause of population decline, it has a role in the reversal of that decline at the farm scale for some species."
You suggest that a large part of the Project's work is considering the effects of predator control. This is absolutely not the case. It is aimed at finding ways to make farming ecologically sound without destroying profitability. Please look at the Allerton Project Research Blog. However, if it is the subject of birds that primarily concerns you, I'd recommend that you read Chris Stoate's article in it of 21st July 2016. This should demonstrate to you that predator control did, indeed, impact songbird populations. You mention not knowing what predators are controlled. These are, of course, those that can legally be controlled - foxes, small mustelids, corvids, grey squirrels and rats. Obviously, raptors were not controlled because it would have been illegal. However, it is a great shame, in my view, that limited experiments can't be licensed which would allow assessment of their (possibly) additional effects on farmland bird populations. I also would like to suggest that it is highly likely that ground nesting species (e.g. sky lark, partridge etc) are more adversely impacted by predation than are songbirds.
You may also like to look at the GWCT NewsBlog of September 2016: "Predation control - two very different debates."
I am very interested in the nutritional requirements of farmland birds and would like to know a lot more about which foodstuffs can meet these requirements as they change throughout the year and how agri-environment schemes could best be tweaked to meet them. I suspect that more information is needed here (although I may be ignorant of what's already out there). I suspect that most ecologists don't know much about nutrition and most nutritionists are not sufficiently au fait with the preferred feed selection by birds of different species. It's intriguing, for example, that some, but not all, species that are declining on farmland are also declining in both rural and suburban gardens. One might have supposed that garden bird feeders would make up for the supposed lack of winter feed on many farms. I have no answers, but will pose a few questions along with the odd statement or two to see if others can enhance my knowledge.
1) Is the mix of feedstuffs offered to garden birds appropriate over winter? Most garden birds won't accept nutritionally balanced compound feeds in the form of crumbs or mini-pellets. Wheat is a man-made cereal. We have, effectively, taken a wild grass seed and increased its starch (carbohydrate) storage capacity while diluting its protein and fat proportions. Fat contains two and a quarter times the energy of carbohydrate. Small birds need a lot of energy to get through cold winter nights (they have very high metabolic rates). Relatively few species take wheat (house sparrows do). Are typical mixes offered adequate in minerals and vitamins? It seems that a good case, for example, could be made out that they tend to be deficient in vitamin A. However, this can be stored in the body for quite long periods.
2) Do artificial feeders in gardens lead to aggregations of birds and encourage spread of disease? Greenfinches come to mind (virus) and, to a lesser extent, other finches. Salmonella is a common infection of garden birds and one unlikely to harm adults. Could it be adversely affecting nestlings?
3) If artificial feeding continues into the breeding season, could it possibly be very harmful? Parent birds may hang round the feeders and satisfy their own needs without foraging for appropriate feed items for their progeny. If they attempt to take seeds from feeders to their nestlings, the latter will die if for no other reason than water deprivation. Dried insects will be as bad. When pheasants hatch, the hen birds have lost a lot of condition during incubation. They take their broods to find insects. This isn't because they know that that is what their chicks need (it is), but because they also need a similar diet themselves to replace lost tissue. If you put a wheat feeder near them during incubation, they wouldn't lose condition and, in consequence, may not take their chicks to good insect areas.
4) Agri-environment schemes encourage the growth of special crops or the leaving of uncultivated areas for the benefit of wildlife. Are these schemes having much effect on farmland birds? Would you get "more bangs for your buck" by putting out and filling artificial feeders along farm tracks?
I think we should be dealing with this whole subject in a less "Punch and Judy" manner. I have never suggested that predator control is the only way to protect endangered species. However, in some situations more than others, it is a very important management tool.