Wednesday, 2 August 2017

Thoughts on concave design

It is not unusual for eggs to move out of Swifts' nests. Sometimes an adult will deliberately pick up an egg and dump it outside the box. However, other times it seems to be accidental as some Swifts have been observed on webcams moving eggs out, either by shuffling or with the head while adjusting nesting material.

We have already shown conclusively that Swifts prefer nest boxes with a concave (here). Although Swifts appear to be most comfortable as they snuggle down in a deep concave, are concaves made by a spherical scoop, with a sloping periphery the best shape?

These thoughts were prompted when swifts in one of my daughter's nest boxes with a concave scoop 85mm diameter and 22mm deep, 2 eggs were moved out in 2016 and then 5 eggs in 2017.

We all know that Swifts are individuals and maybe some Swifts are more clumsy than others, but when this particular concave was replaced by one with a vertical periphery and a shallow depression in the bottom, they went on to incubate the 5th and 6th eggs, hatching one of them (which, at the time of writing is doing well). The replacement concave extended for the whole width of the box.

A discussion on SLN yielded the following experiences:

David Makin had a similar experience to the above with Swifts displacing 4 eggs in 2016 and another 4 in 2017 from a concave 22mm deep, before finally laying a replacement clutch of 2 eggs which they went on to hatch.

Brian Cahalane makes his concaves out of a 30mm thick piece of wood in which he cuts a 100mm diameter hole. The bottom is therefore flat, but Brian paints the bottom with glue and then throws a handful of feathers in. Anything with a flat bottom is not going to cause the eggs to move to the centre, but the feathers may well provide the necessary shape for this. Brian never gets eggs displaced from these concaves, but he has had a small number of eggs displaced from shallower concaves over the years.

Mark Glanville has this experience:
"My scoops are not really cups but made from off-cuts to fit exactly into the box with no gaps between them and the sides. Most of my boxes are 8" wide so I tend to use a piece of wood 8" by 8". They are all homemade with 100mm diameters and I use wood filler to make the edges of the concave smooth. I aim to have at least 2" clearance between the concave and the sides/back of the box. I am sure that having it all on the same level has helped reduce the number of eggs/young being flicked out on changeover duties. The concave depth varies between 12mm and 22mm. No reason for this other than I use the same wood left over from building the box. One thing I do add on the 12mm scoops is a bead of mastic (2-3mm high) around the rim of the concave to give it a bit of a lip. All my scoops are lined with feathers with extra being added around the rim. 

Louise Bentley comments that the Swifts do look most comfortable in her concave, but still they pushed one egg out on their first breeding attempt, which hatched after being replaced by Louise.

Tanya & Edmund Hoare agree with Mark and suggest that more horizontal space around the concave could help, and suggest that a concave in the floor of the box could be a solution.

To add to the above, I have measured a number of natural nests. They are about 85mm diameter, and depth up to ~20mm, and usually the periphery is a wall of feathers. It is not unusual to find a natural nest wedged into a corner or side of the box. There is little in the way of any horizontal surface outside the periphery. Some nests can be quite high.

So what does one conclude about this?


The diameter of a Swift's egg is ~16mm, so any "cliff" over about 8mm should keep the eggs in. 

Therefore I would conclude that a concave with a vertical periphery at least 10mm high (say) and some sort of depression in the bottom is ideal. The depression can either be achieved by machining the right shape (suitable for a product) or by gluing feathers in the bottom (suitable for the DIY enthusiast).

Whether sticking feathers to a concave is worth doing is not known, the experiments have not been done, but certainly feathers scattered in the nest box are readily used by Swifts.

Some horizontal surface around the concave may be a good idea.

As a next step I have had a tool made to make vertically sided shallow scoop concaves with a diameter of 85mm.

The pictures above show the tool and a prototype concave made out of insulation (yes, it is highly inflammable) coated with PVA glue to resist Great Tits and House Sparrows. 

The attraction of this material is that it is very easy to machine.

These concaves are so light that they do need sticking down.

I have had a number of concaves made of this material in boxes over the winter, and none have been damaged so far. The next step is to see what Swifts think of them.







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