Swift Revelations – Citizen science from an armchair. By David Gittens

Some of you may have followed my story of the pair of common swifts that eventually raised
two young in our nest box during the 2020 breeding season. Well, I was curious to know just
how many feeds the adults brought to the youngsters during this time so I carried out a little
mini-study of my own to find out. As the nest box was fitted with a CCTV camera which
recorded the footage to my PC, I was able to watch their activities 24 hours a day at my leisure.
So, during the 44-day period that the chicks were in the nest (19th July to 1st September) I was
able to review the recordings and note the time of day that an adult came to the box with a food
delivery. I also noted when an adult left the nest box too. These two simple pieces of
information yielded not just the answer to my question but a whole lot more. It was a revelation
in fact.
Now I know that stats and charts aren’t everyone’s cup of tea but I hope the following will convey
an insight to the feeding of young swifts. My initial question was answered simply by adding
up the number of feed deliveries that were made during each day and plotting this for every
day of the 44 days the chicks were in the nest (ie hatching to fledging).

You will see that the number of daily feeds varied greatly, from 28 on July 29th and 30th
(equivalent to 14 per chick) to none on 29th August when there was heavy rain all day and the
adults hardly left the nest. Weather would have influenced some other variations but I believe
that others are instinctively natural to the adults. Proper research by others have suggested
swifts control the body-weight of their nestling offspring, allowing them to put on ‘brown’ fat in
the early stages of development (as a safeguard against periods of low food availability due to
poor weather), but then to reduce it in order to drop their weight as they approach fledging. I
suspect some of this variation in feeding reflects this.
It was also possible to determine how much time the adults spent in the nest and how much
time they spent foraging for food. The chart below shows the total number of hours that there
were only ‘one adult’ and ‘no adults’ in the nest box during the daytime foraging period.

You will see that, initially, one adult would stay in the box with the very young chicks in order to
keep them warm. Unlike many young birds, swifts are naked when they hatch and require a
parent to brood them almost constantly. After a few days though, as their down develops, they
can be left for increasingly long periods. By early August (Day 14), they started to show signs
of feathers so could sustain their own body heat, and the time an adult spent in the nest was
generally just during food delivery. In any case, with the chicks growing quickly, their energy
demands required both parents to forage for food. The two large ‘dips’ in the blue line reflect
days of bad weather when at least one adult stayed in the box for extended periods.
Logging the entry and exit times also allowed me to learn other aspects of their behaviour. For
instance, it was clear from the data that these adult swifts did not fly during the night while they
are nesting, which is surprising seeing as they spend the whole of the rest of their lives on the
wing. I also discovered that the adults generally take their first flight of the day at or just after
sunrise and their last return to the nest at sunset with a remarkably close correlation.
Knowing the number of daily feed deliveries and the length of the foraging day (from the first
exit time to the last return time), the average time of an adult’s foraging flight could be
determined. Over the whole 44 day nestling period this turned out to be 53 minutes. In other
words, one or other of the adults would return to the nest box with a feed for the chicks on
average every 53 minutes. Needless to say, this varied quite a bit from day to day. The shortest daily average foraging flight time was 28 minutes and the longest was 2 hours 13 minutes (on
a day when the weather was poor and only a few feeds were delivered). Now, bearing in mind
that the adults were feeding two chicks, these times need to be doubled in order to get the
average times for each chick. For the record, the fastest foraging flight I recorded was just 7
minutes.
I should also point out that an adult would share a feed delivery between both chicks in the first
few days after hatching but soon switched to one delivery for one chick. It should be kept in
mind that the adults had to gather food for themselves as well as for their offspring during these
flights but were never seen to consume their own food in the nest box.
Now let’s look at the exit/entry times data in a different way. Rather than consider the food
delivery for a whole day, we can examine how the deliveries are dispersed throughout the day.
I chose to divide the foraging day into 1-hour sections, starting at 06:30 and ending at 20:30,
and plot how many feeds occurred in each of these time-slots.

The yellow columns represent the first half of the foraging day, and the green the second half.
(The ‘midday point’ was 13:30). Interestingly 50.3% of feeds occurred in the first half of the day
and the remainder in the second half. The distribution of the feeds was very different though.
During the ‘morning’ period the feeds increased more or less continuously, perhaps reflecting
the increasing availability of prey insects as temperatures generally rose. By contrast, the
afternoon saw an immediate and continuous decline in feeds to around 5pm. My thoughts are
that this has little to do with insect availability and possibly more to do with the chicks being
satiated and/or the adults taking time to feed themselves. The last part of the average day saw
a significant increase in feed deliveries, most likely to provide the young swifts with sufficient
sustenance to see them through the night.
Male and female swifts are identical in plumage, to the human eye at least, so it is generally
not possible to separate and compare their relative performance. However, for an 18-day period towards the end of the nestling stage a difference in plumage, due to feather moulting I
believe, was noticeable on one of the birds under the camera’s infrared illumination. I could
not say whether this moulting bird (aka ‘Blotchy’) was male or female, just that it was different
from its partner. Nevertheless, comparison of their feeding performance was now both possible
and interesting.

Although both parents can be seen to gather feeds throughout the day there is some disparity
between them. Initially Parent 1 more-or-less matches Parent 2 until around 10am, but then
Parent two significantly out performs it until late afternoon. In fact, taking the 18-day average
as a whole, Parent 1 gathered 44.8% of the feeds whereas Parent 2 gathered 55.2%. Total
number of feed deliveries for this period was 239. I have since learned that in another similar
study the female swift returned more feeds than the male.
With a bit more analysis, the data I gathered could shine a light on a few more aspects of the
life of a pair of breeding swifts but maybe that’s for another time. I know charts, data and
statistics are not for everyone but I hope they have enabled you to see this part of the swifts’
lives from a different perspective, and maybe answered a few questions. It’s done that for me
but also raised several more. That’s life, I guess.
Putting aside the details of the study, I have to say that the amount of information that can be
obtained from simple data using a CCTV camera was a self-induced revelation to me. All of it
done from the comfort of my armchair – well, the chair in my study. This Citizen Science lark
can be very illuminating.
I do not claim that my observations have revealed anything new or contentious about the
performance of swifts feeding young, but I certainly learned a great deal, and it reinforced my
respect for these remarkable birds. But aren’t they all in their own way?
Oh, and the answer to my question? The adults made a total of 761 feeds deliveries to their
two offspring, an average of 17.3 per day.

 

Swift Factettes
• Swifts are the fastest bird in level flight, travelling at up to 70mph. They hunt for insects
at about one third of this speed. Peregrines are faster but only in a dive.
• Swifts are in an evolutionary group called Apodidae, a branch which separated from
other birds around the time that Tyrannosaurus died out, 65-70 million years ago.
• Our Common Swift (Apus Apus) has 4 toes, none of which face backwards. It therefore
cannot grip to perch like other perching birds.
• Each ‘bolus’ (ball of food delivered from the adult’s mouth) weighs around 1 gram and
can contain 300 – 500 insects.
• Once the young fledge, they will not land again until they breed in 2 or 3 years’ time.
They eat, sleep and even mate on the wing.
• They can fly at very high altitudes, in excess of 10,000 feet.
• The average life of a swift is 5.5 years although some ringed birds are known to have
lived for over 15 years.

Source: https://www.rspb.org.uk/globalassets/downloads/documents/conservation–
sustainability/help-swifts/amazing-swift-facts.pdf

 

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