Parental Feeding of Swiftlets in the Nest
A Citizen Science Study
After a 5 year wait for swifts to nest in one of our north Norfolk nest boxes a pair took residence on 20th May 2020. A failed attempt to breed was followed by a second clutch of 2 eggs which was successful. When the young hatched, I was curious to know just how many feeds the adults brought to the youngsters during the day 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. 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 the time when an adult left the nest box to forage. These two simple pieces of information yielded not just the answer to my question but a whole lot more.
First though was the discovery that, between hatching and fledging, there was a close correlation between the time of the first and last foraging flights of the day, and sunrise/sunset. Unless it was raining, heavily overcast or exceptionally windy the first adult to leave the nest box in the morning would do sovery close to sunrise. Similarly, the last adult to return to the nest box in the evening would generally do so within a couple of minutes of sunset. The period between the first adult leaving the nest to forage in the morning and the last adult returning in the evening is referred to as the ‘foraging period’. No substantial foraging occurred before sunrise or after sunset. It was clear from the data that the swifts adjusted the foraging period for the shortening day length. The chart below illustrates.
Now to the subject of chick feeding frequency. The chart below shows the number of feed deliveries made during each day for every day of the 44 days the chicks were in the nest (i.e. 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 adultshardly left thenest.Weather would haveinfluenced someother variationsbut I believe that others are instinctively natural to the adults. Research by others have suggested parent 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 by underfeeding in order to drop their weight as they approach fledging. I suspect some of this variation in feeding reflects this. Summing the total number of daily feeds for the whole nestling period showed that the adults swifts delivered a total of 761 feeds to their two offspring, an average of 17.3 feeds per day (~8.6 per chick).
Although it was clear that weather had an effect on foraging performance, I found no way to quantify this separately from other factors and believe it to be a highly complex issue. Integrating data from several repeats of this study would significantly help to clarify 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 during the day - the foraging period. The chart below shows the total number of hours that there were only ‘one adult’ in the nest, and ‘no adults’ in the nest during the daytime foraging period. I was not able to determine the various species of insect being fed to the chicks but judging by those that managed to escape in the nest, Pollen Beetle appeared to be a high proportion throughout the study period.
Swift Revelations – David Gittens, February 2021 – Page 2 of 5
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 August (Day 14), the swiftlets started to show signs of feathers and could therefore sustain their own body heat, so the time an adult spent in the nest was generally just for 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.
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). 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 from the first adult leaving the nest box in the morning to it returning with a bolas of insects.
I should also point out that in the first few days after hatching an adult would share a feed delivery between both chicks 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 averages covering the whole 44-day period, shown as a percentage, are presented in the chart below.
of All Feeds
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 during the morning and afternoon 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 feedsup to around 5pm. Mythoughts 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
Swift
– David Gittens, February 2021 – Page 4 of 5
I believe, was noticeable on one of the birds under the camera’s infrared illumination. I could not saywhetherthismoultingbird(aka‘Blotchy’)was maleorfemale,just that itwasdifferent from its partner. Nevertheless, comparison of their feeding performance was now both possible and interesting. The chart is shown below.
Although both parents can be seen to gather food throughout the day there is some performance disparity between them. Initially Parent 1 more-or-less matches Parent 2 until around 10am, but then Parent 2 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. Frequency analysis of the recorded calls of the swifts in this study suggested this was also the case here, ie Parent 1 was male, Parent 2 was female.
For the record, the swiftlets successfully fledged on the morning of 1st September, within 2 hours of each other. The parent birds though didn’t leave until a few days later, resting overnight in the nest box. The male last left the nest box on 4th September and the female on the 6th
I do not claim that my observations have revealed anything new or contentious about the performance of adult swifts feeding young but I do believe that it shows that simple citizen science can yield useful and interesting data, and an insight into the workings of nature.
David Gittens Norfolk, UK– David Gittens, February 2021 – Page 5 of 5
