Swifts and cool weather

The presence of older non-breeding swifts ( we think mainly 2- 3 year olds) can be extremely fickle. One day they are here the next they are gone according to weather conditions. This year is no exception,  they arrived on the evening of June 3rd and departed late the next morning, briefly enlivening our skies as they formed low, fast formations and landing on occupied and unoccupied nest boxes alike. There has been no significant movement of swifts on the east coast so I suspect the missing birds are temporarily at the nearest lakes or reservoirs where food is relatively abundant even in cool conditions.

The village swift population is currently very fragile. Our colony ( now one pair) hangs by a thread.  The unpaired male tree sparrow entered box 2 again on Monday while the parents had left the eggs. It pecked at the replacement clutch, 18 days after destroying the first clutch. It looks like the eggs are intact so with full time incubation we have to hope the tree sparrow cannot strike again- I think we have dodged a bullet! Interestingly this pair waited 72 hours (3 days) between laying the first and second egg. This is probably a response to the current cold June conditions.

There is currently no sign of the older non-breeding swifts that arrived last week- the air has been very cool in northerly winds. Interestingly some colonies in the south of the country seem too be having plenty of non-breeder activity. A few extra degrees and less wind makes a world of difference. The weather today is cold but bright. It’s worth remembering that this weather is just a few degrees below average. We see far more records of weather substantially more than this above average (as this time last year when we were seeing the very high twenties Celsius)- swifts have always had to cope with periods of cool weather but I can’t help wondering with the decline of insects, whether these cool periods are now more critical to swifts. 

Cool weather in June is not unusual and its effects on swifts can be many fold, hence numbers have always fluctuated. Firstly breeding birds have to somehow incubate eggs or brood young and still feed themselves and their chicks. Secondly lack of the second wave of arrivals can mean recruiting new colony members to make up for previous year  breeding birds lost can be delayed or even postponed until the next year. There are many reports coming in of swifts throwing out their own eggs on purpose. Exactly what they know when they do this, we don’t know, but it is more common in cooler weather.

So we are now down to one pair plus one single female who arrived on 20th May – she occasionally brings in a potential mate and mysteriously laid one egg on 6th June. That egg was thrown out very purposefully on 10th June by a visiting male. Hopefully they will yet form our second pair and could still breed, but this will very much depend on the arrival of warmer weather.

The youngest non- breeding birds (in their second calender year) are yet to arrive. We usually expect them around 23rd June here. Low, fast passes increase along with more random prospecting, making this final arrival very distinctive. It is then that we can see peak numbers over colonies and can enjoy the sight of aerial roosting flocks spiralling up into the dusk sky.

Seeing a decline at my colony has been very interesting. I have only ever experienced increase at previous colonies. Instead we have dropped from four pairs in 2022 to just one (possibly two) in 2024 or combining here and the nearby village hall, 7 pairs down to 2 pairs. I wonder what scale this decline is? Maybe our village is just unlucky or for some reason out of favour, but this could be a much wider problem perhaps regional too. I wish I knew- what I can say for sure is that in the last two years arrivals of previous year breeders have been very protracted and fewer than normal are making it back. Clearly no amount of nest boxes helps at the moment but there are plenty available if we were to see new swifts prospect.

Finally, here is a diary of nest box 3 which has had a single occupant since 20th May. Interestingly it shows production of an egg, now ejected, by a female who was not one of a committed roosting pair. I was able to identify this bird as a female after I saw her lay the single egg. She has a very distinctive mark on her right wing so I can continue to monitor her.

20th May – female first arrives in box 3

2nd June – male joins female a few times but doesn’t roost

4th/5th June – the only night spent with another bird in box 3

8th June – egg laid early morning

10th June – male visits and abruptly throws out egg

11th June- male joins her in the nest box 8am

12th June- improved weather with lighter winds. Pair in nest box several times and female nest building

13th/14th June- female still roosting alone

House martin diary

In sharp contrast to swifts house martins are doing well in the village. They have been nest building over the last week or so. These will mainly be first time breeding house martins which form the bulk of the second wave of arrivals from mid May to mid June. Many first time breeders only raise one brood as they have to spend a couple of weeks or so building nests before laying. On our own house we now have five pairs feeding very small young. The visits to nests are now numerous and the air alive with their calls. It’s lovely to watch especially in the absence of aerial swift activity.

Our colony stopped this year at five pairs, which is the same as last year, but I am thrilled that the east side of the village is seeing new pairs building nests. Overall numbers are very healthy compared to the very poor years of 2020-22.

All text and images copyright Jonathan Pomroy 2024

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Swift trio low over the garden- watercolour. Available in my shop .