On finding a Sparrowhawk nest

I always suspected that Sparrowhawks nested at the Cambridge Science Park, however in the many years that I had visited the park I had never found the nest site. The park, which runs adjacent to the back of my garden, consists of 57 office buildings but also three small lakes and a small area of parkland with mature trees. It is currently undergoing a stage of redevelopment, and is thus becoming a busier and noisier setting. It was therefore to my surprise that this would be the year that I found the Sparrowhawk nest.

Adult Sparrowhawks tend to be fairly secretive around the nest site, and thus it can be difficult to locate. There had always been good signs of breeding activity such as adults carrying prey in the breeding season, a regular plucking post and recently fledged juveniles in my nearby garden - so I was sure they bred nearby. However, I had never before actively searched for the nest at the park, and it was therefore pot luck that I stumbled upon it this year.

The Cambridge Science Park

While walking my dog around one of the lakes one early July morning I noticed a dead bird fall out of a nearby tree. On inspecting the thrush-sized carcass it was clear that it had been very recently eaten. Looking directly up into the lone pine tree there was no sign of the predator which had dropped the bird, but there did seem to be a platform of twigs towards the canopy. I moved around to a better vantage point and could make out what looked to be a moderately-sized nest; yet it seemed absent of birds. Was this a plucking post, or could this be a Sparrowhawk nest?

Returning the next day armed with binoculars, it was immediately clear that the platform of twigs was indeed a nest. It was located near the trunk of the pine tree and balanced on an exposed branch, which allowed good views of the entire nest area. Looking in more detail I could see two or more white balls of fluff hunched down among the twigs. Sparrowhawk chicks I thought to myself - I had found the nest!

Despite still being covered in white downy feathers the chicks were quite well developed, suggesting they were likely around two weeks old. I was disappointed with myself because even though I walked this route most days, it had taken me this long to realise I had been passing right next to a Sparrowhawk nest. I suppose this shows how secretive the adults can be around the nest site.

Sparrowhawk chick at presumably 1.5 to 2 weeks old

In subsequent visits I was able to confirm there were two chicks in the nest, one slightly larger than the other. The size difference is likely caused by intervals in egg laying and hence different hatching dates of the young. Though, it may also be suggestive of the sexual dimorphism between Sparrowhawk sexes and indicate that the larger juvenile is a female and the smaller a male. I continued to visit the nest over the next few weeks, observing from a safe distance, and was amazed by the speed of chick development. The chicks' plumage was visibly changing with every visit. By the end of week one the chicks' adult type feathers were growing well, the white fluffy down being mostly moulted, and by the end of week two the chicks more or less resembled adults, with a few comical spots of white down remaining.

Both Sparrowhawk chicks at 4 weeks old and nearly ready to fledge

The adults were rarely seen at the nest in the many hours I was observing. Sometimes the female would drop by, presumably checking for any danger at the nest. I had been mainly watching the latter stages of chick development, so for most of the time the chicks could look after themselves and feed without any adult assistance. Food drops tended to be fairly infrequent but Saimon Clark managed to capture some footage (see here) highlighting how briefly the male visits the nest. Presumably he was terrified of the female – she is much bigger!

At four weeks old the chicks began ‘branching’ and began exploring the surrounding trees, though would regularly return to the nest. A few days later and they were gone. It had been educational to spend time watching these fascinating predatory birds develop as well as enjoyable to share the experience with a few local birders and park regulars. It surprised me how tolerant the birds were of the busy urban setting they had chosen to nest in, but perhaps this highlights that given a suitable habitat nature will use it. 

Below is a video showing the development of the juvenile Sparrowhawks in the two and a half week period I spent regularly watching them.