Chiffchaffs Chasing Other Birds

At this time of year, usually in the garden, I see a piece of bird behaviour that I find puzzling. Namely - Chiffchaffs chasing other birds - Willow warblers also seem prone to do this.

Typically, a Chiffchaff will remain motionless for a brief moment, apparently watching a Blue Tit (the Tits are often on the receiving end). When the Blue tit takes to the air the Chiffchaff will chase it around the garden. Chiffchaffs indulge in this behaviour very enthusiastically and they seem to expend a lot of energy doing it. I think a Chiffchaff coming into the garden in Autumn is more likely to exhibit this behaviour than not. I get the impression that young birds exhibit this behaviour the most.

Back in the 90`s I wrote to Jim Flegg at the BTO about this - he replied "That`s easy - it`s defending a feeding territory." To me this doesn`t seem to ring true for the following reasons:

1) You would at least expect the Chiffchaff to remain, more or less, "in situ" if it were defending a territory. This isn`t the case. Typically a Chiffchaff will engage in a few bouts of chasing, for a few minutes, then isn`t seen again.

2) You would expect this behaviour to be directed at species which were competitors. It`s true the leaf warblers do chase each other and the Blue tit could conceivably be regarded as a competitor - but Chaffinches? Great Spotted Woodpeckers!?

3) In cost-benefit terms, it`s difficult to imagine that the amount of energy expended is warranted by the number of additional food items that are secured - by virtue of a competitor having been ousted

4) Why would the Warbler wait for the victim to fly before chasing it?

Possible explanations:

This possibly IS territorial behaviour being triggered - the mechanism being similar to that which results in the autumnal resumption of singing. The behaviour being "misdirected". This, however, begs further questions. Even if not defending a territory, there ought to be some evolutionary advantage in the behaviour.

Alternatively this could be explained as "play". This isn't to invoke a "just for the fun of it" kind of explanation. A kitten chasing a woollen ball is "playing". What ever the reason for the kitten's behaviour, "play" is a perfectly good word to describe it.

It's fairly easy to image what the  survival value in this kind of behaviour would be - developing co-ordination, developing agility, strengthening muscles, exploring and socialisation - skills that will be used in adulthood.

So do Chiffchaffs play?

1) They hang around for some time prior to migrating as a result of their slow post juvenile moult.

2) Migrants put on the fat they require for the journey, quickly in a short space of time, immediately prior to departing.

3) They will, therefore, have 'spare time' in autumn prior to the intensive fattening process.

4)  'Play seems to be a normal activity with animals which have their basic needs met' (wikipedia) - this probably applies to these autumn, juvenile Chiffchaffs.

5) Juvenile birds are inquisitive and this seems to be very much true for Chiffchaffs - whenever  I've tried 'pishing' - Chiffchaffs seem to be the most eager responders.

6) Waiting for a target to fly is perhaps 'safe play'. There is less chance of being turned on.

7) Autumn is a good time for these play/lessons, better than spring when the imperative to breed is paramount.

So yes 'play' seems to be a pretty good explanation. Furthermore the factors above seem to point reasons why juvenile Chiffchaffs would be particularly 'playful'.





Thanks to those who have contributed their thoughts on this subject.