Revisiting Perrins 1965

In 1965, Christopher Perrins published a paper in the Journal of Animal Ecology, reporting the results of  his study on the population biology of the Great Tit in Wytham Woods, a site made famous by the work initiated by David Lack, Christopher Perrin’s PhD supervisor. Fifty-one years after the paper was published, I asked Chris Perrins about how he got interested in the topic, the changes in Wytham Woods in the last 50 years, and what we have learnt since about great tit population biology.

Citation: Perrins, C. M. (1965). Population fluctuations and clutch-size in the Great Tit, Parus major L. The Journal of Animal Ecology, 601-647.

Date of interview: Questions sent by email on 27th September 2016; responses received by email on 8th October 2016.

 

Hari Sridhar: The work presented in this paper formed part of you PhD under David Lack. Could you please share with us your motivation to work on this topic for your PhD?

Chris Perrins: Lack was the first to ask questions about the evolution of reproductive rates e.g “significance of clutch size” (several papers in Ibis vols 89 & 90), “survival in relation to brood-size in tits (Proc.  Zoo. Soc. Lond 128:313-326) etc. I was intrigued by this work, which was limited by small sample sizes. What I did was to get much larger data sets mainly by a) greatly increasing the study area in Wytham and b) by manipulating brood-size (artificially making more large and small broods by moving chicks at hatching)

 

HS: If you don’t mind my asking, could you tell us why David Lack, who was your PhD supervisor, wasn’t an author on this paper? By today’s practice, that is somewhat unusual.

CP: In those days in the UK. I think this was normal; it was unusual for the supervisor to put his/her name on the papers written by students. For example, H van Balen, a contemporary of mine and a student of Kluijver’s, was the sole author on his main works.

I continued this, only adding my name to a student’s work if a) I had taken a major part in the analysis or b) obtained the grant which funded the work. The putting of supervisor’s name on students papers I think spread from the U.S. Pressure to publish and shortage of funds has more or less forced supervisors to publish with their students. Another, though different, reason for multi-authored papers is that often teams of people with different skills combine on a project.

 

HS: Can you tell us a little more about the influence of H.N. Kluijver on this study?

CP: Really very little. EXCEPT that, of course, Kluijver was the person who fired Lack to switch from robins to tits as a convenient study species. (Strictly, it was not Kluijver who “discovered” tits, but H Wolda who preceded Kluijver).

 

HS:  In the paper you say “Wytham is an island of woodland surrounded by agricultural land and urban areas” and “there was little disturbance, apart from forestry operations”. Does this continue to be an apt description of Wytham woods today?

CP: Basically, this is still true.

 

HS: In what ways has Wytham woods changed from the time when you did this study?

CP: Less forestry intervention, which in some areas has resulted in a more closed canopy; this together with increased grazing by increased numbers of deer has greatly reduced the richness of the ground flora. But Wytham has suffered from many other changes that are occurring throughout the area and indeed UK. Dutch elm disease almost eliminated elm trees. There is an oak disease and an ash disease which may change the wood considerably in the future.

 

HS: Would the places shown in Plate 1 look different today?

CP: No, except we now use a different design of nest-box!

 

HS:  When was the last time you visited the sites used in this study? Do you continue to do field research in these sites?

CP: I continue to visit (I live one km away!); I’m also in contact with students who are currently working on the tits.

 

HS: You say “Only blue and great tits nest commonly in the boxes in Wytham”. Does that continue to remain the case?

CP: Yes, indeed of two of the rare species, marsh tit is still rarer, and redstart no longer breeds in Wytham.

 

HS: You say “The predators [of great tits] are chiefly weasels, Mustela nivalis, grey squirrels, Sciurus carolinensis and great spotted woodpeckers, Dendrocopus major”. Does that continue to remain the case?

CP: We changed nest-boxes to a German design made of cement and sawdust. Nowadays, squirrels and woodpeckers rarely predate broods; weasels are less serious than they used to be, but predation by them fluctuates greatly. Outside the nestling period, sparrowhawks are serious predators, more serious than we realised in this paper.

 

HS: In the 51 years since this study was done, have there been major changes in great tit population size and life-history parameters (breeding season, date of laying, brood size etc.)?

 CP: Great tits have increased a bit, blue tits more. One of the marked changes is that – in line with global warming – the birds breed about two weeks earlier than they used to.

 

HS:  You say “there are two major factors which affect the breeding population of the great tit in Wytham, the survival or loss of the juveniles in the late summer or early autumn, due to a largely unknown factor that may be related to the food supply while the young are in the nest, and the survival of both juveniles and adults through the winter which depends to a large extent on whether there is a beech crop or not, the mortality being higher in years without a crop.”

Today, 51 years after the study was published, would you say that these main conclusions still hold true, more-or-less?

CP: Yes

 

HS: If you were to redo this study today would you do anything differently, given the advances in technology, theory, statistical techniques etc.?

 CP: The study is continuing, using all the technology you mention. The use of tiny PIT tags, which identify the individual when they go to a box or a feeder has expanded many lines of enquiry (the tits are still too small to use conventional radio-tracking devices).

 

 HS: Could you tell us a little more about the role of Mr. J.F. Scott in this study, and your interactions with him?

CP: Another sign of the times!! When this paper was written, the University owned ONE computer. More or less the only people who were allowed to use it were the staff in the Dept. of Statistics (I think that is the right title, might be not quite right); Scott was one of these. Normally, if you had a statistical question, you went to them with the data and they made the analysis; Scott was one of their staff. They did not expect to become authors, though sometimes they did. David Lack used one of these and I worked also with Michael Bulmer who co-authored a couple of papers with me (He was then in the Dept. of Biomathematics).

 

HS: You mention that you put up nest boxes in the gardens of forty houses. Would you know if these boxes still exist and continue to be used by tits?

CP: I’m sure these boxes do not exist, but many people put their own up; tits commonly breed in gardens.

 

HS:  Do you remember why you decided to submit this paper to Journal of Animal Ecology? Was it an obvious choice at that point?

CP: Yes, JAE was the leading Ecology journal in UK.

 

HS: Did this paper have a relatively smooth ride through peer-review?

CP: Yes. Nowadays much more likely to be disputes over statistical analyses!

 

HS: This paper has been cited over a thousand times. Would you know what it is mostly cited for?

CP: I guess a range of things. But most probably relate to 1) general biology of tits (which are widely studied across Europe and as far as Japan!) 2) that the main conclusion still holds true 3) that it was the start of some papers on the timing of breeding seasons 4) the correlations  of population change with winter food supplies.

[For many years JAE published short notes by the authors of papers that had become what they called “Citation Classics”. This paper became one and the note about it was published in Current Contents on Feb 21 1983.]

 

HS: In the 51 years since it was published, have you ever read this paper again?

CP: Usually only in bits, in answer to specific questions, and then often to a later paper rather than this.

 

HS: When you compare it with the papers you write today, what strikes you about it?

CP: It was easier to do this study then than it would be now, and certainly to publish a single paper at that length. Today, it would certainly be sub-divided into several shorter papers.  The stats are amateurish (but remember there were no computers and many of the modern analytical techniques even if they had been invented were not easy or possible to use).

 

HS:  Would you count this as one of your favourites, among all the papers you have published?

CP: Yes, but fortunately there have been a few others!

HS:  What would you say to a student who is about to read this paper today? What should he or she take away from this paper written 51 years ago? Would you add any caveats?

CP: They should read it. But they should also read the more recent papers on each of the specific subjects, to see what else has been done since; reading the latter would be essential before they developed their own research plans. This would be particularly true of brood-size manipulations which create complications of their own. Fortunately, I was one of the first to spot them. See: Perrins, C.M. & Moss, D.  (1975). Reproductive rates in the Great Tit.  J. Anim. Ecol. 44: 695-706.

 

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