Subject: [Others 其他] Are Spring Migrants Less Common Now? [Print This Page] Author: wgeoff Time: 19/12/2012 09:02 Subject: Are Spring Migrants Less Common Now?
This question arose recently on the HKBWS Website and I thought it was sufficiently interesting and important to try to answer.
I chose ten species of Flycatcher, Grey-streaked, Dark-sided, Asia Brown, Ferruginous, Yellow-rumped, Narcissus, Mugimaki, Blue-and-white, Asian and Japanese Paradise as a good selection of spring, autumn and mixed migrants to review and added Japanese Yellow Bunting out of interest in this scarce spring migrant. The next job was to collect data for comparison
Data
I include this section to show how the data was prepared. Some readers may get bored and want to jump to the Results section.
Geoff Carey was able to supply me with the Excel File data used to produce the graphs in The Avifauna and I already had Excel Files for records from 1999 to 2011, sufficient to make a comparison. But the form of the data was a problem. The Avifauna uses a ‘bird-day total by week’ format for migrant species including these above. Whilst this may be valid for The Avifauna years when records for individual locations were less frequent than today, it double-counts numbers substantially when frequent records occur for the same location within a few days of each other, as happens now. So I chose to use the alternative Avifauna method of ‘peak weekly count’ as the weekly count by location by week. All separate locations are then added together (as for ‘bird-day totals’) to give an ‘aggregate weekly count’ by species for each week in the year. Weeks were chosen as in The Avifauna, 1-8, 9-15, 16-23 and 24-month end giving a total of 48 ‘weeks’ in each year.
The change to ‘peak weekly counts’ meant I had to convert the Avifauna data. In most cases, this was possible by reviewing data in Annual Reports. In a few cases, Asian Brown and Asian Paradise Flycatcher and other species for some years, this was not possible. So the data is biased towards higher numbers in the years 1958 to 1998, but, as the results turned out, this was not really a problem.
Results
The graphs below show the Aggregate of Peak Weekly Counts by species for each year from 1958 to 2010, and by looking across from left to right you can compare earlier years with those current.
Review
So, what can we learn from this.
There are only two species showing a definite decline in numbers, Yellow-rumped Flycatcher, an autumn species, and Japanese Yellow Bunting, although the decline for this species is limited to two very good years, 1993 and 1996, when large flocks of up to 15 and 17 respectively occurred, something never since repeated.
Mugimaki shows stable numbers, but the annual figures conceal a change in occurrence through the year, from a mostly wintering species in the early years to a mostly late autumn migrant in recent years. This is mentioned in The Avifauna
All other graphs appear to show increases in numbers – but I think this is due to more consistent recording in the years 2005 onwards than any underlying increase in numbers.
What I can say is, this data does not support the assertion that spring species are becoming rarer. That’s not to say they aren’t, just that our data does not (yet) support it, maybe because of inconsistencies in the recording. So, as always, caution rules and more years data are necessary to support any assertion of spring number decline.
Let me repeat – I am not suggesting here that species numbers are getting higher. It’s just that our records are now more frequent and consistent than previous years, say before 1990, when many species went either unidentified or unseen. But I am saying there's nothing in our records to suggest that species are getting rarer, except Yellow-rumped Flycatcher and Japanese Yellow Bunting.
[ Last edited by wgeoff at 19/12/2012 14:23 ] Author: HKBWS Tung Time: 19/12/2012 11:01
Thanks Geoff. It is very interesting.
Perhaps some statistics could be used to make the analysis more meaningful.
Also, there should have one bias - submission of records. Some species are common and so many birdwatchers would think no need to record or provide information of it, while other species which would be rare before were actually required submission of records. So, it sometimes make a situation that rare species have more records to show trends but common species do not have enough or meaningful information to show trends. Author: wgeoff Time: 19/12/2012 12:30
Yes Tung, you are right - there are many sources of bias in the submitted record data. If we only used submitted records, Tree Sparrow would be one of our rarest birds.
What we should do is, if we find some apparent trend (e.g. declining Yellow-rumped Flycatcher numbers), then find some alternative source of data to confirm or otherwise. For Yellow-rumped Flycatcher, an analysis of historical ringing data (which also has bias, of course), might show a similar trend - does it?
One good thing is, at some locations we are now doing regular and consistent counting, which removes much of the bias e.g. the Mai Po and Long Valley counts, and even the private ones being done at Braemar Hills and Shing Mun (and, dare I say it, Po Toi). We must try to keep these going over a sufficiently long time to show trends. Then we can speak with real authority.
[ Last edited by wgeoff at 19/12/2012 12:34 ] Author: EricB Time: 20/12/2012 05:10
An interesting thread. Dividing the sightings per year by the number of observers submitting records for a given year will produce a fraction that is perhaps more representative of the overall pattern. The identification of some species like Dark sided Flycatcher was poorly known prior to the 80's and some species are probably a better marker of what is going on.Additionally , the discovery of 'new' birding hotspots ,like Po Toi, will undoubtedly skew results.Lord knows I would give large chunks of my anatomy to have birded there in the 70's and 80's.To generate meaningful data you probably are best off looking at a fixed number of sites that have been watched over the whole period.
Anecdotally, over decades in some countries, looking at large falls of birds have given an idea of how much populations have dwindled.If you talk to people in Europe and the USA we never seem to see the falls that were historically described.
Sure, some birds do well if they can adapt or prefer areas which has been altered by man. It is naive to think that over all we are not heading for a steady decline in birds across the board as human population expands and habitat is lost.
Make the most of it. I'm not putting any bets on the situation improving over our lifetime.
Eric
The ringing data will not be useful because of a change in sites and annual variation in effort (number of times people are ringing, how long nets are open, etc.)
I'm not sure that the data presented really shows a decline in Yellow-rumped Flycatcher or Japanese Yellow Bunting. Sure, there are peak years in the 90s when numbers are larger than in recent years, but there is so much annual fluctuation that it is difficult to say whether this is a trend or just chance.
I agree that for many species the apparent increase in recent years is probably related to better reporting. It may be significant that for some species this hasn't occurred (e.g. Mugimaki Flycatcher), which perhaps suggests the increased reporting is balanced by a decline in numbers. But I think it would be difficult to draw any firm conclusions about this. Author: wcaptain Time: 20/12/2012 09:52
For the Japanese Yellow Bunting, as Geoff rightly pointed out that the two exception yrs around 1994 and 1996 (very hard to see exactly which yr), the count per yr is similar and fluctuated between 5 and 15 individuals. I would say they remained low, rather than a decline.
A long term survey of several migrants at a particular site(s) with constant survey effort may answer your question. Reviewing HKBWS bird count just provide a guess for further study, rather than a scientific evidence as Tung pted out that submission of records cannot assumpt to be constant throught yrs. Author: handrew Time: 20/12/2012 16:02
Do photographic records automatically get picked up and counted and if so was that always the case? For example, if someone posts a nice shot of Yellow-rumped flycatcher but does not submit a sighting record, is there a mechanism for adding this to the data? I confess I would not normally record my sightings of say Asian Brown Flycatcher although I guess I should. I do though occasionally take photos if it is a confiding bird. There may well be many images that don't get into the records but then there is the challenge of ensuring the bird is unique and not multiple-counted since many photographers don't disclose in detail where the image was taken. Its a fascinating subject. Our garden has had Black-winged cuckoo-shrike on 4 dates between Nov 30th and 19th December 2012 but I don't know that this is the same bird. I am assuming it is but I can't prove or disprove that. There must be lots of challenges to having data integrity. Author: wgeoff Time: 21/12/2012 09:45
Andrew, the answer to your question is yes. At the beginning of each year, I trawl through all the previous year’s records on the website, including photos and ID requests, and include any interesting and proven records in the records master file for that year. I have been doing this since 2009 (but not before).
Which makes the apparent decline in numbers of species like Yellow-rumped and possibly Mugimaki Flycatchers since the 1990s all the more notable, since both these species are highly valued by photographers.
I think ringing data would be useful to help answer this question, even if it is not perfect (what data is?). Quite large numbers of these species were ringed at MPNR and KFBG in the 1990s – are they still being ringed in these numbers? It may be that ringing data alone is biasing the result. Unfortunately, the HKBR no longer publishes annual ringing data.
[ Last edited by wgeoff at 21/12/2012 11:54 ] Author: ajohn Time: 22/12/2012 15:39
As I have said before, I really don't think that the ringing data will be of any use in this discussion. Most of the species being discussed are trapped infrequently (flycatchers are generally caught less often than other species because of their behaviour). For example, there have only ever been 2 Grey-streaked, 3 Dark-sided and 7 Narcissus ringed by our ringing group (note that other ringers are active in HK, and I don't have their data).
More flycatchers were trapped in the 1990s because there was a lot of ringing carried out in shrubland at that time - there is now more emphasis on reedbeds, where of course flycatchers are caught less frequently. Inevitably this means that less individuals are now trapped & ringed than was the case a few years ago.
My concern is that you are looking to answer the question 'are migrants declining?' and if you look at the ringing data then you are inevitably going to answer that with a 'yes', even though the ringing data are actually of no use in addressing the question. It is not simply that the data are 'not perfect', they are actually misleading.
For what it's worth, the two most frequently trapped flycatcher species in our data set (which is not all ringing data for Hong Kong) are Yellow-rumped (total 140 trapped - peak years 1991-94, when 17-23 were trapped each year - also good years in 1996 and 2000) and Mugimaki (total 135 - peak years 1992-97, when 10-26 trapped each year). Author: wgeoff Time: 22/12/2012 16:42
Thanks for the clarification that ringing data, particularly for Yellow-rumped and Mugimaki Flycatchers, is biased towards the 1990s due to choice of habitat for ringing rather than any actual trends in numbers, something I was not aware of before but was starting to guess. Even so the daily ringing counts are very high, nothing like these numbers are seen nowadays.
Ringing data is included in the 1990s master file data used above, which is why I was interested to know more about it. Taking that data out of the numbers (as well as I can) changes the graphs for Yellow-rumped and Mugimaki Flycatchers substantially - they now look much more like the others, with increases in numbers occurring in years after 2005. And I'm quite sure now that these increases are due to more frequent observations at locations like Po Toi.
So, put simply, the annual master files do not show any negative trends for any species except perhaps Mugimaki but are not very useful as an indicator of trends, and neither apparently is ringing data. We are left with consistent reporting from individual locations, but that will take time to generate any useful results.
[ Last edited by wgeoff at 23/12/2012 06:19 ] Author: wcaptain Time: 28/12/2012 18:06
Regarding the bird report data, assuming birders submitted all their records (so no biased to particular birds), maybe we could make a correction factor to each yr so to eliminate the problem of variation of submission nos.
In Europe, various population indexes have been developed for bird monitoring. Better to some literature researches.
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