This message came from a discussion on the ornithology list ORNITH-L, from
questions I posted about the mortality associated with bird feeding. It's
long, but it's surely of interest to all of us who feed birds (probably =
all of us). Please circulate it among your birding friends who aren't on
tweeters.

>From: "Dunn,Erica [NCR]" <DunnE at msm1s6.sid.ncr.doe.ca>
>To: "'Dennis Paulson'" <dpaulson at ups.edu>
>Subject: RE: Inquiry about birds an use of bird feeders over use of wild food
>Date: Wed, 18 Sep 96 11:28:00 PDT
>Encoding: 323 TEXT
>
> ---
>Dennis: Here is a copy of what I sent to "Natasha", in case you are
>interested. As far as I am aware, my papers on predation and window kills
>at feeders (1993 & 1994) are the only ones of their type. Brittingham and
>Temple have a paper on disease at feeders that is not listed below, but
>which might be cited in one of their papers that is. They found that tray
>feeders were more associated with spread of disease than other types.
> Overall, I agree that bird feeding is more for people than it is for
>birds, and that research is urgently needed on its effects. However, as far
>as I can tell with present data, there is no need to tell people they should
>quit altogether.
> ----------
>
> Dear Ms. Urbistondo:
>
>I have written an article on whether feeders are good/bad for
>birds, which I append below. (It's a bit out of date, but the
>ideas and references may prove useful. However, it deals
>primarily with effects of feeders on bird population size.
>
>I am not aware of any work on influence of feeders on seed
>dispersal. Presumably the question is whether or not feeders
>cause reductions in natural seed-dispersal functions of birds(??)
>My guess (more educated than most, but still a guess) is that
>there would be little such effect. Most evidence suggests that
>birds use feeders only as top-ups, as storage for a rainy day,
>and as emergency food--but not as a primary food supply (see
>references below). The one paper I know of that looks at
>nutritional consequences of relying on feeders is that by Cowie
>and Hinsley (see below), which showed that feeder food is not
>great for nestlings. They also showed, however, that the birds
>were only feeding nestlings with feeder food because of an odd
>combination of circumstances. (Late spring prevented normal
>dispersal from suburban to wilder breeding areas, so birds bred
>in suburbia--where natural foods were scarce. Thus, these pairs
>would have done very poorly even if feeders had not been
>present.)
>
>There are numerous papers in the literature on food choice, which
>suggest that birds are quite sensitive to constituents of the
>foods they eat (protein, fat, secondary compounds, sugar type and
>levels, etc.). Birds evidently change or vary diet readily to
>meet nutritional needs. This literature implies that birds will
>not rely on bird feeders if the foods provided in them are
>incomplete (which they are). As an example, Project FeederWatch
>has found that Black-capped Chickadees, Mourning Doves and House
>Finches are significantly more likely to choose sunflower seeds
>(over millet or milo) in northern (colder areas) than in southern
>regions. This suggests to me that there is a greater need for
>fat in the colder areas--but where fat content of seeds is less
>important (in the south), birds shift their preferences to other
>foods.
>
>Here are a few references (besides ones in the appended article)
>which may prove helpful:
>
>Brittingham, M.C., and S.A. Temple. 1989. Patterns of feeder use
>by Wisconsin birds: a survey of WSO members. Passenger Pigeon
>51:321-324.
>
>Brittingham, M.C., and S.A. Temple. 1992a. Does winter bird
>feeding promote dependency? J. Field Ornithol. 63:190-194.
>
>Brittingham, M.C., and S.A. Temple. 1992b. Use of winter bird
>feeders by Black-capped Chickadees. J. Wildl. Manage. 56:103-110.
>
>Cowie, R.J., and S.A. Hinsley. 1987. Breeding success of blue
>tits and great tits in suburban gardens. Ardea 75:81-90.
>
>Jansson, C., J. Ekman and A. von Bromssen. 1981. Winter mortality
>and food supply in tits Parus spp. Oikos 37:313-322.
>
>Wilson, W.H., Jr. 1994. The distribution of wintering birds in
>central Maine: the interactive effects of landscape and bird
>feeders. J. Field Ornithol. 65:512-519.
>
>
>
>
>[Article printed in 'Bird Observer' Vol. 20(6): 308-314, 1992]
>
>IS WINTER BIRD FEEDING GOOD FOR BIRDS?
>
>Erica H. Dunn, 159 Sapsucker Woods Rd., Ithaca, NY 14850
> (Current address: Canadian Wildlife Service, National
> Wildlife Research Centre, 100 Gamelin Blvd., Hull, Quebec,
> Canada K1A 0H3)
>
> Every person who feeds birds on a regular basis wonders
>occasionally about possible negative effects. Might birds become
>dependent on feeders and lose their natural foraging skills? Do
>feeders lure birds to areas where predation and disease are more
>likely? Alternatively, bird feeding might be too much of a good
>thing. Some authors have suggested that species like Blue Jays
>and Brown-headed Cowbirds benefit so much from bird feeding that
>their nest-robbing and nest-parasitic habits are putting
>increased pressure on other, less common species.
> Despite the incredible food bonanza provided to birds by the
>estimated 50 million North Americans who purchase bird food
>annually, remarkably little research has been carried out on its
>impacts (Shaw and Mangun 1984, Filion et al. 1985). One of the
>best North American studies was done by Margaret Brittingham,
>then a graduate student at the University of Wisconsin
>(Brittingham and Temple 1988). She followed Black-capped
>Chickadees wintering in woodlots with similar characteristics
>except for differing availability of feeders.
> Chickadee flocks without feeder access had lower over-winter
>survival than did chickadees with supplemental food. However,
>the effect was seen only in winters with prolonged, severe cold
>snaps. This suggests that feeding in areas with a milder winter
>climate would have no effect. Moreover, the density of breeding
>birds did not differ between Brittingham's study areas. If the
>"extra" birds that survived because of bird feeding bred at all,
>it must have been through dispersal to other, perhaps less
>suitable areas. Overall, then, the impact of bird feeding on
>chickadees remains obscure.
> Other studies, too, have generated equivocal results (van
>Balen 1980, Desrochers et al. 1988, Kallender 1981, Orell 1989.
>Each research project takes intensive effort over a period of
>years, so data will not accumulate quickly. And even when we
>have solid, clear results, any effect of bird feeding that is
>documented might apply only to the species studied or to a
>particular geographic location.
> We can get around the limitations of single-species studies
>by looking at continent-wide population trends in feeder birds.
>If feeder species have increased or decreased as a group in
>directions that differ from trends in other bird populations, we
>would have strong suggestive evidence that feeders play a role.
>I recently carried out this very analysis.
> For a definition of a "feeder species", I turned to Project
>FeederWatch (a cooperative survey of North American bird
>feeders). Any bird that was reported by at least 25% of
>FeederWatchers within its winter range during a winter was
>considered to be a feeder bird. I then mined the U.S. Fish &
>Wildlife Service's Breeding Bird Survey (BBS) for information on
>population trends during the period 1966-1989. The BBS consists
>of over 2000 road-side counts of birds conducted annually during
>the breeding season, and has been running since 1966. BBS is not
>infallible, but is the best source of data we have on continent-
>wide trends in bird populations over the past 25 years.
> If nothing in particular is happening to bird populations,
>we expect about 50% of species to have increasing trends and 50%
>to be declining, simply by chance. It is quite rare for a
>population trend to show no change at all. Of the 48 feeder
>species in my analysis, 40% were increasing and 56% declined over
>the past 25 years, according to BBS (Table 1). These figures did
>not differ in a statistical sense from the 50:50 ratio expected
>by chance. But when the analysis was restricted to the most
>wide-spread species, those visiting at least 50% of feeders in a
>region, the results showed that significantly more feeder species
>declined than increased (70% vs. 31%, respectively). When I
>considered only population changes that were so large or
>persistent over the 25-year period that the trends could be
>considered biologically important (as opposed to chance events),
>I got the same results. Only among the most widespread species
>was there a difference from the 50:50 ratio. Of the 13
>widespread species with important population changes, 21%
>increased while 77% declined.
> All the feeder species with significant population trends
>are listed in Table 2. It is apparent that the more woodland-
>dependent birds such as nuthatches, woodpeckers and chickadees
>are on the increase side of the ledger, while the declines
>include more open-country and suburban species. Moreover, most
>of the "pest" species are among the decliners, including nest-
>robbers (Blue Jay, grackle, magpie) and nest-site competitors
>(House Sparrow, European Starling). One of the more serious
>pests in terms of its impact on other species is the Brown-headed
>Cowbird. This bird is not included in the table, because its
>declining trend was not significant in statistical terms.
> If we assume that the population changes shown in Table 2
>were a result of winter bird feeding, we might conclude that
>feeding is a bad thing. But when we put the data in a broader
>context, this seems less a concern. Compilations of Breeding Bird
>Survey data for all species show that 92% of grassland-nesting
>birds have declined since 1966, along with 62% of scrub-nesters
>(S. Droege, U.S.F.W.S. unpubl.). Many of the declining species
>in Table 2--grackle, sparrows, thrashers--are members of these
>groups that are decreasing across the board, whether or not the
>constituent species visit feeders.
> Further evidence that bird feeding does not cause excess
>mortality came from a special inquiry undertaken by Project
>FeederWatch. Participants recorded any deaths observed in their
>yards over one winter, providing details on causes and
>surrounding circumstances. The aim was to discover whether
>feeding exposed birds to unusual danger from window collisions,
>disease and predation.
> Window strikes accounted for more deaths near feeders than
>any other factor (close to half of the 2000+ deaths reported). A
>full analysis of this data set (to be published in the Journal of
>Field Ornithology [see Dunn and Tessaglia 1994]) suggested that
>between 1 and 10 birds might be killed annually by striking
>windows at every building in North America. As high as this
>number sounds, it probably represents less than a percent or two
>of all birds alive each fall.
> Predation came a fairly close second in the Project
>FeederWatch study, causing about one-third of reported deaths.
>Sharp-shinned and Cooper's Hawks were the culprits in about 51%
>of kills witnessed, and cats in 29%. Bird-eating hawks make
>about 1-3 prey captures daily (Palmer 1988), but most
>FeederWatchers who witnessed predation at all saw only one or two
>cases over the whole winter. [See Dunn 1993] The conclusion was
>that the majority of hawks use feeders opportunistically, and not
>as a primary food source. In one European study, bird-eating
>hawks were estimated to kill about 10% of all finches passing
>through a particular migratory stopover site in autumn (Lindstrom
>1989). Compared to this level of risk, bird feeders are
>positively safe havens!
> Relatively few of the deaths observed in the FeederWatch
>study were attributed to disease. Most of these were probably
>the results of salmonella infection, in which birds appear
>lethargic, fluff up their feathers and have difficulty breathing
>for a few days prior to death (Terres 1981). Passed through the
>faeces, the disease can spread readily through contaminated bird
>seed. It is seen most often in flocking species when they are
>stressed by severe weather or food shortages.
> Summing up all the sources of mortality reported in this
>FeederWatch study, it was found that only one bird death was
>reported over the winter for every two feeder sites. There is no
>doubt, of course, that many dead birds were not found or
>reported. Nonetheless, natural mortality rates in songbirds of
>35-50% annually would lead us to predict at least 4-5 bird deaths
>over a winter at each FeederWatch home, based on the average
>number of birds present at those feeders and assuming that
>mortality is spread evenly through the year. Actual figures were
>a tenth of that prediction. Even if under-reporting was a severe
>problem, therefore, it appears that feeders do not draw birds
>into an environment that is far more dangerous than the one they
>face in the wild.
> These analyses suggest that bird feeding has not had blanket
>effects on populations of all feeder species. More subtle
>effects may exist, perhaps varying among species (positive for
>some, negative for others). It will take detailed studies on
>individual species, however, to demonstrate such effects. In the
>meantime, you can continue to feed birds with a clear conscience.
>All current evidence suggests you are not unduly upsetting
>natural ecological systems.
>
> REFERENCES CITED
>
>Brittingham, M.C., and S.A. Temple. 1988. Impacts of supplemental
> feeding on survival rates of Black-capped Chickadees.
> Ecology 69: 581-589.
>Desrochers, A., S.J. Hannon, and K.E. Nordin. 1988. Winter
> survival and territory acquisition in a northern population
> of Black-capped Chickadees. Auk 105: 727-736.
>Dunn, E.H. 1993. Bird mortality from striking residential windows
> in winter. J. Field Ornithol. 64(3): 302-309.
>Dunn, E.H., and D.L. Tessaglia. 1994. Predation of birds at
> feeders in winter. J. Field Ornithol. 65(1): 8-16.
>Filion, F.L., A. Jacquemot, and R. Reid. 1985. The importance of
> wildlife to Canadians: An executive overview of the
> recreational economic significance of wildlife. Can. Wildl.
> Serv., Environment Canada, Cat. No. CW66-76/1985E.
>Kallendar, H. 1981. The effects of the provision of food in
> winter on a population of the Great Tit Parus major and the
> Blue Tit Parus caeruleus. Ornis. Scand. 12: 244-248.
>Lindstrom, A. 1989. Finch flock size and risk of hawk predation
> at a migratory stopover site. Auk 106: 225-232.
>Orell, M. 1989. Population fluctuations and survival of Great
> Tits Parus major dependent on food supplied by man in
> winter. Ibis 131: 112-127.
>Palmer, R.S. (Ed.) 1988. Handbook of North American Birds, Vol.
> 4: Diurnal Raptors (Part I). Yale University Press, New
> Haven.
>Shaw, W.W., and W.R. Mangun. 1984. Nonconsumptive use of wildlife
> in the United States. U.S.F.& W.S., Resource Publ. 154: 20
> pp.
>Terres, J.K. 1981. Diseases of birds--how and why some birds die.
> Amer. Birds 35: 255-260.
>van Balen, J.H. 1980. Population fluctuations of the Great Tit
> and feeding conditions in winter. Ardea 68: 143-164.
>
>
> Table 1. Population trends of feeder species in the period 1966-1989,
> according to the Breeding Bird Survey. (Data from S. Droege,
>U.S.F.W.S.
> unpubl.).
>
> Percent of Number of % Species % Species
> Feeders Visited Species Increasing Decreasing
>
> > 25 % 48 40
> 56
>
> > 50 % 29 31
> 70
>
>
>
>Table 2. Feeder species with statistically significant population changes,
> 1966-1989. (Data from S. Droege, U.S.F.W.S. unpubl.).
>
> Percent of Increasing Decreasing
>Feeders Visited Populations Populations
>
>> 75 % Black-capped Chickadee Blue Jay
>
>
> European Starling
>
> American Goldfinch
>
> House Sparrow
>
>50-75 % Hairy Woodpecker Northern Flicker
> White-breasted Nuthatch Northern Mockingbird
>
> Rufous-sided Towhee
>
> Song Sparrow
>
> Common Grackle
>
> Pine Grosbeak
>
>25-50 % Scrub Jay Black-billed
>Magpie
> Red-breasted Nuthatch Brown Thrasher
> Varied Thrush
> White-crowned Sparrow
>
>

Dennis Paulson, Director phone 206-756-3798
Slater Museum of Natural History fax 206-756-3352
University of Puget Sound e-mail dpaulson at ups.edu
Tacoma, WA 98416