Summary
- Loss et al. (2014) “estimate that between 365 and 988 million birds (median = 599 million) are killed annually by building collisions in the U.S. [United States]”. Bird-safe glass is “specially designed to make glass a visible obstacle to birds”, and therefore decrease the number of birds killed by building collisions. In this post, I illustrate that (replacing standard with) bird-safe glass may change the welfare of arthropods much more than that of birds.
- I calculate wild insectivorous birds eat 23.4 k arthropods per bird-year, which implies the birds live for 22.4 bird-minutes for each arthropod they eat.
- I conclude bird-safe glass can easily increase or decrease welfare. I believe it may impact arthropods way more than birds, and I have very little idea about whether it increases or decreases the welfare of arthropods.
- I recommend research on i) the welfare of soil animals and microorganisms, and ii) comparisons of (expected hedonistic) welfare across species. I think progress on ii) is difficult, but necessary to find interventions which robustly increase welfare. I also see lots of room for progress on ii) to change funding decisions even neglecting soil animals and microorganisms. For welfare range proportional to “individual number of neurons”^“exponent”, and “exponent” from 0 to 2, which covers the best guesses that I consider reasonable, the welfare range of shrimps is 10^-12 to 1 times that of humans.
- I am sceptical that targeting non-soil animals is a great way to build capacity to increase the welfare of soil animals later. I believe the most cost-effective ways of building capacity to help any given group of animals will generally be optimised with such animals in mind. I would also expect much more investigation of the extent to which interventions targeting non-soil animals are building capacity to increase the welfare of soil animals if this was key to whether they are increasing or decreasing animal welfare.
Context
Loss et al. (2014) “estimate that between 365 and 988 million birds (median = 599 million) are killed annually by building collisions in the U.S. [United States]”. Bird-safe glass is “specially designed to make glass a visible obstacle to birds”, and therefore decrease the number of birds killed by building collisions. Mal Graham has argued there is significant uncertainty about whether bird-safe glass increases or decreases the welfare of birds and other animals. In this post, I illustrate that bird-safe glass may change the welfare of arthropods much more than that of birds.
Arthropods eaten by birds
Here are my calculations.
I assume the target birds have a mean mass of 26.4 g. I get this from the mean between the means for white-throated sparrows, dark-eyed juncos, ovenbirds, and song sparrows, which are the most commonly killed according to Loss et al. (2014). “Of 92,869 records used for analysis, the species most commonly reported as building kills (collectively representing 35% of all records) were White-throated Sparrow (Zonotrichia albicollis), Dark-eyed Junco (Junco hyemalis), Ovenbird (Seiurus aurocapilla), and Song Sparrow (Melospiza melodia)”. I set the mean mass for each species to the mean between the lower and upper bounds from All About Birds.
I suppose wild insectivorous birds eat 421 Mt of arthropods per year. I get this from a weighted mean of the lower and upper bounds from Nyffeler et al. (2018). I put a weight of 2/3 on 396 Mt, and 1/3 on 472 Mt. They say the “true value” is “most likely at the lower end of this range”. Nyffeler et al. (2018) says “the standing biomass of the global community of insectivorous birds might be on the order of ≈ 3 million tons”. So I infer wild insectivorous birds eat 140 kg of arthropods per year per kg of birds. Combining this with my mean mass for the target birds, I determine that wild insectivorous birds eat 3.70 kg of arthropods per bird-year.
I estimate the target birds eat 6.33 k arthropods per kg of arthropods. I get this for an individual mass of arthropods of 0.158 g, which is the mean of a lognormal distribution with 8th and 99.99th percentile of 0.481 mg and 1.65 g. Gemini 3.1 Pro on 31 March 2026 estimated 8 % of the arthropods eaten by the 4 bird species I mentioned above are lighter than 0.481 mg, and 0.01 % are heavier than 1.65 g. Dark-eyed juncos eat aphids, and these have a mean mass of “0.4809 mg” according to Maia et al. (2020). Song sparrows eat grasshoppers, and I found a fact sheet mentioning a mean mass of female three-banded grasshoppers of “1,654 mg”.
I calculate from the above that wild insectivorous birds eat 23.4 k arthropods per bird-year, which implies the birds live for 22.4 bird-minutes for each arthropod they eat.
Discussion
Bird-safe glass may impact arthropods much more than birds
My intuition based on the numbers above alone is that bird-safe glass may change the welfare of arthropods much more or less than that of birds. I explore this further below, but ultimately arrive at the same conclusion. For simplicity, I ignore changes in the population of arthropods, and in the welfare of birds due to these dying in some other way due to avoiding collisions with buildings.
I guess I am roughly indifferent between 1 s of excruciating pain, like “severe burning in large areas of the body, dismemberment, or extreme torture”, and losing 24 h of fully healthy life. I speculate birds cause 1 s of excruciating pain to each arthropod they eat. So I estimate the decrease in the welfare of each arthropod is equal to that from them losing 24 h of fully healthy life (= 24*60/22.4). This is a loss of 64.3 fully-healthy-arthropod-years per bird-year (= 24*60/22.4).
One can clarify the comparison above using the tentative (expected) welfare ranges in Bob Fischer’s book about comparing welfare across species. That of chickens is 5.48 (= 0.40/0.073) times that of black soldier flies (BSFs). Welfare range is defined there as the difference between the maximum and minimum welfare per unit time among “realistic biological possibilities”. I do not expect the welfare range of the target birds to dramatically differ from 5.48 times that of random arthropods they eat under the methodology of Bob’s book. I would be surprised if the welfare range of the target birds was meaningfully different from that of chickens, or if that of random arthropods they eat was dramatically different from that of BSFs. In addition, I think it is reasonable to assume that individual welfare per fully-healthy-animal-year is proportional to the welfare range. So I suppose 1 fully-healthy-bird-year has 5.48 times as much welfare as 1 fully-healthy-arthropod-year. In this case, the decrease in the welfare of the arthropods eaten by the birds corresponds to a loss of 11.7 fully-healthy-bird-years per bird-year (= 64.3/5.48).
I speculate the lives of the target birds are 50 % as good as fully healthy lives. So I conclude from the above that the effects on arthropods are 23.4 (= 11.7/0.5) times as large as those on the birds.
My practical conclusion is that bird-safe glass may change the welfare of arthropods much more or less than that of birds. The final comparison above suggests the effects on arthropods are much larger than those on birds. Nonetheless, there is large uncertainty in the change in the living time of birds and arthropods, change in the welfare of birds due to these dying in some other way due to avoiding collisions with buildings, change in the welfare of arthropods due to these being eaten by birds instead of dying in some other way, and welfare comparisons across species.
Bird-safe glass can easily increase or decrease welfare
I conclude bird-safe glass can easily increase or decrease welfare. I believe it may impact arthropods way more than birds, and I have very little idea about whether it increases or decreases the welfare of arthropods. I do not know which species of arthropods are the most important to determine the change in the welfare of arthropods given the large uncertainty in welfare comparisons across species. I can see the most important arthropods being the smallest, largest, ones with intermediate mass, or any combination of these. To make matters worse, I have almost no clue about whether any species of wild arthropods has positive or negative lives in a given biome. I am also very uncertain about which arthropods become more or less abundant as a result of changes in the population of birds.
What now?
I recommend research on i) the welfare of soil animals and microorganisms, and ii) comparisons of welfare across species. I think progress on ii) is difficult, but necessary to find interventions which robustly increase welfare. For instance, ones that focus on the greatest sources of suffering across all species. I also see lots of room for progress on ii) to change funding decisions even neglecting soil animals and microorganisms. In Bob’s book, the tentative welfare range of shrimps is 8.0 % of that of humans. However, for welfare range proportional to “individual number of neurons”^“exponent”, and “exponent” from 0 to 2, which covers the best guesses that I consider reasonable, the welfare range of shrimps is 10^-12 (= (10^-6)^2) to 1 times that of humans, as shrimps have 10^-6 times as many neurons as humans.
I would prioritise the above research over the “ecologically inert” interventions which have been proposed so far. I suspect interventions decreasing the pre-slaughter pain of farmed invertebrates are the closest to robustly increasing welfare (in expectation). However, I still do not know whether electrically stunning farmed shrimps increases or decreases welfare due to potentially dominant effects on soil animals and microorganisms. Furthermore, I would say such interventions may increase welfare only negligibly due to their target invertebrates having a super narrow welfare range, as it would be the case if shrimps had a welfare range equal to 10^-12 times that of humans.
Targeting non-soil animals is a great way to build capacity to increase the welfare of soil animals later?
I am sceptical. I believe the most cost-effective ways of building capacity to help any given group of animals will generally be optimised with such animals in mind. I would also expect much more investigation of the extent to which interventions targeting non-soil animals are building capacity to increase the welfare of soil animals if this was key to whether they are increasing or decreasing animal welfare.
Is there any project you think may not impact arthropods and/or soil animals much more than whatever animals are targeted? I feel like exploring this would be far more insightful at this stage.
Hi Jim. No. For all the non-research interventions I am aware of, including all on Rethink Priorities' (RP's) Wild Animal Welfare Intervention Database (WAWID), I think the effects on soil animals or microorganisms may be much larger than those on the target beneficiaries. So I recommend decreasing the uncertainty about the effects on soil animals and microorganisms via research on i) their welfare, and ii) comparisons of welfare across species.
Curious what motivated you to spend time assessing the impact of bird-safe glass on arthropods, specifically, then. Were you hoping to find out that bird effects dominated but found and shared the opposite unsatisfying results? Or maybe you think "here's another example showing how indirect effects on tiny animals may dominate" and that this will convince some people to also prioritize (i) and (ii)? (people who were not convinced by your previous largely-overlapping posts but might by this one?)
I was mostly motivated by this, but I would not be surprised if my post ends up having a very minor effect.
I agree with your logic, but I'm wondering how you psychologically deal with this? I find this type of thinking quite uncomfortable. In a way it taints all of our endeavors. Can anything be about what it was intended to be? Can bird safe glass be about birds?
Here's the most uncomfortable part and what I'm genuinely afraid of: it is the possibility to arrive at negative conclusion about an intervention that is by our very strong intuitions very positive, benevolent and altruistic, and that probably does, indeed help birds.
So if, after research, you arrived at this conclusion that such bird safe glass indeed hurts arthropods more than it helps birds, what would you do with this fact?
Would you advocate for stopping this type of interventions? Would you conclude we should let birds die by crashing into windows?
I personally think this is not a good approach. I think this constant triage is very cruel and cold as it directly puts interests of one group directly against the interest of other group.
My approach is probably more naive and maybe wrong at first sight from the utilitarian point of view, though it maybe be actually good when considered from more sophisticated utilitarian perspectives.
My approach would be to let interventions benefiting birds be about birds without worrying about effects on arthropods, while at the same time trying to directly help arthropods as well, by some other interventions directly aimed at arthropod welfare.
Since you care a lot about arthropods and soil animals and think that their welfare should dominate our moral concerns, maybe it would be valuable to try to think of interventions that could directly help them without hurting other animals or damaging the whole ecosystems.
BTW, my hunch is that they don't have net negative welfare, and even if they do have net negative welfare the solution is not to consider their extermination, but to wait until we're so advanced technologically that we can turn their welfare positive instead of simply eliminating them.
I'm saying this because elimination of certain species that we consider to be suffering, would be a dangerous precedent, that's first, and second it would damage biosphere. Some other animals eat them for food, so if you remove arthropods, you also remove food for those other animals.
So my take is to try to find very conservative ways of helping arthropods directly without eliminating them, without having strong negative effects on other animals and humans, and without causing us to evaluate every other intervention that is focused on other beneficiaries in terms of how it affects arthropods.
So my take is that concern for arthropods and concern for other beneficiaries should not be mixed. It should be two separate things. Both are worthy and valuable, but one should not be judged in terms of other.
Also, interventions that directly help arthropods and soil animals could plausibly have more effects on their welfare than interventions where effects on arthropods and soil animals are just a side effect.
Thank you for the comment, Zlatko.
I sometimes feel a bit demotivated that increasing welfare seems very hard. However, I try to focus on what I can do to improve the situation. I also find comfort in determinism. I already thought I could not contribute to a better or worse world even before learning about effects on soil animals. I believe what I do (or, more precisely, the probabilities of my potential actions) is fully determined by the laws of physics.
I am not confident that bird-safe glass increases the welfare of birds. From Mal's post:
We are always in triage?
Would you advocate for bird-safe glass if it increased the welfare of birds, but robustly increased suffering, and robustly decreased happiness accounting for effects on soil animals and microorganisms?
I can see the total welfare of soil animals being practically negligible or all that matter. For individual welfare per fully-healthy-animal-year proportional to "individual number of neurons"^"exponent", and "exponent" from 0 to 2, which covers the best guesses that I consider reasonable, I estimate that the absolute value of the total welfare of soil ants, termites, springtails, mites, and nematodes is 2.04*10^-5 to 17.9 billion times the total welfare of humans.
I agree. I do not recommend pursuing interventions aiming to change land use, or decrease the welfare of non-soil animals.
Would you advocate for an intervention which harms a group of people A much more than it benefits another group of people B? If not, one should also consider not advocating for an interventions which may harm a group of animals C much more than it benefits another group of animals D?
I agree.
Here's my kind-of logic. Basically it's based on some principles:
that side effects of certain interventions (in this case increased suffering of arthropods) can be compensated in a similar way as you compensate carbon emissions by buying carbon offsets. So if you increase suffering of arthropods by helping birds, you should make sure that you also decrease suffering of arthropods by helping them directly by interventions directly aimed at them. My hope is that such interventions could be much more effective, so that side effects of helping birds becomes a rounding error. This is just a HOPE not a claim that we can actually achieve this.
There are some other considerations but then it would get too long.
Regarding the following:
First, I think even if the pain that ends bird's life in case of predation is indeed much worse than pain caused by hitting a window - 1-2 additional years of life are probably worth it. First of all hitting a window isn't painless either. Second, a bird can survive hitting a window and end up disabled. Third, if the bird is killed by predation, it ends her life, so no matter how painful it was while it lasted, the bird doesn't deal with trauma afterwards. It's bad but lasts very short time. Unsuccessful predation that leaves bird dismembered and traumatized, but alive, is probably much worse.
Now even more importantly, I think we shouldn't even think in this way. If we conclude that extra years of life are net negative for birds, what should we do? Should we go and kill all birds? This is a very negative attitude towards life. I think the good thing about suffering at the end of life is that it isn't endless, and as soon as it ends, the there's nothing more for those birds. It is not remembered it doesn't leave trauma or disability (except in cases of unsuccessful predation) But I guess they should live as long as possible before that. Thinking otherwise would mean that we are in principle supporting painless euthanasia of animals, to protect them in advance from life itself. I think it's not a good way to think about life.
I think there are some higher principles, such as that life is good in principle. And interventions should improve welfare, but not to the detriment of life itself. If some pain is inevitable part of life at this stage of our development, I think it's better to accept it than to rebel against the idea of life itself.
I know this, but I think offsets can help us escape it. There are things that matter for different reasons. Birds matter because we love birds, and we want to help them, and helping them is generally good, if you are looking at the action in itself. Side effects are not immanent to helping birds. So for side effects, you "buy offsets" by helping arthropods directly.
Probably yes, but with buying offsets. I can't logically explain it but I think bird welfare matters for more reasons than just utility calculus. Birds matter in their own right... like they are ends in themselves. They are not means for increasing the amount of pleasure in the Universe. They matter for their own sake, and they have been important for humans for ages, and eating bugs might even be useful... Maybe it is way to keep insect population from exploding, which would likely produce many unhappy insects. So yes, I would help birds anyway, but in case I'm really sure about negative effects on bugs, I would try to eventually offset it by directly helping arthropods by some other intervention. Maybe not immediately, but eventually, helping arthropods would be on my agenda.
No. But I think the two situations are not really analogous.
First of all, all people are in the same category according to most moral theories. Birds and arthropods don't seem to be in the same category. Second interventions that help one group of people and harm other group even more don't seem like they could look good on any intuitive measure. It would seem like some form of exploitation, slavery, war, genocide, or something like this, which doesn't look good.
Third, windows are not a natural part of environment, it's something introduced by us, that directly harms birds. Predation of worms and bugs by birds has always been there and it might have benefits for the birds, for the ecosystem, and perhaps even for the bugs, if it keeps their number in check and avoids overpopulation, which could result in much worse life conditions, hunger, etc... Of course it won't help the insect that's eaten, but it might help the population of insects as whole by controlling their population.