Our immune systems are fascinating and complex, I assume, but as with everything else in our bodies, their effectiveness depends on having adequate resources. If we don’t get enough to eat, we’re less able to fight off infection. Obviously, the same holds true for all other animals, including everyone’s favorite feathered bipeds. That means that if we want to help the local bird population thrive, we should put out bird feeders, right?

Well, not to spoil the ending, but yes, probably. There are, however, some legitimate concerns about widespread use of bird feeders. The first one is that by creating a regular, common feeding place for multiple species, you increase the spread of disease. There’s legitimate concern, especially with the current avian flu pandemic (in birds – it’s not a serious problem for humans yet), and it’s recommended that you periodically clean your bird feeders, for the sake of the birds. Nobody likes eating off dirty dishes, especially when everybody in town has been eating off those same dishes.

There’s also another problem, that I hadn’t really considered – by creating a common feeding spot, you’re also forcing inter-species social dynamics into existence, which could in turn alter the ecosystem:

According to Alex Lees, who, with his colleague Jack Shutt, published the paper in the journal Biological Conservation, the issue is that there are a few species that are now habitual feeder users – familiar garden visitors including great tits and blue tits. And they appear to be receiving a boost from feeding.

“We know from historical research that these species are increasing in number,” says Dr Lees. This could, he says, be at the expense of other “subordinate” birds.

“A blue tit is a dominant species – it tends to win in interactions and fighting for food or quarrelling for nest sites,” explains Dr Lees. “Whereas species like willow and marsh tits are subordinate. They tend to lose those in interactions.

“For willow tits, we know that one of the reasons for the decline is that 40% of their nesting attempts fail because blue tits essentially steal their nesting cavities.”

A constant supply of peanuts and seeds that boosts the number of blue tits and great tits could be helping to drive the continuing decline in the willow tit population. It could also throw off a natural, seasonal ebb and flow in species numbers, Dr Lees says.

“Migrant pied flycatchers are in direct competition with great tits for nesting sites,” he explains. “So, again, by boosting the population of great tits in the UK, we may be tipping the balance in favour of these resident species over those summer migrants.”

As Dr. Lees goes on to say, this won’t necessarily be the case in every ecosystem, because not all birds are going to clash like that. It seems like it’s primarily going to be a problem when bird feeder dynamics work to amplify an existing set of behaviors, so you’re not always going to end up with a tit-à-tit conflict like that. Still, between that and the disease risk, are bird feeders doing more harm than good?

Well, I can’t make broad statements, but based on some research from Sweden, if you live somewhere with harsh winters, then the regular food provided by feeders reduces the severity of birds’ immune response.

A small change in body temperature can be fatal for humans. Small birds, meanwhile, lower their body temperature at night by several degrees during the winter. Just like us, the birds attempt to save energy when it is cold. If they are exposed to infection, the body’s first reaction is to raise its temperature, which clashes with the bird’s simultaneous need to save energy by lowering body temperature.

“We investigated how access to food during winter affected the balancing act between maintaining a low body temperature in order to save energy, and the possibility of raising body temperature in order to fight infection,” says Hannah Watson, biologist Lund University.

The study shows that birds who were fed during the winter did not need to lower their body temperature as much at night as birds who did not have access to feeding tables. They had gathered enough energy to survive a winter night in spite of a having higher body temperature.

When the birds were exposed to a simulated infection, all the birds had essentially the same temperature during a fever. Instead of conserving energy to survive the winter, the birds without access to extra food were forced to use more energy in order to raise their body temperature high enough to battle infection.

“We had expected to find that the birds that had access to birdfeeders would have more energy to fight an infection, and that as a result they would exhibit a stronger fever response. Our results, however, show the opposite – birds that did not have access to a reliable source of food had the strongest reaction to infection. This enabled them to reach the same fever temperature as the birds with extra food,” says Hannah Watson.

Basically, a well-fed body has options, when it comes to fighting infection, that a malnourished body does not.

But if you’re like me, you may be wondering what it means to expose a bird to a “simulated infection”. While it did make me wonder about how one would convince a bird it had been exposed to a disease, and must therefor be sick, I figured I’d just go to the actual paper, since it’s freely available, and see what they actually did. Basically, they injected a substance from the cell wall of an E. coli bacteria, which caused the birds’ immune systems to react to the presence of a “disease”, without any actual risk of an infection. I doubt the birds appreciated any of their involvement in this study, but I think it’s a neat trick, and a cool way to study how immune systems work without any actual infection.

I think most people who feed birds do it because they like seeing the birds at the feeder. Growing up, my grandparents had several bird feeders right outside their dining room window, and I have fond memories of watching the birds while I ate breakfast during holiday visits. My parents also have bird feeders, and in the years they’ve been up, the diversity of birds coming by seems to have increased.

I’m not going to make recommendations about how you interact with your local bird community, but for me, at least, the knock-on effects of it are not something I’ve thought about much. All in all, I think the problems caused by bird feeders pale in comparison to what we’re doing to the planet as a whole, and how that is affecting the birds. The lesson I’m gonna close with is that if you ever see birds at your feeder during the harsh weather, and feel good knowing they’ve got something to eat, you can rest assured that your feelings are supported by science, and you really are helping them.