Is It Hunger or Lights Attracting Birds to Cities? Chicago A Case Study

                If All Chicago Lights Out, Migrants Still Land in Chicago

It is often stated as a fact that “artificial light… lures (migrating birds)…. into urban centers.” Therefore, reducing metropolitan light glow should be a major conservation goal. Cities, after all, with many windows and cats are not a safe place for birds. This analysis concludes that it is hunger, not lights, which is attracting migrating birds to Chicago.

A 2020 study by a coalition of  Illinois Avian Conservation organizations observed that birds land in Chicago “looking for food and shelter.” Are they “lured by lights” or “looking for food and shelter?”

Testing the “Light Attracts” Thesis. In this short paper the Chicago metro area is used to test the thesis that birds are “lured by artificial lights” to land in metro and this is to their detriment. Chicago is on a principal flyway and the location of widely reported bird death events.

SUMMARY

Hunger, not lights, lures birds into the metro Chicago area. Chicago provides much more food for migrants than the adjacent areas.This is the case because metro Chicago has much more tree cover than the adjacent areas. (water and farmlands) Trees are a major source of food which songbirds must have to survive migration. Since birds are landing in metro Chicago to feed, a campaign in lower lights will be in vain. They will still come to feed as they always have. Ironically, it would benefits the birds if they were they lured by light to metro Chicago.

Best Option  Protecting birds at hot spots in the Chicago area is the best conservation option – not turning off lights. Birds concentrate where they find food. If they are threatened by windows near those sites, a collision prevention system on those windows can reduce collision mortality by 95% — as shown at the McCormick Center. Increasing tree cover should also be a major priority.

Migrants Attracted to Feed and to Rest

A multi-year study by Illinois ornithological experts concluded that migrants land in the Chicago metro area to feed just before dawn to refuel for the long journey ahead. As the Bird Conservation Network Breeding Bird Trends report stated: 

The green spaces of the Chicago Wilderness area provide important stopover habitat for birds that would otherwise find themselves over farmland, heavily urbanized areas, or Lake Michigan when dawn arrives and they are looking for food and shelter. p. 12. (The term wilderness include metro Chicago.)   https://www.bcnbirds.org/wp-content/uploads/Breeding-Bird-Trends-in-the-Chicago-Region-1999-2020.pdf

Does New Biological Research Disagree?

A study published in 2023 is cited as proving the light glow is the culprit (Horton, 2023). The study’s headline is  Artificial light at night is a top predictor of bird migration stopover density. Does this new paper mean that the local experts are wrong? A careful reading of the paper actually affirms that the local experts are right – birds are attracted to feeding habitat at night as they migrate.

The key to understanding this paper are the charts on page 3 and the text on page 5. The bar chart shows that vegetated cover is about as important as light as an attractant when the cover elements are combined. The pie chart below says that land cover is more predictive than sky glow. Vegetated cover in this case is trees and bushes where migrants find the food they need to coninue their migration.(Grassland birds and waterbirds are not discussed here.)

Horton, as a good scientist, recognizes that alternatives to light glow attracting birds should be considered. He states on page 5.

 ” Specifically, are these bright areas, which may fit our definition of a stopover hotspot, a result of attraction from artificial lights or important ecological regions, or the combination of the two?”

Food Is Strongest Attractant  The strongest attractant for a migrating bird, indeed of any bird, is habitat where they can find food. It is far stronger than light. This is illustrated by the fact that six species account for 50% of all strikes at the McCormick Center. Thirty percent were sparrows. “Striking heterogeneity” is how a study described it. (Winger, 2019) If birds are attracted to light as a class, one would not expect this striking heterogeneity.(The Winger study did not address overall light glow, only window lights.)

In contrast, all birds are driven to eat, only some are attracted to lights. Hunger is a primal urge which explains bird behavior far better than lights. An attractant that explains the behavior of all birds is per se preferable to an attractant that only explains the behavior of some birds.

Chicago: “Important Ecological Region?”   

Is Chicago “an important ecological region” to which birds will be attracted?”   Regions that provide food are ecologically important. Why do birds migrate through Chicago? First, because they have always followed this migration path. Millions fly over without landing– but why do some land in metro Chicago? They land to find the food they need to continue migrating. The primary foods for most passerines during migration are insects and fruit. Because of their vegetated mass, trees feed many insects and also bear fruit. They are the best place for a bird to seek food. They find those trees in metro Chicago. The Chicago metro area is not an expanse of concrete, it is actually a relatively heavily forested area. That is why Chicago is an “ecologically important” area and birds land there. (Wetlands and grasslands are also very important for different species).

Chicago Best Stopover Site For Birds. Thus, fully understood, the new research confirms what the Illinois experts observed. Migrants are attracted to the land Chicago metro area because it is  an “important ecological  region” where they can find food. As the Bird Conservation Network Breeding Bird Trends report stated: 

when dawn arrives and they are looking for food and shelter.”

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Chicago Is The Best Target For Migrants  The Chicago regional map shows why the metro region is the best option for birds. The option to the east is open water – not much food for passerines. The option to the west beyond the metro area is crop land – which many studies have found is very poor habitat for songbird migrants. Chicago is where the birds will land because that is where the food is. This image shows that canopy-cover (trees)- is much higher in Chicago than in the adjacent land. (Grassland birds are a different matter.)

Chicago Region Trees Initiative Priority Maps

Counterintuitively, Chicago is the best habitat in Illinois for birds. Indeed, Cornell’s Living Birds reported that “Breeding birds are doing better in the Chicago region than the rest of Illinois, according to a decades-long survey of the region’s natural areas conducted by a local conservation coalition.”

Showing dead birds in the concrete canyons is good media, not good science.

Is Chicago an outlier? Probably not. In the central flyway, metro areas are often surrounded or bordered by farmland. Most migration paths are adjacent to water. Many metro areas also have extensive tree cover. Each ecological region needs to be assessed on its own terms.

The policy implications are clear. Even if herculean efforts succeed in lowering the sky glow in metro Chicago, birds will be attracted to Chicago to find food. Lowering a metro area sky glow to some lower level, is a quixotic quest. We do not even know if there is a safe level.

Better Option: Stopover Sites

Chicago also points the way to the best strategy to protect the birds attracted to it.  It is to install collision prevention systems to prevent deadly bird window crashes. When such a system was installed on Chicago’s most problematic building it reduced collision deaths by 95%. Thus,  a better option is to concentrate on protecting migrating birds at the stopover sites within the metropolitan area where more birds spend far more time than they do flying. Birds face the greatest risk where they spend the most time. They often stop over for several days at a time.  At these “hotspots” they face the dangers of both window collisions and cats. Unlike downtowns, it is also feasible to reduce window lights in limited areas.

Let’s protect them at those stopover sites.