Older urban blackbirds’ nests held more human-made strips

A loose plastic strip on a pavement can become part of a bird’s home. That is worth noticing because human litter does not merely surround urban wildlife: animals may sort it, carry it and build it into the places where they raise their young.

Researchers examined 81 European blackbird nests associated with 88 breeding attempts by 44 females in Szczecin, Poland, between 2014 and 2021. They dismantled every nest and inventoried its human-made objects. For the 61 nests that could be separated into base, cup and lining, they also recorded which layer contained each item. The remaining 20 nests were examined whole, so they could contribute to the inventory but not to that layer-by-layer comparison.

The results, reported September 21 in Scientific Reports, reveal a measurable pattern without establishing why the birds produced it.

Human-made material appeared in 89% of the nests. The researchers counted 805 plastic fragments, alongside less common pieces of foil, ribbon, paper, tissue, wire, rubber and cotton. Artificial nest material, in other words, was not synonymous with plastic.

Nearly every human-made item was shaped like a strip. Among the 61 separable nests, the objects occurred most often in the base. Yet all the human-made material together represented an average of only 0.7% of nest mass. These were common additions, not nests substantially rebuilt from rubbish.

Three associations, translated

The study’s statistical model expressed changes on a logarithmic scale. Converting its coefficients into percentages makes their size easier to picture:

• Each additional year of female age was associated with about 16% more expected human-made pieces.
• A laying date 30 days later was associated with about 16% more expected pieces.
• Successful breeding attempts were associated with about 45% more expected pieces than failed attempts.

Anyone with a scientific calculator can reproduce those translations. Enter exp(0.15) - 1, exp(0.005 × 30) - 1 and exp(0.37) - 1. The results are approximately 0.162, 0.162 and 0.448, which become 16%, 16% and 45% after multiplying by 100 and rounding.

Those percentages describe expected counts in a model, not changes observed in every individual nest. They are also associations, not causal effects. The findings do not show that another birthday makes a female deliberately seek litter, that the strips strengthen a nest or that adding them improves breeding success.

What might age be measuring?

Experience is an appealing explanation. An older female has had more opportunities to encounter and handle unfamiliar materials. But age can accompany other differences too: where a bird holds territory, which objects are available there, when it breeds or qualities the study could not completely separate.

The laying-date result leaves similar possibilities open. Available materials may change during the season, or later breeders may differ from earlier ones for reasons unrelated to the calendar. A completed nest records what a bird carried home; it does not preserve all the objects she encountered and rejected.

The association with successful breeding is equally easy to overread. An experienced female or a favorable territory could contribute both to breeding success and to a larger collection of artificial pieces. The study also found no relationship between the presence of plastic strings and foundation strength. That analysis did not test whether the total number of human-made items predicted strength. Item number was also not associated with the female’s seasonal fledgling total or the duration of a failed attempt.

This is the revealing limit of dismantling a nest. It can show what was used, how much was present and—with the 61 separable nests—where an object was placed. It cannot recover intention from the finished structure or distinguish deliberate preference from convenient availability.

A strip and a loop are not the same hazard

Earlier studies show why material, shape and setting matter.

A long-running Danish study found that blackbird use of plastic rose after observations began in 1966, peaked in the 1970s and later declined. Nests containing plastic experienced greater predation outdoors but not inside buildings, according to the study in Avian Research. The reported consequence therefore depended on where the nest was built.

A study of American crows documented another kind of risk. Researchers monitored 195 nestlings in 106 nests across urban and agricultural areas in California. Of 54 nests later dissected, 85.2% contained human-made material. Eleven of the 195 nestlings became entangled, and the odds of entanglement increased 7.55-fold for each additional metre of human-made material in a nest, the researchers reported in PLOS ONE.

That crow study does not demonstrate an equivalent danger from the predominantly strip-shaped objects found in the blackbird nests. The species, materials, object dimensions and nest structures differ. Its useful lesson is narrower: “human-made” is not a mechanical property. A short fragment tucked into a base and a long loop of twine can interact with a bird in very different ways.

Keeping loose string, ribbon, fishing line and agricultural twine out of the environment removes objects already associated with entanglement risk. The new blackbird study supplies another reason to notice such debris: even when it forms less than 1% of a nest’s mass, it can become a regular and patterned part of urban bird architecture.

The available blackbird paper is a peer-reviewed accepted manuscript released early and still subject to editing before its final version. Its clearest discovery is not that older females “prefer plastic.” It is that older females’ nests contained more human-made pieces—and that even a carefully dismantled home can reveal more about a bird’s selections than about the reasons behind them.