How are people managing invasive birds at nest boxes?

Photo © Allen Wilkinson

In 2018, NestWatch circulated a survey to investigate how many people had experience with non-native birds in their nest boxes. We were interested in finding out peoples’ knowledge of House Sparrows and European Starlings, and their attitudes towards managing them, especially if people encountered these non-native birds in the nest boxes they monitor. We are pleased to report that the findings have now been published online.

Witnessing Competition

We received 871 fully completed responses from NestWatchers and other people who monitor nest boxes. Nearly one-third of the respondents (30.3%) reported having witnessed a House Sparrow or European Starling usurping the nest of a native bird in 2018. In this context, usurping means to take over an active nest, causing it to fail. The people who witnessed this reported varying levels of competition by species. The two species which were most impacted by this competition were Northern Flicker (35.3%) and American Kestrel (23.0%), both of which are experiencing widespread population declines. In our study, sample size was small for Northern Flickers, but another study of natural cavity nests found that European Starlings were the largest source of nest failure for flickers (Tomasevic & Marzluff, 2017). Multiple sources of evidence suggest that competition from invasive species is a problem for flickers in both natural nest sites and nest boxes (at least in urban populations where starlings are most abundant). American Kestrels can sometimes outcompete starlings for nest cavities (McClure et al. 2015), but our nest box data revealed that kestrels are not always the winner. Unfortunately, it is not yet possible to exclude starlings from boxes made for Northern Flickers and American Kestrels because starlings are similar in size to these two vulnerable species.

Chickadees (all species combined) were the third most-impacted species, at 16.5% usurpation. Since they do not migrate, and because they are smaller in body size, chickadees may be in direct conflict with House Sparrows as these species seek out the same size nest cavity at the same time of year. For bluebirds (all species combined), 9.3% of nests were usurped, predominately by House Sparrows (starlings not being able to fit in standard bluebird boxes). We suspect that the usurpation rate is lower for bluebirds because more people are proactively managing for bluebirds, which are a beloved and iconic species.

Nest Usurpation Varies By Species
Nest Usurpation Varies By Species

Proportion of nest attempts by native host species usurped by non-native European Starlings and House Sparrows. Asterisks above columns indicate that House Sparrows and European Starlings usurped nests at statistically different levels; no asterisks indicate similar levels of impact from starlings and sparrows. Nest samples: n = 3,156 bluebirds; n = 1,468 Tree Swallows; n = 412 Purple Martins; n = 81 American Kestrels; n = 439 House Wrens; n = 19 Northern Flickers; n = 164 chickadees; n = 37 titmice; and n = 46 Bewick’s/Carolina Wrens.

Management Behaviors

Just over a third of our survey respondents reported that they did not manage for invasive species in 2018 (36.9%). But of those who did, eviction and repelling techniques were the most popular (e.g., removing nests, removing eggs, reducing entrance hole sizes, using Sparrow Spookers, etc.). Of those who managed their nest boxes actively (as opposed to preemptively), 33.4% were willing to trap and euthanize invasive species. Because removing nests and eggs may actually increase nest site takeovers if sparrows simply relocate to the nearest occupied box, this finding highlights the need to develop more effective non-lethal management techniques.

House Sparrow Egg Management
House Sparrow Egg Management

Nest monitors may choose from several methods of managing invasive species, from removing nests and eggs to euthanasia.

Our data reflect that the biggest difference between nest monitors who manage against House Sparrows and European Starlings (using any technique) and those who don’t comes down to whether or not they witnessed the failure of a nest that was taken over by either a starling or a sparrow. That is, those who experienced a takeover were 9.6 times more likely to manage than those who did not witness this in 2018. Additionally, the higher the respondent ranked the overall threat of non-native species to native birds, the more likely they were to manage invasive species. Interestingly, NestWatch participants were nearly twice as likely to take management action as non-participants, perhaps because of the desire to avoid reporting this type of failure. Gender, age, number of nest boxes, and number of years’ experience did not influence whether or not people managed their nest boxes to avoid usurpation by non-native species.

Key Takeaways:

  • Although our results only reflect nests in boxes and cannot speak to natural nests, competition from non-native species was evident across nine species groups: Northern Flicker, American Kestrel, Purple Martin, Tree Swallow, chickadees, titmice, House Wren, Bewick’s/Carolina Wren, and bluebirds.
  • Nest box monitors are engaged in a variety of management tasks that can support the nesting success of native birds. Non-lethal techniques are more popular, and need to be evaluated for their long-term efficacy.
  • NestWatch staff will be updating the data entry site to better record when a nest has been taken over by another species. Participants will soon see a new outcome option, and the ability to specify which species has taken over an active nest (including instances where native birds have taken over another native species’ nest).

NestWatch staff would like to thank all of the respondents who shared their experiences with us through the survey. Your responses give valuable insight into the social aspects of current ecological issues. Thank you!


References:

  • Bailey R. L., H. A. Faulkner-Grant, V. Y. Martin, T. B. Phillips, and D. N. Bonter. 2020. Nest usurpation by non-native birds and the role of people in nest box management. Conservation Science and Practice 2020: e185. https://doi.org/10.1111/csp2.185
  • McClure C. J. W., D. M. Hilleary, and D. P. Spurling. 2015. American Kestrels actively exclude European Starlings from using a nest box. Journal of Raptor Research 49: 231–233.
  • Tomasevic J. A., and J. M. Marzluff. 2017. Cavity nesting birds along an urban-wildland gradient: is human facilitation structuring the bird community? Urban Ecosystems 20: 435–448.

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