Mass changes of six migratory passerine species during stopover at a small, isolated urban patch

Background/Question/Methods

Habitats where birds can replenish energy spent during flight are crucial to successfully complete their migratory journey. However, migratory birds are increasingly flying over regions in North America that are extensively urbanized and with high levels of fragmentation and degradation of woodland habitat. It has been established that large city parks can provide suitable stopover habitat where migratory birds are able to gain in body mass. However, the argument for effective conservation of urban habitat for migrants is still not well established since potential urban stopovers are being overlooked because of their relatively small size. During fall and spring of 2010-2013, we investigated a 0.3 acre wooded plot located within the campus of Rutgers University in the city of Newark, NJ. Specifically, we aimed to determine if such small, isolated urban wooded patch functions as a suitable habitat where migratory birds gain in body mass during the estimated stopover time.  To explore this question, birds were trapped using mist-nets and each bird was banded, measured (weight and wing length), and its fat content visually assessed.  Individuals that were recaptured were weighed again and their fat contents reassessed.  

Results/Conclusions

A total of 2659 individual birds representing 71 different species were banded during the study period.  Six species, Common Yellowthroat (Geothlypis trichas), Gray Catbird (Dumetella carolinensis), Hermit Thrush (Catharus guttatus), Ovenbird (Seiurus aurocapillus), Swamp Sparrow (Melospiza georgiana), and White-throated Sparrow (Zonotrichia albicollis) yielded 187 recaptures corresponding to an average recapture rate of 7% (5-47%). Using dates of initial and final captures, it was estimated that the average stopover for the six species was 4 days during spring and 5 days during fall.  Analysis of body mass indicates that ground-foragers gained in body mass during stopover but the gains vary throughout duration of stopover and migratory season. Common Yellowthroat, a foliage gleaner, showed a decrease in body weight during spring and fall stopovers at the urban patch. These findings suggest that certain foraging guilds may be able to benefit from isolated, small, urban stopovers while others might not.