## 4 Discussion  
### Canada Goose Abundance Hypothesis  
During the reference period (February 2022-December 2023), 74% of all H5N1 outbreaks in Iowa were reported, supporting the evaluation of Canada goose abundance during this time. The significant bivariate results indicating an inverse relationship between Canada goose abundance and H5N1 risk were unexpected, given the species' potential as a vector for transmission. Multivariate analysis confirmed the current unexpected results, as Canada goose abundance between the 25 th and 75 th percentiles was also associated with reduced risk of H5N1 outbreaks. One possible biological mechanism underlying this association is that Canada geese, due to their large size and territorial behavior, may displace other waterforb species that are more efficient transmitters of HPAI. Their dominance in shared habitats could reduce interspecies interactions and limit opportunities for viral transmission, despite their own potential to carry the virus. However, without access to detailed farm location data or significant findings for geese abundance above the 75 th percentile, we interpret these results with caution. It remains unclear whether higher goose abundance is associated with increased or decreased risk. Given that the Canada goose is among the most abundant waterforb species in North America and has demonstrated the ability to carry HPAI, we initially hypothesized that intermediate or high abundance would be associated with increased outbreak risk. The lack of support for this hypothesis raises the possibility that other vectors, such as rodents, may play a more prominent role in H5N1 transmission. Rodents frequently inhabit environments used by waterforb (e.g., inland water bodies) and can infiltrate poultry farms with compromised fencing [30]. Their anatomy allows them to collect and transport contaminated organic material, which could be deposited in areas where domestic birds are housed. These characteristics suggest rodents may act as effective mechanical vectors of HPAI. Our findings are consistent with recent studies that did not identify Canada geese as primary vectors of HPAI [16; 28]. For example, infected mallard ducks have been shown to \begin{table}
\begin{tabular}{c c c c} \hline
**Variables/Covariates** & **RR** & **95 CI\%** & _p-Value_ \\ \hline Canada goose abundance & & & \\ between 25 th and 75 th & 0.18 & \textless{}0.01, 37.12 & 0.52 \\ percentiles & & & \\ Canada goose abundance & & & \\ above 75 th percentile & \textgreater{}100 & \textless{}0.01, \textgreater{}100 & 0.95 \\ Mallard duck abundance & & & \\ between 25 th and 75 th & \textless{}0.01 & \textless{}0.01, \textgreater{}100 & 0.94 \\ percentiles & & & \\ Mallard duck abundance & 1.73 & \textless{}0.01, \textgreater{}100 & 0.99 \\ above 75 th percentile & 8.51 & 0.29, 17.84 & 0.21 \\ Swey goose abundance & & & \\ above 75 th percentile & 0.18 & \textless{}0.01, 3.85 & 0.28 \\ Wood duck abundance & & & \\ between 25 th and 75 th & \textgreater{}100 & \textless{}0.01, \textgreater{}100 & 0.92 \\ percentiles & & & \\ Wood duck abundance & & & \\ above 75 th percentile & \textgreater{}100 & \textless{}0.01, \textgreater{}100 & 0.95 \\ \hline \end{tabular}
\end{table} 
Table 5: _Cont._maintain migratory behavior while shedding H5N1 and exhibit greater viral tropism than Canada geese [16; 28]. These traits allow mallards to travel long distances while remaining infectious, a capacity not yet demonstrated in Canada geese. Our results further support the notion that Canada geese may not play the same role in HPAI transmission dynamics as mallards. Although our study yielded statistically significant inverse results, the role of Canada geese in H5N1 transmission remains inconclusive. This uncertainty stems from the lack of significant associations between both low and high Canada goose abundance and HPAI occurrence.