Why are pole reversals not more detrimental to life?
There are several factors to consider. The main one is the atmosphere (especially if you want to compare Mars with the Earth's during magnetic reversals). Earth's atmosphere is a formidable shield against solar wind and cosmic radiation. Each type of radiation have a different penetration, but in general the radiation dose associated to each type of radiation decays exponentially at lower elevations. For example for ionizing radiation the relationship is

$E(z)=E_0 \left(0.21 e^{-1.649 z} + 0.79 e^{-0.4528 z}\right)$

Where $E$ is the dose, $E_0$ is the dose ate sea level and z is the altitude (reference). The following figure illustrate how the radiation dose drop at lower elevations. But if you extend that curve to 100 km you will be able to grasp how much of the incoming radiation is filtered by the atmosphere.



Figure: Cosmic radiation exposure as a function of altitude, latitude 50° north, taken from the European Nuclear Society.

Therefore even in the absence of magnetic field, only a small fraction of the radiation would make it to the surface. And note that reversals only means a weakening of the magnetic field, not a complete disappearance. What can disappear is the dipole component of the magnetic field, not the magnetic field as a whole.

It is important to note also that the magnetic field is very important to prevent atmospheric erosion. Therefore, a permanent lack of magnetic field can generate the loss of the atmosphere (likely what happened to Mars). Conversely, Earth's magnetic field is one of the reasons we have a very thick atmosphere compared to Mars, and during the brief periods of magnetic field weakening (due to pole reversals), our thick atmosphere still provided a quite effective protection.

The fact that space radiation doesn't penetrate much can be observed in many different ways, one of them is noting that auroras (a.k.a. northern lights) usually never happen below 90 km of elevation.

Moreover, the reason why auroras happen only at high latitudes (i.e. near the poles), is due to the "geomagnetic latitude effect" that describe how radiations dose rates are minimal at the equator and maximal at the geomagnetic poles. Nevertheless, the atmosphere absorbs most of that extra radiation and life thrives in the arctic forest without any problems.

Also, the atmosphere would not be just a passive filter, it could create its own magnetic field as some studies suggest. Such studies argue that as long as we have atmosphere, the solar wind can induced a magnetic field around the Earth even in the absence of internal magnetic field (reference)

The other important point is: Would the paleontologic record of migratory birds populations and magnetic-dependant microbial life be good enough to detect a catastrophic population reduction on migratory birds and some microbial life that happened 780,000 years ago? I'm not an expert but I would be skeptical. In fact, 780,000 years seems to be enough time for the evolution whole new species and the rise of the migratory behaviour from scratch. More recent reversal events like the Laschamp event 41,000 years ago were much quicker than 1,000 years, and even so, I wonder how well we could reconstruct migratory birds populations that far back (considering that we have trouble reconstructing human populations back then).

Regarding migratory birds, you say that after the reversal the birds would migrate in the wrong direction. And that is quite questionable. In the lack of any point of reference, using an improvised compass to know the north-south direction is easy, but figuring out which side is north and which is south is much more difficult. Therefore, maybe birds use other means to figure that out which side of their compasses is north, means that don't depend on the magnetic field. For example, the side at which sunrise happen or something like that.

Finally, microbes evolve and adapt very quickly, so even assuming that some (or many) of them would die during reversals, I'm sure others would take their place. At the end the ocean microbiota is nutrient-limited, and all the nutrients not used by the dying species will be used by other species that can thrive with a weaker magnetic field. So I wouldn't be so quick to think that such event would lead to a food chain collapse.